Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use the parentheses, the simple (but occasionally
-surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
+surprising) rule is this: It I<looks> like a function, therefore it I<is> a
function, and precedence doesn't matter. Otherwise it's a list
operator or unary operator, and precedence does matter. And whitespace
between the function and left parenthesis doesn't count--so you need to
of the same name (like chown(2), fork(2), closedir(2), etc.) all return
true when they succeed and C<undef> otherwise, as is usually mentioned
in the descriptions below. This is different from the C interfaces,
-which return C<-1> on failure. Exceptions to this rule are C<wait()>,
-C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
+which return C<-1> on failure. Exceptions to this rule are C<wait>,
+C<waitpid>, and C<syscall>. System calls also set the special C<$!>
variable on failure. Other functions do not, except accidentally.
=head2 Perl Functions by Category
arranged by category. Some functions appear in more
than one place.
-=over
+=over 4
=item Functions for SCALARs or strings
=item Keywords related to scoping
-C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
+C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
=item Miscellaneous functions
-C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
+C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
C<scalar>, C<undef>, C<wantarray>
=item Functions for processes and process groups
C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
-C<socket>, C<socketpair>
+C<sockatmark>, C<socket>, C<socketpair>
=item System V interprocess communication functions
=item Functions new in perl5
C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
-C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
-C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
+C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
+C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
* - C<sub> was a keyword in perl4, but in perl5 it is an
C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
-C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
-C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
+C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>,
+C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
+C<times>, C<truncate>, C<umask>, C<unlink>,
C<utime>, C<wait>, C<waitpid>
For more information about the portability of these functions, see
operator takes one argument, either a filename or a filehandle, and
tests the associated file to see if something is true about it. If the
argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
-Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
+Unless otherwise documented, it returns C<1> for true and C<''> for false, or
the undefined value if the file doesn't exist. Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator. The
-O File is owned by real uid.
-e File exists.
- -z File has zero size.
- -s File has nonzero size (returns size).
+ -z File has zero size (is empty).
+ -s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-g File has setgid bit set.
-k File has sticky bit set.
- -T File is a text file.
- -B File is a binary file (opposite of -T).
+ -T File is an ASCII text file.
+ -B File is a "binary" file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
Example:
while (<>) {
- chop;
+ chomp;
next unless -f $_; # ignore specials
#...
}
The C<-T> and C<-B> switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
-characters with the high bit set. If too many strange characters (E<gt>30%)
+characters with the high bit set. If too many strange characters (>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
containing null in the first block is considered a binary file. If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
-rather than the first block. Both C<-T> and C<-B> return TRUE on a null
+rather than the first block. Both C<-T> and C<-B> return true on a null
file, or a file at EOF when testing a filehandle. Because you have to
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.
-If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
+If any of the file tests (or either the C<stat> or C<lstat> operators) are given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call. (This doesn't work with C<-t>, and you need to remember
=item accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call
-does. Returns the packed address if it succeeded, FALSE otherwise.
+does. Returns the packed address if it succeeded, false otherwise.
See the example in L<perlipc/"Sockets: Client/Server Communication">.
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptor, as determined by the
+value of $^F. See L<perlvar/$^F>.
+
=item alarm SECONDS
=item alarm
on the previous timer.
For delays of finer granularity than one second, you may use Perl's
-four-arugment version of select() leaving the first three arguments
-undefined, or you might be able to use the C<syscall()> interface to
-access setitimer(2) if your system supports it. The Time::HiRes module
-from CPAN may also prove useful.
+four-argument version of select() leaving the first three arguments
+undefined, or you might be able to use the C<syscall> interface to
+access setitimer(2) if your system supports it. The Time::HiRes
+module (from CPAN, and starting from Perl 5.8 part of the standard
+distribution) may also prove useful.
-It is usually a mistake to intermix C<alarm()>
-and C<sleep()> calls.
+It is usually a mistake to intermix C<alarm> and C<sleep> calls.
+(C<sleep> may be internally implemented in your system with C<alarm>)
-If you want to use C<alarm()> to time out a system call you need to use an
-C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
+If you want to use C<alarm> to time out a system call you need to use an
+C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
-restart system calls on some systems. Using C<eval()>/C<die()> always works,
+restart system calls on some systems. Using C<eval>/C<die> always works,
modulo the caveats given in L<perlipc/"Signals">.
eval {
Returns the arctangent of Y/X in the range -PI to PI.
-For the tangent operation, you may use the C<POSIX::tan()>
+For the tangent operation, you may use the C<Math::Trig::tan>
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
=item bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call
-does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
+does. Returns true if it succeeded, false otherwise. NAME should be a
packed address of the appropriate type for the socket. See the examples in
L<perlipc/"Sockets: Client/Server Communication">.
+=item binmode FILEHANDLE, DISCIPLINE
+
=item binmode FILEHANDLE
-Arranges for the file to be read or written in "binary" mode in operating
-systems that distinguish between binary and text files. Files that
-are not in binary mode have CR LF sequences translated to LF on input
-and LF translated to CR LF on output. Binmode has no effect under
-many sytems, but in MS-DOS and similarly archaic systems, it may be
-imperative--otherwise your MS-DOS-damaged C library may mangle your file.
-The key distinction between systems that need C<binmode()> and those
-that don't is their text file formats. Systems like Unix, MacOS, and
-Plan9 that delimit lines with a single character, and that encode that
-character in C as C<"\n">, do not need C<binmode()>. The rest may need it.
-If FILEHANDLE is an expression, the value is taken as the name of the
-filehandle.
-
-If the system does care about it, using it when you shouldn't is just as
-perilous as failing to use it when you should. Fortunately for most of
-us, you can't go wrong using binmode() on systems that don't care about
-it, though.
+Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
+on systems where the run-time libraries distinguish between binary and
+text files. If FILEHANDLE is an expression, the value is taken as the
+name of the filehandle. DISCIPLINE can be either of C<":raw"> for
+binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
+omitted, it defaults to C<":raw">.
+
+binmode() should be called after open() but before any I/O is done on
+the filehandle.
+
+On many systems binmode() currently has no effect, but in future, it
+will be extended to support user-defined input and output disciplines.
+On some systems binmode() is necessary when you're not working with a
+text file. For the sake of portability it is a good idea to always use
+it when appropriate, and to never use it when it isn't appropriate.
+
+In other words: Regardless of platform, use binmode() on binary
+files, and do not use binmode() on text files.
+
+The C<open> pragma can be used to establish default disciplines.
+See L<open>.
+
+The operating system, device drivers, C libraries, and Perl run-time
+system all work together to let the programmer treat a single
+character (C<\n>) as the line terminator, irrespective of the external
+representation. On many operating systems, the native text file
+representation matches the internal representation, but on some
+platforms the external representation of C<\n> is made up of more than
+one character.
+
+Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
+character to end each line in the external representation of text (even
+though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
+on Unix and most VMS files). Consequently binmode() has no effect on
+these operating systems. In other systems like OS/2, DOS and the various
+flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
+what's stored in text files are the two characters C<\cM\cJ>. That means
+that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
+disk will be converted to C<\n> on input, and any C<\n> in your program
+will be converted back to C<\cM\cJ> on output. This is what you want for
+text files, but it can be disastrous for binary files.
+
+Another consequence of using binmode() (on some systems) is that
+special end-of-file markers will be seen as part of the data stream.
+For systems from the Microsoft family this means that if your binary
+data contains C<\cZ>, the I/O subsystem will regard it as the end of
+the file, unless you use binmode().
+
+binmode() is not only important for readline() and print() operations,
+but also when using read(), seek(), sysread(), syswrite() and tell()
+(see L<perlport> for more details). See the C<$/> and C<$\> variables
+in L<perlvar> for how to manually set your input and output
+line-termination sequences.
=item bless REF,CLASSNAME
This function tells the thingy referenced by REF that it is now an object
in the CLASSNAME package. If CLASSNAME is omitted, the current package
-is used. Because a C<bless()> is often the last thing in a constructor.
+is used. Because a C<bless> is often the last thing in a constructor,
it returns the reference for convenience. Always use the two-argument
version if the function doing the blessing might be inherited by a
derived class. See L<perltoot> and L<perlobj> for more about the blessing
Returns the context of the current subroutine call. In scalar context,
returns the caller's package name if there is a caller, that is, if
-we're in a subroutine or C<eval()> or C<require()>, and the undefined value
+we're in a subroutine or C<eval> or C<require>, and the undefined value
otherwise. In list context, returns
($package, $filename, $line) = caller;
print a stack trace. The value of EXPR indicates how many call frames
to go back before the current one.
- ($package, $filename, $line, $subroutine,
- $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
+ ($package, $filename, $line, $subroutine, $hasargs,
+ $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
-Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
-call, but an C<eval()>. In such a case additional elements C<$evaltext> and
+Here $subroutine may be C<(eval)> if the frame is not a subroutine
+call, but an C<eval>. In such a case additional elements $evaltext and
C<$is_require> are set: C<$is_require> is true if the frame is created by a
-C<require> or C<use> statement, C<$evaltext> contains the text of the
-C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
-C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
+C<require> or C<use> statement, $evaltext contains the text of the
+C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
+$filename is C<(eval)>, but $evaltext is undefined. (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
-frame.
+frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
+frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
+was compiled with. The C<$hints> and C<$bitmask> values are subject to
+change between versions of Perl, and are not meant for external use.
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable C<@DB::args> to be the
arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before
-C<caller()> had a chance to get the information. That means that C<caller(N)>
+C<caller> had a chance to get the information. That means that C<caller(N)>
might not return information about the call frame you expect it do, for
-C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
-previous time C<caller()> was called.
+C<< N > 1 >>. In particular, C<@DB::args> might have information from the
+previous time C<caller> was called.
=item chdir EXPR
Changes the working directory to EXPR, if possible. If EXPR is omitted,
-changes to the user's home directory. Returns TRUE upon success,
-FALSE otherwise. See the example under C<die()>.
+changes to the directory specified by C<$ENV{HOME}>, if set; if not,
+changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
+set, C<chdir> does nothing. It returns true upon success, false
+otherwise. See the example under C<die>.
=item chmod LIST
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
+You can also import the symbolic C<S_I*> constants from the Fcntl
+module:
+
+ use Fcntl ':mode';
+
+ chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
+ # This is identical to the chmod 0755 of the above example.
+
=item chomp VARIABLE
=item chomp LIST
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline. When in paragraph
mode (C<$/ = "">), it removes all trailing newlines from the string.
+When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
+a reference to an integer or the like, see L<perlvar>) chomp() won't
+remove anything.
If VARIABLE is omitted, it chomps C<$_>. Example:
while (<>) {
# ...
}
+If VARIABLE is a hash, it chomps the hash's values, but not its keys.
+
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
=item chop
Chops off the last character of a string and returns the character
-chopped. It's used primarily to remove the newline from the end of an
-input record, but is much more efficient than C<s/\n//> because it neither
+chopped. It is much more efficient than C<s/.$//s> because it neither
scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
-Example:
+If VARIABLE is a hash, it chops the hash's values, but not its keys.
- while (<>) {
- chop; # avoid \n on last field
- @array = split(/:/);
- #...
- }
-
-You can actually chop anything that's an lvalue, including an assignment:
-
- chop($cwd = `pwd`);
- chop($answer = <STDIN>);
+You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the
-last C<chop()> is returned.
+last C<chop> is returned.
-Note that C<chop()> returns the last character. To return all but the last
+Note that C<chop> returns the last character. To return all but the last
character, use C<substr($string, 0, -1)>.
=item chown LIST
Changes the owner (and group) of a list of files. The first two
-elements of the list must be the I<NUMERICAL> uid and gid, in that order.
-Returns the number of files successfully changed.
+elements of the list must be the I<numeric> uid and gid, in that
+order. A value of -1 in either position is interpreted by most
+systems to leave that value unchanged. Returns the number of files
+successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
- chop($user = <STDIN>);
+ chomp($user = <STDIN>);
print "Files: ";
- chop($pattern = <STDIN>);
+ chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups. On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.
+On POSIX systems, you can detect this condition this way:
+
+ use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
+ $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
=item chr NUMBER
Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
-chr(0x263a) is a Unicode smiley face (but only within the scope of
-a C<use utf8>). For the reverse, use L</ord>.
+chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
+characters higher than 127 are encoded in Unicode; if you don't want
+this, temporarily C<use bytes> or use C<pack("C*",...)>
+
+For the reverse, use L</ord>.
See L<utf8> for more about Unicode.
If NUMBER is omitted, uses C<$_>.
This function works like the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
-begin with a C<"/"> by your process and all its children. (It doesn't
+begin with a C</> by your process and all its children. (It doesn't
change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is
-omitted, does a C<chroot()> to C<$_>.
+omitted, does a C<chroot> to C<$_>.
=item close FILEHANDLE
=item close
-Closes the file or pipe associated with the file handle, returning TRUE
+Closes the file or pipe associated with the file handle, returning true
only if stdio successfully flushes buffers and closes the system file
-descriptor. Closes the currently selected filehandle if the argument
+descriptor. Closes the currently selected filehandle if the argument
is omitted.
You don't have to close FILEHANDLE if you are immediately going to do
-another C<open()> on it, because C<open()> will close it for you. (See
-C<open()>.) However, an explicit C<close()> on an input file resets the line
-counter (C<$.>), while the implicit close done by C<open()> does not.
+another C<open> on it, because C<open> will close it for you. (See
+C<open>.) However, an explicit C<close> on an input file resets the line
+counter (C<$.>), while the implicit close done by C<open> does not.
-If the file handle came from a piped open C<close()> will additionally
-return FALSE if one of the other system calls involved fails or if the
+If the file handle came from a piped open C<close> will additionally
+return false if one of the other system calls involved fails or if the
program exits with non-zero status. (If the only problem was that the
-program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
+program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
also waits for the process executing on the pipe to complete, in case you
-want to look at the output of the pipe afterwards, and
+want to look at the output of the pipe afterwards, and
implicitly puts the exit status value of that command into C<$?>.
+Prematurely closing the read end of a pipe (i.e. before the process
+writing to it at the other end has closed it) will result in a
+SIGPIPE being delivered to the writer. If the other end can't
+handle that, be sure to read all the data before closing the pipe.
+
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
=item closedir DIRHANDLE
-Closes a directory opened by C<opendir()> and returns the success of that
+Closes a directory opened by C<opendir> and returns the success of that
system call.
DIRHANDLE may be an expression whose value can be used as an indirect
=item connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect system call
-does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
+does. Returns true if it succeeded, false otherwise. NAME should be a
packed address of the appropriate type for the socket. See the examples in
L<perlipc/"Sockets: Client/Server Communication">.
statement).
C<last>, C<next>, or C<redo> may appear within a C<continue>
-block. C<last> and C<redo> will behave as if they had been executed within
-the main block. So will C<next>, but since it will execute a C<continue>
+block. C<last> and C<redo> will behave as if they had been executed within
+the main block. So will C<next>, but since it will execute a C<continue>
block, it may be more entertaining.
while (EXPR) {
### last always comes here
Omitting the C<continue> section is semantically equivalent to using an
-empty one, logically enough. In that case, C<next> goes directly back
+empty one, logically enough. In that case, C<next> goes directly back
to check the condition at the top of the loop.
=item cos EXPR
+=item cos
+
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
takes cosine of C<$_>.
-For the inverse cosine operation, you may use the C<POSIX::acos()>
+For the inverse cosine operation, you may use the C<Math::Trig::acos()>
function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
the password file for lousy passwords, amongst other things. Only the
guys wearing white hats should do this.
-Note that C<crypt()> is intended to be a one-way function, much like breaking
+Note that C<crypt> is intended to be a one-way function, much like breaking
eggs to make an omelette. There is no (known) corresponding decrypt
function. As a result, this function isn't all that useful for
cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted
text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
-allows your code to work with the standard C<crypt()> and with more
+allows your code to work with the standard C<crypt> and with more
exotic implementations. When choosing a new salt create a random two
character string whose characters come from the set C<[./0-9A-Za-z]>
(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
Of course, typing in your own password to whoever asks you
for it is unwise.
+The L<crypt> function is unsuitable for encrypting large quantities
+of data, not least of all because you can't get the information
+back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
+on your favorite CPAN mirror for a slew of potentially useful
+modules.
+
=item dbmclose HASH
-[This function has been largely superseded by the C<untie()> function.]
+[This function has been largely superseded by the C<untie> function.]
Breaks the binding between a DBM file and a hash.
-=item dbmopen HASH,DBNAME,MODE
+=item dbmopen HASH,DBNAME,MASK
-[This function has been largely superseded by the C<tie()> function.]
+[This function has been largely superseded by the C<tie> function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
-hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
-argument is I<NOT> a filehandle, even though it looks like one). DBNAME
+hash. HASH is the name of the hash. (Unlike normal C<open>, the first
+argument is I<not> a filehandle, even though it looks like one). DBNAME
is the name of the database (without the F<.dir> or F<.pag> extension if
any). If the database does not exist, it is created with protection
-specified by MODE (as modified by the C<umask()>). If your system supports
-only the older DBM functions, you may perform only one C<dbmopen()> in your
+specified by MASK (as modified by the C<umask>). If your system supports
+only the older DBM functions, you may perform only one C<dbmopen> in your
program. In older versions of Perl, if your system had neither DBM nor
-ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
+ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
sdbm(3).
If you don't have write access to the DBM file, you can only read hash
variables, not set them. If you want to test whether you can write,
-either use file tests or try setting a dummy hash entry inside an C<eval()>,
+either use file tests or try setting a dummy hash entry inside an C<eval>,
which will trap the error.
-Note that functions such as C<keys()> and C<values()> may return huge lists
-when used on large DBM files. You may prefer to use the C<each()>
+Note that functions such as C<keys> and C<values> may return huge lists
+when used on large DBM files. You may prefer to use the C<each>
function to iterate over large DBM files. Example:
# print out history file offsets
other values. (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and C<"0">, which are all equally
false.) Note that since C<undef> is a valid scalar, its presence
-doesn't I<necessarily> indicate an exceptional condition: C<pop()>
+doesn't I<necessarily> indicate an exceptional condition: C<pop>
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.
-You may also use C<defined()> to check whether a subroutine exists, by
-saying C<defined &func> without parentheses. On the other hand, use
-of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
-produce intuitive results, and should probably be avoided.
+You may also use C<defined(&func)> to check whether subroutine C<&func>
+has ever been defined. The return value is unaffected by any forward
+declarations of C<&foo>. Note that a subroutine which is not defined
+may still be callable: its package may have an C<AUTOLOAD> method that
+makes it spring into existence the first time that it is called -- see
+L<perlsub>.
+
+Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
+used to report whether memory for that aggregate has ever been
+allocated. This behavior may disappear in future versions of Perl.
+You should instead use a simple test for size:
+
+ if (@an_array) { print "has array elements\n" }
+ if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash. Use L</exists> for the latter
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
-Note: Many folks tend to overuse C<defined()>, and then are surprised to
+Note: Many folks tend to overuse C<defined>, and then are surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values. For example, if you say
matched something that happened to be zero characters long. This is all
very above-board and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer. So you
-should use C<defined()> only when you're questioning the integrity of what
+should use C<defined> only when you're questioning the integrity of what
you're trying to do. At other times, a simple comparison to C<0> or C<""> is
what you want.
-Currently, using C<defined()> on an entire array or hash reports whether
-memory for that aggregate has ever been allocated. So an array you set
-to the empty list appears undefined initially, and one that once was full
-and that you then set to the empty list still appears defined. You
-should instead use a simple test for size:
-
- if (@an_array) { print "has array elements\n" }
- if (%a_hash) { print "has hash members\n" }
-
-Using C<undef()> on these, however, does clear their memory and then report
-them as not defined anymore, but you shouldn't do that unless you don't
-plan to use them again, because it saves time when you load them up
-again to have memory already ready to be filled. The normal way to
-free up space used by an aggregate is to assign the empty list.
-
-This counterintuitive behavior of C<defined()> on aggregates may be
-changed, fixed, or broken in a future release of Perl.
-
See also L</undef>, L</exists>, L</ref>.
=item delete EXPR
-Deletes the specified key(s) and their associated values from a hash.
-For each key, returns the deleted value associated with that key, or
-the undefined value if there was no such key. Deleting from C<$ENV{}>
-modifies the environment. Deleting from a hash tied to a DBM file
-deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
-doesn't necessarily return anything.)
+Given an expression that specifies a hash element, array element, hash slice,
+or array slice, deletes the specified element(s) from the hash or array.
+In the case of an array, if the array elements happen to be at the end,
+the size of the array will shrink to the highest element that tests
+true for exists() (or 0 if no such element exists).
+
+Returns each element so deleted or the undefined value if there was no such
+element. Deleting from C<$ENV{}> modifies the environment. Deleting from
+a hash tied to a DBM file deletes the entry from the DBM file. Deleting
+from a C<tie>d hash or array may not necessarily return anything.
+
+Deleting an array element effectively returns that position of the array
+to its initial, uninitialized state. Subsequently testing for the same
+element with exists() will return false. Note that deleting array
+elements in the middle of an array will not shift the index of the ones
+after them down--use splice() for that. See L</exists>.
-The following deletes all the values of a hash:
+The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
-And so does this:
+ foreach $index (0 .. $#ARRAY) {
+ delete $ARRAY[$index];
+ }
+
+And so do these:
- delete @HASH{keys %HASH}
+ delete @HASH{keys %HASH};
+
+ delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list
-or undefining it:
+or undefining %HASH or @ARRAY:
+
+ %HASH = (); # completely empty %HASH
+ undef %HASH; # forget %HASH ever existed
- %hash = (); # completely empty %hash
- undef %hash; # forget %hash every existed
+ @ARRAY = (); # completely empty @ARRAY
+ undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the final
-operation is a hash element lookup or hash slice:
+operation is a hash element, array element, hash slice, or array slice
+lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
+ delete $ref->[$x][$y][$index];
+ delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
+
=item die LIST
-Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
-the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
-C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
-is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
-C<$@> and the C<eval()> is terminated with the undefined value. This makes
-C<die()> the way to raise an exception.
+Outside an C<eval>, prints the value of LIST to C<STDERR> and
+exits with the current value of C<$!> (errno). If C<$!> is C<0>,
+exits with the value of C<<< ($? >> 8) >>> (backtick `command`
+status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
+an C<eval(),> the error message is stuffed into C<$@> and the
+C<eval> is terminated with the undefined value. This makes
+C<die> the way to raise an exception.
Equivalent examples:
die() can also be called with a reference argument. If this happens to be
trapped within an eval(), $@ contains the reference. This behavior permits
a more elaborate exception handling implementation using objects that
-maintain arbitary state about the nature of the exception. Such a scheme
+maintain arbitrary state about the nature of the exception. Such a scheme
is sometimes preferable to matching particular string values of $@ using
regular expressions. Here's an example:
}
}
-Since perl will stringify uncaught exception messages before displaying
+Because perl will stringify uncaught exception messages before displaying
them, you may want to overload stringification operations on such custom
exception objects. See L<overload> for details about that.
-You can arrange for a callback to be run just before the C<die()> does
-its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
-will be called with the error text and can change the error message, if
-it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
-setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
-
-Note that the C<$SIG{__DIE__}> hook is currently called even inside
-eval()ed blocks/strings! If one wants the hook to do nothing in such
-situations, put
+You can arrange for a callback to be run just before the C<die>
+does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
+handler will be called with the error text and can change the error
+message, if it sees fit, by calling C<die> again. See
+L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
+L<"eval BLOCK"> for some examples. Although this feature was meant
+to be run only right before your program was to exit, this is not
+currently the case--the C<$SIG{__DIE__}> hook is currently called
+even inside eval()ed blocks/strings! If one wants the hook to do
+nothing in such situations, put
die @_ if $^S;
-as the first line of the handler (see L<perlvar/$^S>). Because this
-promotes action at a distance, this counterintuitive behavior may be fixed
-in a future release.
+as the first line of the handler (see L<perlvar/$^S>). Because
+this promotes strange action at a distance, this counterintuitive
+behavior may be fixed in a future release.
=item do BLOCK
is just like
- scalar eval `cat stat.pl`;
+ eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current
filename for error messages, searches the @INC libraries, and updates
evaluated.
Note that inclusion of library modules is better done with the
-C<use()> and C<require()> operators, which also do automatic error checking
+C<use> and C<require> operators, which also do automatic error checking
and raise an exception if there's a problem.
You might like to use C<do> to read in a program configuration
file. Manual error checking can be done this way:
- # read in config files: system first, then user
+ # read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
- "$ENV{HOME}/.someprogrc")
+ "$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
=item dump
-This causes an immediate core dump. Primarily this is so that you can
-use the B<undump> program to turn your core dump into an executable binary
-after having initialized all your variables at the beginning of the
-program. When the new binary is executed it will begin by executing a
-C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
-it as a goto with an intervening core dump and reincarnation. If C<LABEL>
-is omitted, restarts the program from the top. WARNING: Any files
-opened at the time of the dump will NOT be open any more when the
-program is reincarnated, with possible resulting confusion on the part
-of Perl. See also B<-u> option in L<perlrun>.
-
-Example:
-
- #!/usr/bin/perl
- require 'getopt.pl';
- require 'stat.pl';
- %days = (
- 'Sun' => 1,
- 'Mon' => 2,
- 'Tue' => 3,
- 'Wed' => 4,
- 'Thu' => 5,
- 'Fri' => 6,
- 'Sat' => 7,
- );
-
- dump QUICKSTART if $ARGV[0] eq '-d';
-
- QUICKSTART:
- Getopt('f');
-
-This operator is largely obsolete, partly because it's very hard to
-convert a core file into an executable, and because the real perl-to-C
-compiler has superseded it.
+This function causes an immediate core dump. See also the B<-u>
+command-line switch in L<perlrun>, which does the same thing.
+Primarily this is so that you can use the B<undump> program (not
+supplied) to turn your core dump into an executable binary after
+having initialized all your variables at the beginning of the
+program. When the new binary is executed it will begin by executing
+a C<goto LABEL> (with all the restrictions that C<goto> suffers).
+Think of it as a goto with an intervening core dump and reincarnation.
+If C<LABEL> is omitted, restarts the program from the top.
+
+B<WARNING>: Any files opened at the time of the dump will I<not>
+be open any more when the program is reincarnated, with possible
+resulting confusion on the part of Perl.
+
+This function is now largely obsolete, partly because it's very
+hard to convert a core file into an executable, and because the
+real compiler backends for generating portable bytecode and compilable
+C code have superseded it.
+
+If you're looking to use L<dump> to speed up your program, consider
+generating bytecode or native C code as described in L<perlcc>. If
+you're just trying to accelerate a CGI script, consider using the
+C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
+You might also consider autoloading or selfloading, which at least
+make your program I<appear> to run faster.
=item each HASH
When called in list context, returns a 2-element list consisting of the
key and value for the next element of a hash, so that you can iterate over
-it. When called in scalar context, returns the key for only the "next"
-element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
-false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
-for this reason.)
+it. When called in scalar context, returns only the key for the next
+element in the hash.
Entries are returned in an apparently random order. The actual random
order is subject to change in future versions of perl, but it is guaranteed
-to be in the same order as either the C<keys()> or C<values()> function
+to be in the same order as either the C<keys> or C<values> function
would produce on the same (unmodified) hash.
When the hash is entirely read, a null array is returned in list context
-(which when assigned produces a FALSE (C<0>) value), and C<undef> in
-scalar context. The next call to C<each()> after that will start iterating
-again. There is a single iterator for each hash, shared by all C<each()>,
-C<keys()>, and C<values()> function calls in the program; it can be reset by
+(which when assigned produces a false (C<0>) value), and C<undef> in
+scalar context. The next call to C<each> after that will start iterating
+again. There is a single iterator for each hash, shared by all C<each>,
+C<keys>, and C<values> function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating C<keys HASH> or
C<values HASH>. If you add or delete elements of a hash while you're
-iterating over it, you may get entries skipped or duplicated, so don't.
+iterating over it, you may get entries skipped or duplicated, so
+don't. Exception: It is always safe to delete the item most recently
+returned by C<each()>, which means that the following code will work:
+
+ while (($key, $value) = each %hash) {
+ print $key, "\n";
+ delete $hash{$key}; # This is safe
+ }
The following prints out your environment like the printenv(1) program,
only in a different order:
print "$key=$value\n";
}
-See also C<keys()>, C<values()> and C<sort()>.
+See also C<keys>, C<values> and C<sort>.
=item eof FILEHANDLE
Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open. FILEHANDLE may be an expression whose value
gives the real filehandle. (Note that this function actually
-reads a character and then C<ungetc()>s it, so isn't very useful in an
+reads a character and then C<ungetc>s it, so isn't very useful in an
interactive context.) Do not read from a terminal file (or call
-C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
+C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
as terminals may lose the end-of-file condition if you do.
-An C<eof> without an argument uses the last file read as argument.
-Using C<eof()> with empty parentheses is very different. It indicates
-the pseudo file formed of the files listed on the command line, i.e.,
-C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)> loop to
-detect the end of only the last file. Use C<eof(ARGV)> or eof without the
-parentheses to test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
+An C<eof> without an argument uses the last file read. Using C<eof()>
+with empty parentheses is very different. It refers to the pseudo file
+formed from the files listed on the command line and accessed via the
+C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
+as a normal filehandle is, an C<eof()> before C<< <> >> has been
+used will cause C<@ARGV> to be examined to determine if input is
+available.
+
+In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
+detect the end of each file, C<eof()> will only detect the end of the
+last file. Examples:
# reset line numbering on each input file
while (<>) {
- next if /^\s*#/; # skip comments
+ next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators return false values when they run out of data, or if there
-was an error.
+input operators typically return C<undef> when they run out of data, or if
+there was an error.
=item eval EXPR
In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program. The value of the expression (which is itself
determined within scalar context) is first parsed, and if there weren't any
-errors, executed in the context of the current Perl program, so that any
-variable settings or subroutine and format definitions remain afterwards.
-Note that the value is parsed every time the eval executes. If EXPR is
-omitted, evaluates C<$_>. This form is typically used to delay parsing
-and subsequent execution of the text of EXPR until run time.
+errors, executed in the lexical context of the current Perl program, so
+that any variable settings or subroutine and format definitions remain
+afterwards. Note that the value is parsed every time the eval executes.
+If EXPR is omitted, evaluates C<$_>. This form is typically used to
+delay parsing and subsequent execution of the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the eval itself was parsed--and executed
in void, scalar, or list context, depending on the context of the eval itself.
See L</wantarray> for more on how the evaluation context can be determined.
-If there is a syntax error or runtime error, or a C<die()> statement is
-executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
+If there is a syntax error or runtime error, or a C<die> statement is
+executed, an undefined value is returned by C<eval>, and C<$@> is set to the
error message. If there was no error, C<$@> is guaranteed to be a null
-string. Beware that using C<eval()> neither silences perl from printing
+string. Beware that using C<eval> neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
L</warn> and L<perlvar>.
-Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
-determining whether a particular feature (such as C<socket()> or C<symlink()>)
+Note that, because C<eval> traps otherwise-fatal errors, it is useful for
+determining whether a particular feature (such as C<socket> or C<symlink>)
is implemented. It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
warn $@ if $@;
This is especially significant, given that C<__DIE__> hooks can call
-C<die()> again, which has the effect of changing their error messages:
+C<die> again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
print $@ if $@; # prints "bar lives here"
}
-Because this promotes action at a distance, this counterintuive behavior
+Because this promotes action at a distance, this counterintuitive behavior
may be fixed in a future release.
-With an C<eval()>, you should be especially careful to remember what's
+With an C<eval>, you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in
-the variable C<$x>. (Although case 2 has misleading double quotes making
+the variable $x. (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).) Cases 3
and 4 likewise behave in the same way: they run the code C<'$x'>, which
-does nothing but return the value of C<$x>. (Case 4 is preferred for
+does nothing but return the value of $x. (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.) Case 5 is a place where
-normally you I<WOULD> like to use double quotes, except that in this
+normally you I<would> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.
=item exec PROGRAM LIST
-The C<exec()> function executes a system command I<AND NEVER RETURNS> -
-use C<system()> instead of C<exec()> if you want it to return. It fails and
-returns FALSE only if the command does not exist I<and> it is executed
+The C<exec> function executes a system command I<and never returns>--
+use C<system> instead of C<exec> if you want it to return. It fails and
+returns false only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).
-Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
-warns you if there is a following statement which isn't C<die()>, C<warn()>,
-or C<exit()> (if C<-w> is set - but you always do that). If you
-I<really> want to follow an C<exec()> with some other statement, you
+Since it's a common mistake to use C<exec> instead of C<system>, Perl
+warns you if there is a following statement which isn't C<die>, C<warn>,
+or C<exit> (if C<-w> is set - but you always do that). If you
+I<really> want to follow an C<exec> with some other statement, you
can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
the entire argument is passed to the system's command shell for parsing
(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
-words and passed directly to C<execvp()>, which is more efficient. Note:
-C<exec()> and C<system()> do not flush your output buffer, so you may need to
-set C<$|> to avoid lost output. Examples:
+words and passed directly to C<execvp>, which is more efficient.
+Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
be subject to its quirks and capabilities. See L<perlop/"`STRING`">
for details.
-Using an indirect object with C<exec()> or C<system()> is also more secure.
-This usage forces interpretation of the arguments as a multivalued list,
-even if the list had just one argument. That way you're safe from the
-shell expanding wildcards or splitting up words with whitespace in them.
+Using an indirect object with C<exec> or C<system> is also more
+secure. This usage (which also works fine with system()) forces
+interpretation of the arguments as a multivalued list, even if the
+list had just one argument. That way you're safe from the shell
+expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
didn't--it tried to run a program literally called I<"echo surprise">,
didn't find it, and set C<$?> to a non-zero value indicating failure.
-Note that C<exec()> will not call your C<END> blocks, nor will it call
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before the exec, but this may not be supported on some platforms
+(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
+in English) or call the C<autoflush()> method of C<IO::Handle> on any
+open handles in order to avoid lost output.
+
+Note that C<exec> will not call your C<END> blocks, nor will it call
any C<DESTROY> methods in your objects.
=item exists EXPR
-Returns TRUE if the specified hash key exists in its hash array, even
-if the corresponding value is undefined.
+Given an expression that specifies a hash element or array element,
+returns true if the specified element in the hash or array has ever
+been initialized, even if the corresponding value is undefined. The
+element is not autovivified if it doesn't exist.
- print "Exists\n" if exists $array{$key};
- print "Defined\n" if defined $array{$key};
- print "True\n" if $array{$key};
+ print "Exists\n" if exists $hash{$key};
+ print "Defined\n" if defined $hash{$key};
+ print "True\n" if $hash{$key};
-A hash element can be TRUE only if it's defined, and defined if
+ print "Exists\n" if exists $array[$index];
+ print "Defined\n" if defined $array[$index];
+ print "True\n" if $array[$index];
+
+A hash or array element can be true only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
+Given an expression that specifies the name of a subroutine,
+returns true if the specified subroutine has ever been declared, even
+if it is undefined. Mentioning a subroutine name for exists or defined
+does not count as declaring it. Note that a subroutine which does not
+exist may still be callable: its package may have an C<AUTOLOAD>
+method that makes it spring into existence the first time that it is
+called -- see L<perlsub>.
+
+ print "Exists\n" if exists &subroutine;
+ print "Defined\n" if defined &subroutine;
+
Note that the EXPR can be arbitrarily complicated as long as the final
-operation is a hash key lookup:
+operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
-Although the last element will not spring into existence just because
-its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
-and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
-existence test for a $key element. This happens anywhere the arrow
-operator is used, including even
+ if (exists $ref->{A}->{B}->[$ix]) { }
+ if (exists $hash{A}{B}[$ix]) { }
+
+ if (exists &{$ref->{A}{B}{$key}}) { }
+
+Although the deepest nested array or hash will not spring into existence
+just because its existence was tested, any intervening ones will.
+Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
+into existence due to the existence test for the $key element above.
+This happens anywhere the arrow operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
second--glance appear to be an lvalue context may be fixed in a future
release.
+See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
+on how exists() acts when used on a pseudo-hash.
+
+Use of a subroutine call, rather than a subroutine name, as an argument
+to exists() is an error.
+
+ exists ⊂ # OK
+ exists &sub(); # Error
+
=item exit EXPR
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
-See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
+See also C<die>. If EXPR is omitted, exits with C<0> status. The only
universally recognized values for EXPR are C<0> for success and C<1>
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running. For example, exiting
69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
the mailer to return the item undelivered, but that's not true everywhere.
-Don't use C<exit()> to abort a subroutine if there's any chance that
-someone might want to trap whatever error happened. Use C<die()> instead,
-which can be trapped by an C<eval()>.
+Don't use C<exit> to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use C<die> instead,
+which can be trapped by an C<eval>.
-The exit() function does not always exit immediately. It calls any
+The exit() function does not always exit immediately. It calls any
defined C<END> routines first, but these C<END> routines may not
-themselves abort the exit. Likewise any object destructors that need to
+themselves abort the exit. Likewise any object destructors that need to
be called are called before the real exit. If this is a problem, you
can call C<POSIX:_exit($status)> to avoid END and destructor processing.
-See L<perlsub> for details.
+See L<perlmod> for details.
=item exp EXPR
=item exp
-Returns I<e> (the natural logarithm base) to the power of EXPR.
+Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.
=item fcntl FILEHANDLE,FUNCTION,SCALAR
use Fcntl;
first to get the correct constant definitions. Argument processing and
-value return works just like C<ioctl()> below.
+value return works just like C<ioctl> below.
For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
-You don't have to check for C<defined()> on the return from C<fnctl()>.
-Like C<ioctl()>, it maps a C<0> return from the system call into "C<0>
-but true" in Perl. This string is true in boolean context and C<0>
+You don't have to check for C<defined> on the return from C<fnctl>.
+Like C<ioctl>, it maps a C<0> return from the system call into
+C<"0 but true"> in Perl. This string is true in boolean context and C<0>
in numeric context. It is also exempt from the normal B<-w> warnings
on improper numeric conversions.
-Note that C<fcntl()> will produce a fatal error if used on a machine that
+Note that C<fcntl> will produce a fatal error if used on a machine that
doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.
Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open. This is mainly useful for constructing
-bitmaps for C<select()> and low-level POSIX tty-handling operations.
+bitmaps for C<select> and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.
-You can use this to find out whether two handles refer to the
+You can use this to find out whether two handles refer to the
same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
- }
+ }
+
+(Filehandles connected to memory objects via new features of C<open> may
+return undefined even though they are open.)
+
=item flock FILEHANDLE,OPERATION
-Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE
-for success, FALSE on failure. Produces a fatal error if used on a
+Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
+for success, false on failure. Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
-C<flock()> is Perl's portable file locking interface, although it locks
+C<flock> is Perl's portable file locking interface, although it locks
only entire files, not records.
Two potentially non-obvious but traditional C<flock> semantics are
that it waits indefinitely until the lock is granted, and that its locks
B<merely advisory>. Such discretionary locks are more flexible, but offer
-fewer guarantees. This means that files locked with C<flock()> may be
-modified by programs that do not also use C<flock()>. See L<perlport>,
+fewer guarantees. This means that files locked with C<flock> may be
+modified by programs that do not also use C<flock>. See L<perlport>,
your port's specific documentation, or your system-specific local manpages
for details. It's best to assume traditional behavior if you're writing
portable programs. (But if you're not, you should as always feel perfectly
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
-you can use the symbolic names if import them from the Fcntl module,
+you can use the symbolic names if you import them from the Fcntl module,
either individually, or as a group using the ':flock' tag. LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
-releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
-LOCK_EX then C<flock()> will return immediately rather than blocking
+releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
+LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.
-Note also that some versions of C<flock()> cannot lock things over the
-network; you would need to use the more system-specific C<fcntl()> for
+Note also that some versions of C<flock> cannot lock things over the
+network; you would need to use the more system-specific C<fcntl> for
that. If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch C<-Ud_flock> to the F<Configure> program when you configure
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.
-Note: unflushed buffers remain unflushed in both processes, which means
-you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
-method of C<IO::Handle> to avoid duplicate output.
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before forking the child process, but this may not be supported
+on some platforms (see L<perlport>). To be safe, you may need to set
+C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
+C<IO::Handle> on any open handles in order to avoid duplicate output.
-If you C<fork()> without ever waiting on your children, you will
+If you C<fork> without ever waiting on your children, you will
accumulate zombies. On some systems, you can avoid this by setting
C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, then the remote server (such as, say, a CGI script or a
-backgrounded job launced from a remote shell) won't think you're done.
+backgrounded job launched from a remote shell) won't think you're done.
You should reopen those to F</dev/null> if it's any issue.
=item format
-Declare a picture format for use by the C<write()> function. For
+Declare a picture format for use by the C<write> function. For
example:
format Something =
too. It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
-Eventually, when a C<write()> is done, the contents of
+Eventually, when a C<write> is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
yourself and then set C<$^A> back to C<"">. Note that a format typically
-does one C<formline()> per line of form, but the C<formline()> function itself
+does one C<formline> per line of form, but the C<formline> function itself
doesn't care how many newlines are embedded in the PICTURE. This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.
-Be careful if you put double quotes around the picture, because an "C<@>"
+Be careful if you put double quotes around the picture, because an C<@>
character may be taken to mean the beginning of an array name.
-C<formline()> always returns TRUE. See L<perlform> for other examples.
+C<formline> always returns true. See L<perlform> for other examples.
=item getc FILEHANDLE
Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.
-The C<POSIX::getattr()> function can do this more portably on
+The C<POSIX::getattr> function can do this more portably on
systems purporting POSIX compliance. See also the C<Term::ReadKey>
module from your nearest CPAN site; details on CPAN can be found on
L<perlmodlib/CPAN>.
Implements the C library function of the same name, which on most
systems returns the current login from F</etc/utmp>, if any. If null,
-use C<getpwuid()>.
+use C<getpwuid>.
$login = getlogin || getpwuid($<) || "Kilroy";
-Do not consider C<getlogin()> for authentication: it is not as
-secure as C<getpwuid()>.
+Do not consider C<getlogin> for authentication: it is not as
+secure as C<getpwuid>.
=item getpeername SOCKET
use Socket;
$hersockaddr = getpeername(SOCK);
- ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
+ ($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
a PID of C<0> to get the current process group for the
current process. Will raise an exception if used on a machine that
doesn't implement getpgrp(2). If PID is omitted, returns process
-group of current process. Note that the POSIX version of C<getpgrp()>
+group of current process. Note that the POSIX version of C<getpgrp>
does not accept a PID argument, so only C<PID==0> is truly portable.
=item getppid
(If the entry doesn't exist you get a null list.)
+The exact meaning of the $gcos field varies but it usually contains
+the real name of the user (as opposed to the login name) and other
+information pertaining to the user. Beware, however, that in many
+system users are able to change this information and therefore it
+cannot be trusted and therefore the $gcos is tainted (see
+L<perlsec>). The $passwd and $shell, user's encrypted password and
+login shell, are also tainted, because of the same reason.
+
In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.) For example:
$name = getgrent();
#etc.
-In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are
-special cases in the sense that in many systems they are unsupported.
-If the C<$quota> is unsupported, it is an empty scalar. If it is
-supported, it usually encodes the disk quota. If the C<$comment>
-field is unsupported, it is an empty scalar. If it is supported it
-usually encodes some administrative comment about the user. In some
-systems the $quota field may be C<$change> or C<$age>, fields that have
-to do with password aging. In some systems the C<$comment> field may
-be C<$class>. The C<$expire> field, if present, encodes the expiration
-period of the account or the password. For the availability and the
-exact meaning of these fields in your system, please consult your
-getpwnam(3) documentation and your F<pwd.h> file. You can also find
-out from within Perl what your C<$quota> and C<$comment> fields mean
-and whether you have the C<$expire> field by using the C<Config> module
-and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
-and C<d_pwexpire>. Shadow password files are only supported if your
-vendor has implemented them in the intuitive fashion that calling the
-regular C library routines gets the shadow versions if you're running
-under privilege. Those that incorrectly implement a separate library
-call are not supported.
-
-The C<$members> value returned by I<getgr*()> is a space separated list of
+In I<getpw*()> the fields $quota, $comment, and $expire are special
+cases in the sense that in many systems they are unsupported. If the
+$quota is unsupported, it is an empty scalar. If it is supported, it
+usually encodes the disk quota. If the $comment field is unsupported,
+it is an empty scalar. If it is supported it usually encodes some
+administrative comment about the user. In some systems the $quota
+field may be $change or $age, fields that have to do with password
+aging. In some systems the $comment field may be $class. The $expire
+field, if present, encodes the expiration period of the account or the
+password. For the availability and the exact meaning of these fields
+in your system, please consult your getpwnam(3) documentation and your
+F<pwd.h> file. You can also find out from within Perl what your
+$quota and $comment fields mean and whether you have the $expire field
+by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
+C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
+files are only supported if your vendor has implemented them in the
+intuitive fashion that calling the regular C library routines gets the
+shadow versions if you're running under privilege or if there exists
+the shadow(3) functions as found in System V ( this includes Solaris
+and Linux.) Those systems which implement a proprietary shadow password
+facility are unlikely to be supported.
+
+The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.
For the I<gethost*()> functions, if the C<h_errno> variable is supported in
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
- $straddr = inet_ntoa($iaddr");
+ $straddr = inet_ntoa($iaddr);
-If you get tired of remembering which element of the return list contains
-which return value, by-name interfaces are also provided in modules:
-C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
-C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
-normal built-in, replacing them with versions that return objects with
-the appropriate names for each field. For example:
+If you get tired of remembering which element of the return list
+contains which return value, by-name interfaces are provided
+in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
+C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
+and C<User::grent>. These override the normal built-ins, supplying
+versions that return objects with the appropriate names
+for each field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
-Even though it looks like they're the same method calls (uid),
-they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
+Even though it looks like they're the same method calls (uid),
+they aren't, because a C<File::stat> object is different from
+a C<User::pwent> object.
=item getsockname SOCKET
-Returns the packed sockaddr address of this end of the SOCKET connection.
+Returns the packed sockaddr address of this end of the SOCKET connection,
+in case you don't know the address because you have several different
+IPs that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
- ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
+ ($port, $myaddr) = sockaddr_in($mysockaddr);
+ printf "Connect to %s [%s]\n",
+ scalar gethostbyaddr($myaddr, AF_INET),
+ inet_ntoa($myaddr);
=item getsockopt SOCKET,LEVEL,OPTNAME
Returns the value of EXPR with filename expansions such as the
standard Unix shell F</bin/csh> would do. This is the internal function
-implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
-If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
+implementing the C<< <*.c> >> operator, but you can use it directly.
+If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
discussed in more detail in L<perlop/"I/O Operators">.
+Beginning with v5.6.0, this operator is implemented using the standard
+C<File::Glob> extension. See L<File::Glob> for details.
+
=item gmtime EXPR
-Converts a time as returned by the time function to a 9-element array
+Converts a time as returned by the time function to a 8-element list
with the time localized for the standard Greenwich time zone.
Typically used as follows:
- # 0 1 2 3 4 5 6 7 8
- ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
+ # 0 1 2 3 4 5 6 7
+ ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
gmtime(time);
-All array elements are numeric, and come straight out of a struct tm.
-In particular this means that C<$mon> has the range C<0..11> and C<$wday>
-has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
-number of years since 1900, that is, C<$year> is C<123> in year 2023,
-I<not> simply the last two digits of the year. If you assume it is,
-then you create non-Y2K-compliant programs--and you wouldn't want to do
-that, would you?
+All list elements are numeric, and come straight out of the C `struct
+tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
+specified time. $mday is the day of the month, and $mon is the month
+itself, in the range C<0..11> with 0 indicating January and 11
+indicating December. $year is the number of years since 1900. That
+is, $year is C<123> in year 2023. $wday is the day of the week, with
+0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
+the year, in the range C<0..364> (or C<0..365> in leap years.)
+
+Note that the $year element is I<not> simply the last two digits of
+the year. If you assume it is, then you create non-Y2K-compliant
+programs--and you wouldn't want to do that, would you?
+
+The proper way to get a complete 4-digit year is simply:
+
+ $year += 1900;
-If EXPR is omitted, does C<gmtime(time())>.
+And to get the last two digits of the year (e.g., '01' in 2001) do:
-In scalar context, returns the ctime(3) value:
+ $year = sprintf("%02d", $year % 100);
+
+If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
+
+In scalar context, C<gmtime()> returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
-Also see the C<timegm()> function provided by the C<Time::Local> module,
+Also see the C<timegm> function provided by the C<Time::Local> module,
and the strftime(3) function available via the POSIX module.
This scalar value is B<not> locale dependent (see L<perllocale>), but
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a C<foreach> loop. It
also can't be used to go into a construct that is optimized away,
-or to get out of a block or subroutine given to C<sort()>.
+or to get out of a block or subroutine given to C<sort>.
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
-construct such as C<last> or C<die()>. The author of Perl has never felt the
+construct such as C<last> or C<die>. The author of Perl has never felt the
need to use this form of C<goto> (in Perl, that is--C is another matter).
The C<goto-EXPR> form expects a label name, whose scope will be resolved
goto ("FOO", "BAR", "GLARCH")[$i];
-The C<goto-&NAME> form is highly magical, and substitutes a call to the
-named subroutine for the currently running subroutine. This is used by
-C<AUTOLOAD> subroutines that wish to load another subroutine and then
-pretend that the other subroutine had been called in the first place
-(except that any modifications to C<@_> in the current subroutine are
-propagated to the other subroutine.) After the C<goto>, not even C<caller()>
-will be able to tell that this routine was called first.
+The C<goto-&NAME> form is quite different from the other forms of C<goto>.
+In fact, it isn't a goto in the normal sense at all, and doesn't have
+the stigma associated with other gotos. Instead, it
+substitutes a call to the named subroutine for the currently running
+subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
+another subroutine and then pretend that the other subroutine had been
+called in the first place (except that any modifications to C<@_>
+in the current subroutine are propagated to the other subroutine.)
+After the C<goto>, not even C<caller> will be able to tell that this
+routine was called first.
+
+NAME needn't be the name of a subroutine; it can be a scalar variable
+containing a code reference, or a block which evaluates to a code
+reference.
=item grep BLOCK LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value consisting of those
-elements for which the expression evaluated to TRUE. In scalar
-context, returns the number of times the expression was TRUE.
+elements for which the expression evaluated to true. In scalar
+context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
@foo = grep {!/^#/} @bar; # weed out comments
-Note that, because C<$_> is a reference into the list value, it can
-be used to modify the elements of the array. While this is useful and
-supported, it can cause bizarre results if the LIST is not a named array.
+Note that C<$_> is an alias to the list value, so it can be used to
+modify the elements of the LIST. While this is useful and supported,
+it can cause bizarre results if the elements of LIST are not variables.
Similarly, grep returns aliases into the original list, much as a for
loop's index variable aliases the list elements. That is, modifying an
-element of a list returned by grep (for example, in a C<foreach>, C<map()>
-or another C<grep()>) actually modifies the element in the original list.
+element of a list returned by grep (for example, in a C<foreach>, C<map>
+or another C<grep>) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.
-See also L</map> for an array composed of the results of the BLOCK or EXPR.
+See also L</map> for a list composed of the results of the BLOCK or EXPR.
=item hex EXPR
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
+Hex strings may only represent integers. Strings that would cause
+integer overflow trigger a warning.
+
=item import
-There is no builtin C<import()> function. It is just an ordinary
+There is no builtin C<import> function. It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
-names to another module. The C<use()> function calls the C<import()> method
-for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
+names to another module. The C<use> function calls the C<import> method
+for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
=item index STR,SUBSTR,POSITION
numbers can sometimes produce counterintuitive results. For example,
C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
because it's really more like -268.99999999999994315658 instead. Usually,
-the C<sprintf()>, C<printf()>, or the C<POSIX::floor> and C<POSIX::ceil>
+the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
functions will serve you better than will int().
=item ioctl FILEHANDLE,FUNCTION,SCALAR
to get the correct function definitions. If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
-own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
+own, based on your C header files such as F<< <sys/ioctl.h> >>.
(There is a Perl script called B<h2ph> that comes with the Perl kit that
may help you in this, but it's nontrivial.) SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
-will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
+will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value. To guarantee this to be
-TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
-functions are useful for manipulating the values of structures used by
-C<ioctl()>. The following example sets the erase character to DEL.
-
- require 'ioctl.ph';
- $getp = &TIOCGETP;
- die "NO TIOCGETP" if $@ || !$getp;
- $sgttyb_t = "ccccs"; # 4 chars and a short
- if (ioctl(STDIN,$getp,$sgttyb)) {
- @ary = unpack($sgttyb_t,$sgttyb);
- $ary[2] = 127;
- $sgttyb = pack($sgttyb_t,@ary);
- ioctl(STDIN,&TIOCSETP,$sgttyb)
- || die "Can't ioctl: $!";
- }
+true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
+functions may be needed to manipulate the values of structures used by
+C<ioctl>.
-The return value of C<ioctl()> (and C<fcntl()>) is as follows:
+The return value of C<ioctl> (and C<fcntl>) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
-Thus Perl returns TRUE on success and FALSE on failure, yet you can
+Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:
The special string "C<0> but true" is exempt from B<-w> complaints
about improper numeric conversions.
+Here's an example of setting a filehandle named C<REMOTE> to be
+non-blocking at the system level. You'll have to negotiate C<$|>
+on your own, though.
+
+ use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
+
+ $flags = fcntl(REMOTE, F_GETFL, 0)
+ or die "Can't get flags for the socket: $!\n";
+
+ $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
+ or die "Can't set flags for the socket: $!\n";
+
=item join EXPR,LIST
Joins the separate strings of LIST into a single string with fields
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
-See L</split>.
+Beware that unlike C<split>, C<join> doesn't take a pattern as its
+first argument. Compare L</split>.
=item keys HASH
-Returns a list consisting of all the keys of the named hash. (In a
+Returns a list consisting of all the keys of the named hash. (In
scalar context, returns the number of keys.) The keys are returned in
an apparently random order. The actual random order is subject to
change in future versions of perl, but it is guaranteed to be the same
-order as either the C<values()> or C<each()> function produces (given
+order as either the C<values> or C<each> function produces (given
that the hash has not been modified). As a side effect, it resets
HASH's iterator.
@keys = keys %ENV;
@values = values %ENV;
- while ($#keys >= 0) {
+ while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
print $key, '=', $ENV{$key}, "\n";
}
-To sort a hash by value, you'll need to use a C<sort()> function.
+The returned values are copies of the original keys in the hash, so
+modifying them will not affect the original hash. Compare L</values>.
+
+To sort a hash by value, you'll need to use a C<sort> function.
Here's a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
-As an lvalue C<keys()> allows you to increase the number of hash buckets
+As an lvalue C<keys> allows you to increase the number of hash buckets
allocated for the given hash. This can gain you a measure of efficiency if
you know the hash is going to get big. (This is similar to pre-extending
an array by assigning a larger number to $#array.) If you say
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
-C<keys()> in this way (but you needn't worry about doing this by accident,
+C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).
-See also C<each()>, C<values()> and C<sort()>.
+See also C<each>, C<values> and C<sort>.
-=item kill LIST
+=item kill SIGNAL, LIST
-Sends a signal to a list of processes. The first element of
-the list must be the signal to send. Returns the number of
-processes successfully signaled.
+Sends a signal to a list of processes. Returns the number of
+processes successfully signaled (which is not necessarily the
+same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
-Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
+If SIGNAL is zero, no signal is sent to the process. This is a
+useful way to check that the process is alive and hasn't changed
+its UID. See L<perlport> for notes on the portability of this
+construct.
+
+Unlike in the shell, if SIGNAL is negative, it kills
process groups instead of processes. (On System V, a negative I<PROCESS>
number will also kill process groups, but that's not portable.) That
means you usually want to use positive not negative signals. You may also
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.
+Note that a block by itself is semantically identical to a loop
+that executes once. Thus C<last> can be used to effect an early
+exit out of such a block.
+
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
Returns an lowercased version of EXPR. This is the internal function
implementing the C<\L> escape in double-quoted strings.
-Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
+and L<utf8>.
If EXPR is omitted, uses C<$_>.
=item length
Returns the length in characters of the value of EXPR. If EXPR is
-omitted, returns length of C<$_>. Note that this cannot be used on
+omitted, returns length of C<$_>. Note that this cannot be used on
an entire array or hash to find out how many elements these have.
For that, use C<scalar @array> and C<scalar keys %hash> respectively.
=item link OLDFILE,NEWFILE
-Creates a new filename linked to the old filename. Returns TRUE for
-success, FALSE otherwise.
+Creates a new filename linked to the old filename. Returns true for
+success, false otherwise.
=item listen SOCKET,QUEUESIZE
-Does the same thing that the listen system call does. Returns TRUE if
-it succeeded, FALSE otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
+Does the same thing that the listen system call does. Returns true if
+it succeeded, false otherwise. See the example in
+L<perlipc/"Sockets: Client/Server Communication">.
=item local EXPR
-You really probably want to be using C<my()> instead, because C<local()> isn't
-what most people think of as "local". See L<perlsub/"Private Variables
-via my()"> for details.
+You really probably want to be using C<my> instead, because C<local> isn't
+what most people think of as "local". See
+L<perlsub/"Private Variables via my()"> for details.
A local modifies the listed variables to be local to the enclosing
block, file, or eval. If more than one value is listed, the list must
=item localtime EXPR
-Converts a time as returned by the time function to a 9-element array
+Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone. Typically used as
follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
-All array elements are numeric, and come straight out of a struct tm.
-In particular this means that C<$mon> has the range C<0..11> and C<$wday>
-has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
-number of years since 1900, that is, C<$year> is C<123> in year 2023,
-and I<not> simply the last two digits of the year. If you assume it is,
-then you create non-Y2K-compliant programs--and you wouldn't want to do
-that, would you?
+All list elements are numeric, and come straight out of the C `struct
+tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
+specified time. $mday is the day of the month, and $mon is the month
+itself, in the range C<0..11> with 0 indicating January and 11
+indicating December. $year is the number of years since 1900. That
+is, $year is C<123> in year 2023. $wday is the day of the week, with
+0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
+the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
+is true if the specified time occurs during daylight savings time,
+false otherwise.
+
+Note that the $year element is I<not> simply the last two digits of
+the year. If you assume it is, then you create non-Y2K-compliant
+programs--and you wouldn't want to do that, would you?
+
+The proper way to get a complete 4-digit year is simply:
+
+ $year += 1900;
+
+And to get the last two digits of the year (e.g., '01' in 2001) do:
-If EXPR is omitted, uses the current time (C<localtime(time)>).
+ $year = sprintf("%02d", $year % 100);
-In scalar context, returns the ctime(3) value:
+If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
+
+In scalar context, C<localtime()> returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is B<not> locale dependent, see L<perllocale>, but
-instead a Perl builtin. Also see the C<Time::Local> module, and the
-strftime(3) and mktime(3) function available via the POSIX module. To
-get somewhat similar but locale dependent date strings, set up your
-locale environment variables appropriately (please see L<perllocale>)
-and try for example:
+instead a Perl builtin. Also see the C<Time::Local> module
+(to convert the second, minutes, hours, ... back to seconds since the
+stroke of midnight the 1st of January 1970, the value returned by
+time()), and the strftime(3) and mktime(3) functions available via the
+POSIX module. To get somewhat similar but locale dependent date
+strings, set up your locale environment variables appropriately
+(please see L<perllocale>) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
+=item lock
+
+ lock I<THING>
+
+This function places an advisory lock on a variable, subroutine,
+or referenced object contained in I<THING> until the lock goes out
+of scope. This is a built-in function only if your version of Perl
+was built with threading enabled, and if you've said C<use Threads>.
+Otherwise a user-defined function by this name will be called. See
+L<Thread>.
+
=item log EXPR
=item log
Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
returns log of C<$_>. To get the log of another base, use basic algebra:
-The base-N log of a number is is equal to the natural log of that number
+The base-N log of a number is equal to the natural log of that number
divided by the natural log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
- }
+ }
See also L</exp> for the inverse operation.
-=item lstat FILEHANDLE
-
=item lstat EXPR
=item lstat
-Does the same thing as the C<stat()> function (including setting the
+Does the same thing as the C<stat> function (including setting the
special C<_> filehandle) but stats a symbolic link instead of the file
the symbolic link points to. If symbolic links are unimplemented on
-your system, a normal C<stat()> is done.
+your system, a normal C<stat> is done.
If EXPR is omitted, stats C<$_>.
=item map EXPR,LIST
-Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
-element) and returns the list value composed of the results of each such
-evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
-may produce zero, one, or more elements in the returned value.
-
-In scalar context, returns the total number of elements so generated.
+Evaluates the BLOCK or EXPR for each element of LIST (locally setting
+C<$_> to each element) and returns the list value composed of the
+results of each such evaluation. In scalar context, returns the
+total number of elements so generated. Evaluates BLOCK or EXPR in
+list context, so each element of LIST may produce zero, one, or
+more elements in the returned value.
@chars = map(chr, @nums);
$hash{getkey($_)} = $_;
}
-Note that, because C<$_> is a reference into the list value, it can
-be used to modify the elements of the array. While this is useful and
-supported, it can cause bizarre results if the LIST is not a named array.
+Note that C<$_> is an alias to the list value, so it can be used to
+modify the elements of the LIST. While this is useful and supported,
+it can cause bizarre results if the elements of LIST are not variables.
Using a regular C<foreach> loop for this purpose would be clearer in
most cases. See also L</grep> for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.
-=item mkdir FILENAME,MODE
+C<{> starts both hash references and blocks, so C<map { ...> could be either
+the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
+ahead for the closing C<}> it has to take a guess at which its dealing with
+based what it finds just after the C<{>. Usually it gets it right, but if it
+doesn't it won't realize something is wrong until it gets to the C<}> and
+encounters the missing (or unexpected) comma. The syntax error will be
+reported close to the C<}> but you'll need to change something near the C<{>
+such as using a unary C<+> to give perl some help:
+
+ %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
+ %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
+ %hash = map { ("\L$_", 1) } @array # this also works
+ %hash = map { lc($_), 1 } @array # as does this.
+ %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
+
+ %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
+
+or to force an anon hash constructor use C<+{>
+
+ @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
+
+and you get list of anonymous hashes each with only 1 entry.
+
+=item mkdir FILENAME,MASK
+
+=item mkdir FILENAME
Creates the directory specified by FILENAME, with permissions
-specified by MODE (as modified by C<umask>). If it succeeds it
-returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
+specified by MASK (as modified by C<umask>). If it succeeds it
+returns true, otherwise it returns false and sets C<$!> (errno).
+If omitted, MASK defaults to 0777.
-In general, it is better to create directories with permissive MODEs,
+In general, it is better to create directories with permissive MASK,
and let the user modify that with their C<umask>, than it is to supply
-a restrictive MODE and give the user no way to be more permissive.
+a restrictive MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance). The perlfunc(1) entry on
-C<umask> discusses the choice of MODE in more detail.
+C<umask> discusses the choice of MASK in more detail.
+
+Note that according to the POSIX 1003.1-1996 the FILENAME may have any
+number of trailing slashes. Some operating and filesystems do not get
+this right, so Perl automatically removes all trailing slashes to keep
+everyone happy.
=item msgctl ID,CMD,ARG
first to get the correct constant definitions. If CMD is C<IPC_STAT>,
then ARG must be a variable which will hold the returned C<msqid_ds>
-structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
-true" for zero, or the actual return value otherwise. See also
-C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
+structure. Returns like C<ioctl>: the undefined value for error,
+C<"0 but true"> for zero, or the actual return value otherwise. See also
+L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
=item msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue
-id, or the undefined value if there is an error. See also C<IPC::SysV>
-and C<IPC::SysV::Msg> documentation.
-
-=item msgsnd ID,MSG,FLAGS
-
-Calls the System V IPC function msgsnd to send the message MSG to the
-message queue ID. MSG must begin with the long integer message type,
-which may be created with C<pack("l", $type)>. Returns TRUE if
-successful, or FALSE if there is an error. See also C<IPC::SysV>
-and C<IPC::SysV::Msg> documentation.
+id, or the undefined value if there is an error. See also
+L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
-SIZE. Note that if a message is received, the message type will be
-the first thing in VAR, and the maximum length of VAR is SIZE plus the
-size of the message type. Returns TRUE if successful, or FALSE if
-there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
+SIZE. Note that when a message is received, the message type as a
+native long integer will be the first thing in VAR, followed by the
+actual message. This packing may be opened with C<unpack("l! a*")>.
+Taints the variable. Returns true if successful, or false if there is
+an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
+C<IPC::SysV::Msg> documentation.
+
+=item msgsnd ID,MSG,FLAGS
+
+Calls the System V IPC function msgsnd to send the message MSG to the
+message queue ID. MSG must begin with the native long integer message
+type, and be followed by the length of the actual message, and finally
+the message itself. This kind of packing can be achieved with
+C<pack("l! a*", $type, $message)>. Returns true if successful,
+or false if there is an error. See also C<IPC::SysV>
+and C<IPC::SysV::Msg> documentation.
=item my EXPR
-A C<my()> declares the listed variables to be local (lexically) to the
-enclosing block, file, or C<eval()>. If
+=item my EXPR : ATTRIBUTES
+
+A C<my> declares the listed variables to be local (lexically) to the
+enclosing block, file, or C<eval>. If
more than one value is listed, the list must be placed in parentheses. See
L<perlsub/"Private Variables via my()"> for details.
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.
+Note that a block by itself is semantically identical to a loop
+that executes once. Thus C<next> will exit such a block early.
+
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
$val = oct($val) if $val =~ /^0/;
-If EXPR is omitted, uses C<$_>. This function is commonly used when
-a string such as C<644> needs to be converted into a file mode, for
-example. (Although perl will automatically convert strings into
-numbers as needed, this automatic conversion assumes base 10.)
+If EXPR is omitted, uses C<$_>. To go the other way (produce a number
+in octal), use sprintf() or printf():
+
+ $perms = (stat("filename"))[2] & 07777;
+ $oct_perms = sprintf "%lo", $perms;
+
+The oct() function is commonly used when a string such as C<644> needs
+to be converted into a file mode, for example. (Although perl will
+automatically convert strings into numbers as needed, this automatic
+conversion assumes base 10.)
=item open FILEHANDLE,EXPR
+=item open FILEHANDLE,MODE,EXPR
+
+=item open FILEHANDLE,MODE,EXPR,LIST
+
=item open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates it with
-FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
-name of the real filehandle wanted. If EXPR is omitted, the scalar
+FILEHANDLE. If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
+assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression,
+its value is used as the name of the real filehandle wanted. (This is considered a symbolic
+reference, so C<use strict 'refs'> should I<not> be in effect.)
+
+If EXPR is omitted, the scalar
variable of the same name as the FILEHANDLE contains the filename.
-(Note that lexical variables--those declared with C<my()>--will not work
-for this purpose; so if you're using C<my()>, specify EXPR in your call
+(Note that lexical variables--those declared with C<my>--will not work
+for this purpose; so if you're using C<my>, specify EXPR in your call
to open.) See L<perlopentut> for a kinder, gentler explanation of opening
files.
-If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
-If the filename begins with C<'E<gt>'>, the file is truncated and opened for
-output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
-the file is opened for appending, again being created if necessary.
-You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
-you want both read and write access to the file; thus C<'+E<lt>'> is almost
-always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
+If three or more arguments are specified then the mode of opening and the file name
+are separate. If MODE is C<< '<' >> or nothing, the file is opened for input.
+If MODE is C<< '>' >>, the file is truncated and opened for
+output, being created if necessary. If MODE is C<<< '>>' >>>,
+the file is opened for appending, again being created if necessary.
+You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
+you want both read and write access to the file; thus C<< '+<' >> is almost
+always preferred for read/write updates--the C<< '+>' >> mode would clobber the
file first. You can't usually use either read-write mode for updating
textfiles, since they have variable length records. See the B<-i>
switch in L<perlrun> for a better approach. The file is created with
permissions of C<0666> modified by the process' C<umask> value.
-The prefix and the filename may be separated with spaces.
-These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
-C<'w+'>, C<'a'>, and C<'a+'>.
+These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
+C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
+
+In the 2-arguments (and 1-argument) form of the call the mode and
+filename should be concatenated (in this order), possibly separated by
+spaces. It is possible to omit the mode in these forms if the mode is
+C<< '<' >>.
If the filename begins with C<'|'>, the filename is interpreted as a
command to which output is to be piped, and if the filename ends with a
C<'|'>, the filename is interpreted as a command which pipes output to
us. See L<perlipc/"Using open() for IPC">
-for more examples of this. (You are not allowed to C<open()> to a command
+for more examples of this. (You are not allowed to C<open> to a command
+that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
+and L<perlipc/"Bidirectional Communication with Another Process">
+for alternatives.)
+
+For three or more arguments if MODE is C<'|-'>, the filename is interpreted as a
+command to which output is to be piped, and if MODE is
+C<'-|'>, the filename is interpreted as a command which pipes output to
+us. In the 2-arguments (and 1-argument) form one should replace dash
+(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
+for more examples of this. (You are not allowed to C<open> to a command
that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
-and L<perlipc/"Bidirectional Communication"> for alternatives.)
+and L<perlipc/"Bidirectional Communication"> for alternatives.) In 3+ arg form of
+pipe opens then if LIST is specified (extra arguments after the command name) then
+LIST becomes arguments to the command invoked if the platform supports it.
+The meaning of C<open> with more than three arguments for non-pipe modes
+is not yet specified. Experimental "layers" may give extra LIST arguments meaning.
+
+In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
+and opening C<< '>-' >> opens STDOUT.
-Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
-nonzero upon success, the undefined value otherwise. If the C<open()>
+Open returns
+nonzero upon success, the undefined value otherwise. If the C<open>
involved a pipe, the return value happens to be the pid of the
subprocess.
If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</binmode> for tips for
-dealing with this. The key distinction between systems that need C<binmode()>
+dealing with this. The key distinction between systems that need C<binmode>
and those that don't is their text file formats. Systems like Unix, MacOS, and
Plan9, which delimit lines with a single character, and which encode that
-character in C as C<"\n">, do not need C<binmode()>. The rest need it.
+character in C as C<"\n">, do not need C<binmode>. The rest need it.
+
+In the three argument form MODE may also contain a list of IO "layers" (see L<open> and
+L<PerlIO> for more details) to be applied to the handle. This can be used to achieve the
+effect of C<binmode> as well as more complex behaviours.
When opening a file, it's usually a bad idea to continue normal execution
-if the request failed, so C<open()> is frequently used in connection with
-C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
+if the request failed, so C<open> is frequently used in connection with
+C<die>. Even if C<die> won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
modules that can help with that problem)) you should always check
-the return value from opening a file. The infrequent exception is when
+the return value from opening a file. The infrequent exception is when
working with an unopened filehandle is actually what you want to do.
+As a special case the 3 arg form with a read/write mode and the third argument
+being C<undef>:
+
+ open(TMP, "+>", undef) or die ...
+
+opens a filehandle to an anonymous temporary file.
+
+
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
- open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
+ open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
- open(DBASE, '+<dbase.mine') # open for update
+ open(DBASE, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
- open(ARTICLE, "caesar <$article |") # decrypt article
+ open(DBASE, '+<dbase.mine') # ditto
+ or die "Can't open 'dbase.mine' for update: $!";
+
+ open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
- open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
+ open(ARTICLE, "caesar <$article |") # ditto
+ or die "Can't start caesar: $!";
+
+ open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
}
You may also, in the Bourne shell tradition, specify an EXPR beginning
-with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
+with C<< '>&' >>, in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) to be
-duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
-C<+E<gt>E<gt>>, and C<+E<lt>>. The
+duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
+C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
-stdio buffers.)
+stdio buffers.) If you use the 3 arg form then you can pass either a number,
+the name of a filehandle or the normal "reference to a glob".
+
Here is a script that saves, redirects, and restores STDOUT and
STDERR:
#!/usr/bin/perl
- open(OLDOUT, ">&STDOUT");
+ open(my $oldout, ">&", \*STDOUT);
open(OLDERR, ">&STDERR");
- open(STDOUT, ">foo.out") || die "Can't redirect stdout";
- open(STDERR, ">&STDOUT") || die "Can't dup stdout";
+ open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
+ open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
-If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
-equivalent of C's C<fdopen()> of that file descriptor; this is more
-parsimonious of file descriptors. For example:
+If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
+do an equivalent of C's C<fdopen> of that file descriptor; this is
+more parsimonious of file descriptors. For example:
open(FILEHANDLE, "<&=$fd")
-If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
+or
+
+ open(FILEHANDLE, "<&=", $fd)
+
+Note that if Perl is using the standard C libraries' fdopen() then on
+many UNIX systems, fdopen() is known to fail when file descriptors
+exceed a certain value, typically 255. If you need more file
+descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
+
+You can see whether Perl has been compiled with PerlIO or not by
+running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
+is C<define>, you have PerlIO, otherwise you don't.
+
+If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
+with 2-arguments (or 1-argument) form of open(), then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and C<0> within the child
process. (Use C<defined($pid)> to determine whether the open was successful.)
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.
-The following pairs are more or less equivalent:
+The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
- open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
+ open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
+ open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
+ open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
- open(FOO, "-|") || exec 'cat', '-n', $file;
+ open(FOO, '-|', "cat -n '$file'");
+ open(FOO, '-|') || exec 'cat', '-n', $file;
+ open(FOO, '-|', "cat", '-n', $file);
+
+The last example in each block shows the pipe as "list form", which is
+not yet supported on all platforms.
See L<perlipc/"Safe Pipe Opens"> for more examples of this.
-NOTE: On any operation that may do a fork, any unflushed buffers remain
-unflushed in both processes, which means you may need to set C<$|> to
-avoid duplicate output. On systems that support a close-on-exec flag on
-files, the flag will be set for the newly opened file descriptor as
-determined by the value of $^F. See L<perlvar/$^F>.
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before any operation that may do a fork, but this may not be
+supported on some platforms (see L<perlport>). To be safe, you may need
+to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
+of C<IO::Handle> on any open handles.
+
+On systems that support a
+close-on-exec flag on files, the flag will be set for the newly opened
+file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?>.
-The filename passed to open will have leading and trailing
+The filename passed to 2-argument (or 1-argument) form of open()
+will have leading and trailing
whitespace deleted, and the normal redirection characters
-honored. This property, known as "magic open",
+honored. This property, known as "magic open",
can often be used to good effect. A user could specify a filename of
F<"rsh cat file |">, or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
-However, to open a file with arbitrary weird characters in it, it's
-necessary to protect any leading and trailing whitespace:
+Use 3-argument form to open a file with arbitrary weird characters in it,
+
+ open(FOO, '<', $file);
+
+otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
-If you want a "real" C C<open()> (see L<open(2)> on your system), then you
-should use the C<sysopen()> function, which involves no such magic. This is
+(this may not work on some bizarre filesystems). One should
+conscientiously choose between the I<magic> and 3-arguments form
+of open():
+
+ open IN, $ARGV[0];
+
+will allow the user to specify an argument of the form C<"rsh cat file |">,
+but will not work on a filename which happens to have a trailing space, while
+
+ open IN, '<', $ARGV[0];
+
+will have exactly the opposite restrictions.
+
+If you want a "real" C C<open> (see L<open(2)> on your system), then you
+should use the C<sysopen> function, which involves no such magic (but
+may use subtly different filemodes than Perl open(), which is mapped
+to C fopen()). This is
another way to protect your filenames from interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
- print HANDLE "stuff $$\n");
+ print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
=item opendir DIRHANDLE,EXPR
-Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
-C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
+Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
+C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
=item ord EXPR
EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
See L<utf8> for more about Unicode.
+=item our EXPR
+
+=item our EXPR : ATTRIBUTES
+
+An C<our> declares the listed variables to be valid globals within
+the enclosing block, file, or C<eval>. That is, it has the same
+scoping rules as a "my" declaration, but does not create a local
+variable. If more than one value is listed, the list must be placed
+in parentheses. The C<our> declaration has no semantic effect unless
+"use strict vars" is in effect, in which case it lets you use the
+declared global variable without qualifying it with a package name.
+(But only within the lexical scope of the C<our> declaration. In this
+it differs from "use vars", which is package scoped.)
+
+An C<our> declaration declares a global variable that will be visible
+across its entire lexical scope, even across package boundaries. The
+package in which the variable is entered is determined at the point
+of the declaration, not at the point of use. This means the following
+behavior holds:
+
+ package Foo;
+ our $bar; # declares $Foo::bar for rest of lexical scope
+ $bar = 20;
+
+ package Bar;
+ print $bar; # prints 20
+
+Multiple C<our> declarations in the same lexical scope are allowed
+if they are in different packages. If they happened to be in the same
+package, Perl will emit warnings if you have asked for them.
+
+ use warnings;
+ package Foo;
+ our $bar; # declares $Foo::bar for rest of lexical scope
+ $bar = 20;
+
+ package Bar;
+ our $bar = 30; # declares $Bar::bar for rest of lexical scope
+ print $bar; # prints 30
+
+ our $bar; # emits warning
+
+An C<our> declaration may also have a list of attributes associated
+with it. B<WARNING>: This is an experimental feature that may be
+changed or removed in future releases of Perl. It should not be
+relied upon.
+
+The only currently recognized attribute is C<shared> which indicates
+that a single copy of the global is to be used by all interpreters
+should the program happen to be running in a multi-interpreter
+environment. (The default behaviour would be for each interpreter to
+have its own copy of the global.) In such an environment, this
+attribute also has the effect of making the global readonly.
+Examples:
+
+ our @EXPORT : shared = qw(foo);
+ our %EXPORT_TAGS : shared = (bar => [qw(aa bb cc)]);
+ our $VERSION : shared = "1.00";
+
+Multi-interpreter environments can come to being either through the
+fork() emulation on Windows platforms, or by embedding perl in a
+multi-threaded application. The C<shared> attribute does nothing in
+all other environments.
+
=item pack TEMPLATE,LIST
-Takes an array or list of values and packs it into a binary structure,
-returning the string containing the structure. The TEMPLATE is a
+Takes a LIST of values and converts it into a string using the rules
+given by the TEMPLATE. The resulting string is the concatenation of
+the converted values. Typically, each converted value looks
+like its machine-level representation. For example, on 32-bit machines
+a converted integer may be represented by a sequence of 4 bytes.
+
+The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:
a A string with arbitrary binary data, will be null padded.
- A An ascii string, will be space padded.
+ A An ASCII string, will be space padded.
Z A null terminated (asciz) string, will be null padded.
- b A bit string (ascending bit order, like vec()).
- B A bit string (descending bit order).
+ b A bit string (ascending bit order inside each byte, like vec()).
+ B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
- what a local C compiler calls 'short'.)
+ what a local C compiler calls 'short'. If you want
+ native-length shorts, use the '!' suffix.)
i A signed integer value.
I An unsigned integer value.
- (This 'integer' is _at least_ 32 bits wide. Its exact
+ (This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int',
and may even be larger than the 'long' described in
the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
- what a local C compiler calls 'long'.)
+ what a local C compiler calls 'long'. If you want
+ native-length longs, use the '!' suffix.)
- n A short in "network" (big-endian) order.
- N A long in "network" (big-endian) order.
- v A short in "VAX" (little-endian) order.
- V A long in "VAX" (little-endian) order.
+ n An unsigned short in "network" (big-endian) order.
+ N An unsigned long in "network" (big-endian) order.
+ v An unsigned short in "VAX" (little-endian) order.
+ V An unsigned long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
q A signed quad (64-bit) value.
Q An unsigned quad value.
- (Available only if your system supports 64-bit integer values
- _and_ if Perl has been compiled to support those.
+ (Quads are available only if your system supports 64-bit
+ integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
f A single-precision float in the native format.
=item *
Each letter may optionally be followed by a number giving a repeat
-count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
-C<"H">, and C<"P"> the pack function will gobble up that many values from
+count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
+C<H>, and C<P> the pack function will gobble up that many values from
the LIST. A C<*> for the repeat count means to use however many items are
-left.
+left, except for C<@>, C<x>, C<X>, where it is equivalent
+to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
+same).
+
+When used with C<Z>, C<*> results in the addition of a trailing null
+byte (so the packed result will be one longer than the byte C<length>
+of the item).
+
+The repeat count for C<u> is interpreted as the maximal number of bytes
+to encode per line of output, with 0 and 1 replaced by 45.
=item *
-The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
+The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
string of length count, padding with nulls or spaces as necessary. When
-unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
-after the first null, and C<"a"> returns data verbatim.
+unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
+after the first null, and C<a> returns data verbatim. When packing,
+C<a>, and C<Z> are equivalent.
+
+If the value-to-pack is too long, it is truncated. If too long and an
+explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
+by a null byte. Thus C<Z> always packs a trailing null byte under
+all circumstances.
=item *
-Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
+Likewise, the C<b> and C<B> fields pack a string that many bits long.
+Each byte of the input field of pack() generates 1 bit of the result.
+Each result bit is based on the least-significant bit of the corresponding
+input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
+C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
+
+Starting from the beginning of the input string of pack(), each 8-tuple
+of bytes is converted to 1 byte of output. With format C<b>
+the first byte of the 8-tuple determines the least-significant bit of a
+byte, and with format C<B> it determines the most-significant bit of
+a byte.
+
+If the length of the input string is not exactly divisible by 8, the
+remainder is packed as if the input string were padded by null bytes
+at the end. Similarly, during unpack()ing the "extra" bits are ignored.
+
+If the input string of pack() is longer than needed, extra bytes are ignored.
+A C<*> for the repeat count of pack() means to use all the bytes of
+the input field. On unpack()ing the bits are converted to a string
+of C<"0">s and C<"1">s.
=item *
-The C<"h"> and C<"H"> fields pack a string that many nybbles long.
+The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
+representable as hexadecimal digits, 0-9a-f) long.
+
+Each byte of the input field of pack() generates 4 bits of the result.
+For non-alphabetical bytes the result is based on the 4 least-significant
+bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
+bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
+C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
+is compatible with the usual hexadecimal digits, so that C<"a"> and
+C<"A"> both generate the nybble C<0xa==10>. The result for bytes
+C<"g".."z"> and C<"G".."Z"> is not well-defined.
+
+Starting from the beginning of the input string of pack(), each pair
+of bytes is converted to 1 byte of output. With format C<h> the
+first byte of the pair determines the least-significant nybble of the
+output byte, and with format C<H> it determines the most-significant
+nybble.
+
+If the length of the input string is not even, it behaves as if padded
+by a null byte at the end. Similarly, during unpack()ing the "extra"
+nybbles are ignored.
+
+If the input string of pack() is longer than needed, extra bytes are ignored.
+A C<*> for the repeat count of pack() means to use all the bytes of
+the input field. On unpack()ing the bits are converted to a string
+of hexadecimal digits.
=item *
-The C<"p"> type packs a pointer to a null-terminated string. You are
+The C<p> type packs a pointer to a null-terminated string. You are
responsible for ensuring the string is not a temporary value (which can
potentially get deallocated before you get around to using the packed result).
-The C<"P"> type packs a pointer to a structure of the size indicated by the
-length. A NULL pointer is created if the corresponding value for C<"p"> or
-C<"P"> is C<undef>.
+The C<P> type packs a pointer to a structure of the size indicated by the
+length. A NULL pointer is created if the corresponding value for C<p> or
+C<P> is C<undef>, similarly for unpack().
+
+=item *
+
+The C</> template character allows packing and unpacking of strings where
+the packed structure contains a byte count followed by the string itself.
+You write I<length-item>C</>I<string-item>.
+
+The I<length-item> can be any C<pack> template letter,
+and describes how the length value is packed.
+The ones likely to be of most use are integer-packing ones like
+C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
+and C<N> (for Sun XDR).
+
+The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
+For C<unpack> the length of the string is obtained from the I<length-item>,
+but if you put in the '*' it will be ignored.
+
+ unpack 'C/a', "\04Gurusamy"; gives 'Guru'
+ unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
+ pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
+
+The I<length-item> is not returned explicitly from C<unpack>.
+
+Adding a count to the I<length-item> letter is unlikely to do anything
+useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
+I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
+which Perl does not regard as legal in numeric strings.
=item *
-The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
-immediately followed by a C<"!"> to signify native shorts or longs--as
-you can see from above for example a bare C<"l"> does mean exactly 32
-bits, the native C<long> (as seen by the local C compiler) may be
-larger. This is an issue mainly in 64-bit platforms. You can see
-whether using C<"!"> makes any difference by
+The integer types C<s>, C<S>, C<l>, and C<L> may be
+immediately followed by a C<!> suffix to signify native shorts or
+longs--as you can see from above for example a bare C<l> does mean
+exactly 32 bits, the native C<long> (as seen by the local C compiler)
+may be larger. This is an issue mainly in 64-bit platforms. You can
+see whether using C<!> makes any difference by
print length(pack("s")), " ", length(pack("s!")), "\n";
print length(pack("l")), " ", length(pack("l!")), "\n";
-C<"i!"> and C<"I!"> also work but only because of completeness;
-they are identical to C<"i"> and C<"I">.
+C<i!> and C<I!> also work but only because of completeness;
+they are identical to C<i> and C<I>.
-The actual sizes (in bytes) of native shorts, ints, and longs on
-the platform where Perl was built are also available via L<Config>:
+The actual sizes (in bytes) of native shorts, ints, longs, and long
+longs on the platform where Perl was built are also available via
+L<Config>:
- use Config;
- print $Config{shortsize}, "\n";
- print $Config{intsize}, "\n";
- print $Config{longsize}, "\n";
+ use Config;
+ print $Config{shortsize}, "\n";
+ print $Config{intsize}, "\n";
+ print $Config{longsize}, "\n";
+ print $Config{longlongsize}, "\n";
+
+(The C<$Config{longlongsize}> will be undefine if your system does
+not support long longs.)
=item *
-The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
+The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
are inherently non-portable between processors and operating systems
because they obey the native byteorder and endianness. For example a
-4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
+4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
(arranged in and handled by the CPU registers) into bytes as
-
- 0x12 0x34 0x56 0x78 # little-endian
- 0x78 0x56 0x34 0x12 # big-endian
-
-Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
-everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
-Power, and Cray are big-endian. MIPS can be either: Digital used it
-in little-endian mode, SGI uses it in big-endian mode.
-
-The names `big-endian' and `little-endian' are joking references to
+
+ 0x12 0x34 0x56 0x78 # big-endian
+ 0x78 0x56 0x34 0x12 # little-endian
+
+Basically, the Intel and VAX CPUs are little-endian, while everybody
+else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
+Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
+used/uses them in little-endian mode; SGI/Cray uses them in big-endian
+mode.
+
+The names `big-endian' and `little-endian' are comic references to
the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
-the egg-eating habits of the lilliputs.
-
-Some systems may even have weird byte orders such as
-
+the egg-eating habits of the Lilliputians.
+
+Some systems may have even weirder byte orders such as
+
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
-
+
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
and C<'87654321'> are big-endian.
-If you want portable packed integers use the formats C<"n">, C<"N">,
-C<"v">, and C<"V">, their byte endianness and size is known.
+If you want portable packed integers use the formats C<n>, C<N>,
+C<v>, and C<V>, their byte endianness and size are known.
+See also L<perlport>.
=item *
made. This means that packed floating point data written on one machine
may not be readable on another - even if both use IEEE floating point
arithmetic (as the endian-ness of the memory representation is not part
-of the IEEE spec).
+of the IEEE spec). See also L<perlport>.
Note that Perl uses doubles internally for all numeric calculation, and
converting from double into float and thence back to double again will
lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
-equal C<$foo>).
+equal $foo).
+
+=item *
+
+If the pattern begins with a C<U>, the resulting string will be treated
+as Unicode-encoded. You can force UTF8 encoding on in a string with an
+initial C<U0>, and the bytes that follow will be interpreted as Unicode
+characters. If you don't want this to happen, you can begin your pattern
+with C<C0> (or anything else) to force Perl not to UTF8 encode your
+string, and then follow this with a C<U*> somewhere in your pattern.
+
+=item *
+
+You must yourself do any alignment or padding by inserting for example
+enough C<'x'>es while packing. There is no way to pack() and unpack()
+could know where the bytes are going to or coming from. Therefore
+C<pack> (and C<unpack>) handle their output and input as flat
+sequences of bytes.
+
+=item *
+
+A comment in a TEMPLATE starts with C<#> and goes to the end of line.
+
+=item *
+
+If TEMPLATE requires more arguments to pack() than actually given, pack()
+assumes additional C<""> arguments. If TEMPLATE requires less arguments
+to pack() than actually given, extra arguments are ignored.
=back
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
+ # note: the above examples featuring "C" and "c" are true
+ # only on ASCII and ASCII-derived systems such as ISO Latin 1
+ # and UTF-8. In EBCDIC the first example would be
+ # $foo = pack("CCCC",193,194,195,196);
+
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
-The same template may generally also be used in unpack().
+ $foo = pack('sx2l', 12, 34);
+ # short 12, two zero bytes padding, long 34
+ $bar = pack('s@4l', 12, 34);
+ # short 12, zero fill to position 4, long 34
+ # $foo eq $bar
-=item package
+The same template may generally also be used in unpack().
=item package NAMESPACE
+=item package
+
Declares the compilation unit as being in the given namespace. The scope
of the package declaration is from the declaration itself through the end
-of the enclosing block, file, or eval (the same as the C<my()> operator).
+of the enclosing block, file, or eval (the same as the C<my> operator).
All further unqualified dynamic identifiers will be in this namespace.
A package statement affects only dynamic variables--including those
-you've used C<local()> on--but I<not> lexical variables, which are created
-with C<my()>. Typically it would be the first declaration in a file to
+you've used C<local> on--but I<not> lexical variables, which are created
+with C<my>. Typically it would be the first declaration in a file to
be included by the C<require> or C<use> operator. You can switch into a
package in more than one place; it merely influences which symbol table
is used by the compiler for the rest of that block. You can refer to
=item pop
Pops and returns the last value of the array, shortening the array by
-one element. Has a similar effect to
+one element. Has an effect similar to
- $tmp = $ARRAY[$#ARRAY--];
+ $ARRAY[$#ARRAY--]
-If there are no elements in the array, returns the undefined value.
-If ARRAY is omitted, pops the C<@ARGV> array in the main program, and
-the C<@_> array in subroutines, just like C<shift()>.
+If there are no elements in the array, returns the undefined value
+(although this may happen at other times as well). If ARRAY is
+omitted, pops the C<@ARGV> array in the main program, and the C<@_>
+array in subroutines, just like C<shift>.
=item pos SCALAR
=item pos
Returns the offset of where the last C<m//g> search left off for the variable
-is in question (C<$_> is used when the variable is not specified). May be
+in question (C<$_> is used when the variable is not specified). May be
modified to change that offset. Such modification will also influence
the C<\G> zero-width assertion in regular expressions. See L<perlre> and
L<perlop>.
=item print
-Prints a string or a comma-separated list of strings. Returns TRUE
-if successful. FILEHANDLE may be a scalar variable name, in which case
-the variable contains the name of or a reference to the filehandle, thus
-introducing one level of indirection. (NOTE: If FILEHANDLE is a variable
-and the next token is a term, it may be misinterpreted as an operator
+Prints a string or a list of strings. Returns true if successful.
+FILEHANDLE may be a scalar variable name, in which case the variable
+contains the name of or a reference to the filehandle, thus introducing
+one level of indirection. (NOTE: If FILEHANDLE is a variable and
+the next token is a term, it may be misinterpreted as an operator
unless you interpose a C<+> or put parentheses around the arguments.)
-If FILEHANDLE is omitted, prints by default to standard output (or to the
-last selected output channel--see L</select>). If LIST is also omitted,
-prints C<$_> to the currently selected output channel. To set the default
-output channel to something other than STDOUT use the select operation.
-Note that, because print takes a LIST, anything in the LIST is evaluated
-in list context, and any subroutine that you call will have one or
-more of its expressions evaluated in list context. Also be careful
-not to follow the print keyword with a left parenthesis unless you want
-the corresponding right parenthesis to terminate the arguments to the
-print--interpose a C<+> or put parentheses around all the arguments.
+If FILEHANDLE is omitted, prints by default to standard output (or
+to the last selected output channel--see L</select>). If LIST is
+also omitted, prints C<$_> to the currently selected output channel.
+To set the default output channel to something other than STDOUT
+use the select operation. The current value of C<$,> (if any) is
+printed between each LIST item. The current value of C<$\> (if
+any) is printed after the entire LIST has been printed. Because
+print takes a LIST, anything in the LIST is evaluated in list
+context, and any subroutine that you call will have one or more of
+its expressions evaluated in list context. Also be careful not to
+follow the print keyword with a left parenthesis unless you want
+the corresponding right parenthesis to terminate the arguments to
+the print--interpose a C<+> or put parentheses around all the
+arguments.
Note that if you're storing FILEHANDLES in an array or other expression,
you will have to use a block returning its value instead:
Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
(the output record separator) is not appended. The first argument
-of the list will be interpreted as the C<printf()> format. If C<use locale> is
+of the list will be interpreted as the C<printf> format. If C<use locale> is
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale. See L<perllocale>.
-Don't fall into the trap of using a C<printf()> when a simple
-C<print()> would do. The C<print()> is more efficient and less
+Don't fall into the trap of using a C<printf> when a simple
+C<print> would do. The C<print> is more efficient and less
error prone.
=item prototype FUNCTION
If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
name for Perl builtin. If the builtin is not I<overridable> (such as
C<qw//>) or its arguments cannot be expressed by a prototype (such as
-C<system()>) returns C<undef> because the builtin does not really behave
+C<system>) returns C<undef> because the builtin does not really behave
like a Perl function. Otherwise, the string describing the equivalent
prototype is returned.
=item quotemeta
-Returns the value of EXPR with all non-alphanumeric
+Returns the value of EXPR with all non-"word"
characters backslashed. (That is, all characters not matching
C</[A-Za-z_0-9]/> will be preceded by a backslash in the
returned string, regardless of any locale settings.)
Returns a random fractional number greater than or equal to C<0> and less
than the value of EXPR. (EXPR should be positive.) If EXPR is
-omitted, the value C<1> is used. Automatically calls C<srand()> unless
-C<srand()> has already been called. See also C<srand()>.
+omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
+unless C<srand> has already been called. See also C<srand>.
+
+Apply C<int()> to the value returned by C<rand()> if you want random
+integers instead of random fractional numbers. For example,
+
+ int(rand(10))
+
+returns a random integer between C<0> and C<9>, inclusive.
(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
=item read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the
-specified FILEHANDLE. Returns the number of bytes actually read,
-C<0> at end of file, or undef if there was an error. SCALAR will be grown
-or shrunk to the length actually read. An OFFSET may be specified to
-place the read data at some other place than the beginning of the
-string. This call is actually implemented in terms of stdio's fread(3)
-call. To get a true read(2) system call, see C<sysread()>.
+specified FILEHANDLE. Returns the number of bytes actually read, C<0>
+at end of file, or undef if there was an error. SCALAR will be grown
+or shrunk to the length actually read. If SCALAR needs growing, the
+new bytes will be zero bytes. An OFFSET may be specified to place
+the read data into some other place in SCALAR than the beginning.
+The call is actually implemented in terms of stdio's fread(3) call.
+To get a true read(2) system call, see C<sysread>.
=item readdir DIRHANDLE
-Returns the next directory entry for a directory opened by C<opendir()>.
+Returns the next directory entry for a directory opened by C<opendir>.
If used in list context, returns all the rest of the entries in the
directory. If there are no more entries, returns an undefined value in
scalar context or a null list in list context.
-If you're planning to filetest the return values out of a C<readdir()>, you'd
+If you're planning to filetest the return values out of a C<readdir>, you'd
better prepend the directory in question. Otherwise, because we didn't
-C<chdir()> there, it would have been testing the wrong file.
+C<chdir> there, it would have been testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
context (i.e. file slurp mode), and when an empty file is read, it
returns C<''> the first time, followed by C<undef> subsequently.
-This is the internal function implementing the C<E<lt>EXPRE<gt>>
-operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
+This is the internal function implementing the C<< <EXPR> >>
+operator, but you can use it directly. The C<< <EXPR> >>
operator is discussed in more detail in L<perlop/"I/O Operators">.
$line = <STDIN>;
=item recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH bytes of
-data into variable SCALAR from the specified SOCKET filehandle.
-Actually does a C C<recvfrom()>, so that it can return the address of the
-sender. Returns the undefined value if there's an error. SCALAR will
-be grown or shrunk to the length actually read. Takes the same flags
-as the system call of the same name.
-See L<perlipc/"UDP: Message Passing"> for examples.
+data into variable SCALAR from the specified SOCKET filehandle. SCALAR
+will be grown or shrunk to the length actually read. Takes the same
+flags as the system call of the same name. Returns the address of the
+sender if SOCKET's protocol supports this; returns an empty string
+otherwise. If there's an error, returns the undefined value. This call
+is actually implemented in terms of recvfrom(2) system call. See
+L<perlipc/"UDP: Message Passing"> for examples.
=item redo LABEL
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.
+Note that a block by itself is semantically identical to a loop
+that executes once. Thus C<redo> inside such a block will effectively
+turn it into a looping construct.
+
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
=item ref
-Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
+Returns a true value if EXPR is a reference, false otherwise. If EXPR
is not specified, C<$_> will be used. The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:
- REF
SCALAR
ARRAY
HASH
CODE
+ REF
GLOB
+ LVALUE
If the referenced object has been blessed into a package, then that package
-name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
+name is returned instead. You can think of C<ref> as a C<typeof> operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
print "r is a reference to something that isa hash.\n";
- }
+ }
See also L<perlref>.
=item rename OLDNAME,NEWNAME
-Changes the name of a file. Returns C<1> for success, C<0> otherwise.
+Changes the name of a file; an existing file NEWNAME will be
+clobbered. Returns true for success, false otherwise.
+
Behavior of this function varies wildly depending on your system
implementation. For example, it will usually not work across file system
boundaries, even though the system I<mv> command sometimes compensates
open files, or pre-existing files. Check L<perlport> and either the
rename(2) manpage or equivalent system documentation for details.
+=item require VERSION
+
=item require EXPR
=item require
Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
-supplied. If EXPR is numeric, demands that the current version of Perl
-(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
+supplied.
+
+If a VERSION is specified as a literal of the form v5.6.1,
+demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
+at least as recent as that version, at run time. (For compatibility
+with older versions of Perl, a numeric argument will also be interpreted
+as VERSION.) Compare with L</use>, which can do a similar check at
+compile time.
+
+ require v5.6.1; # run time version check
+ require 5.6.1; # ditto
+ require 5.005_03; # float version allowed for compatibility
Otherwise, demands that a library file be included if it hasn't already
been included. The file is included via the do-FILE mechanism, which is
-essentially just a variety of C<eval()>. Has semantics similar to the following
+essentially just a variety of C<eval>. Has semantics similar to the following
subroutine:
sub require {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
+ $INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
+ delete $INC{$filename} if $@ || !$result;
die $@ if $@;
die "$filename did not return true value" unless $result;
- $INC{$filename} = $realfilename;
return $result;
}
Note that the file will not be included twice under the same specified
-name. The file must return TRUE as the last statement to indicate
+name. The file must return true as the last statement to indicate
successful execution of any initialization code, so it's customary to
-end such a file with "C<1;>" unless you're sure it'll return TRUE
-otherwise. But it's better just to put the "C<1;>", in case you add more
+end such a file with C<1;> unless you're sure it'll return true
+otherwise. But it's better just to put the C<1;>, in case you add more
statements.
If EXPR is a bareword, the require assumes a "F<.pm>" extension and
In other words, if you try this:
- require Foo::Bar; # a splendid bareword
+ require Foo::Bar; # a splendid bareword
-The require function will actually look for the "F<Foo/Bar.pm>" file in the
+The require function will actually look for the "F<Foo/Bar.pm>" file in the
directories specified in the C<@INC> array.
But if you try this:
#or
require "Foo::Bar"; # not a bareword because of the ""
-The require function will look for the "F<Foo::Bar>" file in the @INC array and
-will complain about not finding "F<Foo::Bar>" there. In this case you can do:
+The require function will look for the "F<Foo::Bar>" file in the @INC array and
+will complain about not finding "F<Foo::Bar>" there. In this case you can do:
eval "require $class";
=item return
-Returns from a subroutine, C<eval()>, or C<do FILE> with the value
+Returns from a subroutine, C<eval>, or C<do FILE> with the value
given in EXPR. Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
-may vary from one execution to the next (see C<wantarray()>). If no EXPR
+may vary from one execution to the next (see C<wantarray>). If no EXPR
is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in a void context.
=item rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the
-C<readdir()> routine on DIRHANDLE.
+C<readdir> routine on DIRHANDLE.
=item rindex STR,SUBSTR,POSITION
=item rmdir
Deletes the directory specified by FILENAME if that directory is empty. If it
-succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
+succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
FILENAME is omitted, uses C<$_>.
=item s///
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.
-Wince C<scalar> is unary operator, if you accidentally use for EXPR a
+Because C<scalar> is unary operator, if you accidentally use for EXPR a
parenthesized list, this behaves as a scalar comma expression, evaluating
all but the last element in void context and returning the final element
evaluated in scalar context. This is seldom what you want.
=item seek FILEHANDLE,POSITION,WHENCE
-Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
+Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
FILEHANDLE may be an expression whose value gives the name of the
filehandle. The values for WHENCE are C<0> to set the new position to
-POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
-set it to EOF plus POSITION (typically negative). For WHENCE you may
-use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
-C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
+POSITION, C<1> to set it to the current position plus POSITION, and
+C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
+you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
+(start of the file, current position, end of the file) from the Fcntl
+module. Returns C<1> upon success, C<0> otherwise.
-If you want to position file for C<sysread()> or C<syswrite()>, don't use
-C<seek()> -- buffering makes its effect on the file's system position
-unpredictable and non-portable. Use C<sysseek()> instead.
+If you want to position file for C<sysread> or C<syswrite>, don't use
+C<seek>--buffering makes its effect on the file's system position
+unpredictable and non-portable. Use C<sysseek> instead.
Due to the rules and rigors of ANSI C, on some systems you have to do a
seek whenever you switch between reading and writing. Amongst other
This is also useful for applications emulating C<tail -f>. Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
-seek() to reset things. The C<seek()> doesn't change the current position,
+seek() to reset things. The C<seek> doesn't change the current position,
but it I<does> clear the end-of-file condition on the handle, so that the
-next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
+next C<< <FILE> >> makes Perl try again to read something. We hope.
If that doesn't work (some stdios are particularly cantankerous), then
you may need something more like this:
=item seekdir DIRHANDLE,POS
-Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
-must be a value returned by C<telldir()>. Has the same caveats about
+Sets the current position for the C<readdir> routine on DIRHANDLE. POS
+must be a value returned by C<telldir>. Has the same caveats about
possible directory compaction as the corresponding system library
routine.
Returns the currently selected filehandle. Sets the current default
filehandle for output, if FILEHANDLE is supplied. This has two
-effects: first, a C<write()> or a C<print()> without a filehandle will
+effects: first, a C<write> or a C<print> without a filehandle will
default to this FILEHANDLE. Second, references to variables related to
output will refer to this output channel. For example, if you have to
set the top of form format for more than one output channel, you might
=item select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit masks specified, which
-can be constructed using C<fileno()> and C<vec()>, along these lines:
+can be constructed using C<fileno> and C<vec>, along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
-Most systems do not bother to return anything useful in C<$timeleft>, so
-calling select() in scalar context just returns C<$nfound>.
+Most systems do not bother to return anything useful in $timeleft, so
+calling select() in scalar context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is
in seconds, which may be fractional. Note: not all implementations are
-capable of returning theC<$timeleft>. If not, they always return
-C<$timeleft> equal to the supplied C<$timeout>.
+capable of returning the$timeleft. If not, they always return
+$timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
-B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
-or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
-then only on POSIX systems. You have to use C<sysread()> instead.
+B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
+or <FH>) with C<select>, except as permitted by POSIX, and even
+then only on POSIX systems. You have to use C<sysread> instead.
=item semctl ID,SEMNUM,CMD,ARG
-Calls the System V IPC function C<semctl()>. You'll probably have to say
+Calls the System V IPC function C<semctl>. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or
GETALL, then ARG must be a variable which will hold the returned
-semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
-undefined value for error, "C<0> but true" for zero, or the actual return
-value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
+semid_ds structure or semaphore value array. Returns like C<ioctl>:
+the undefined value for error, "C<0 but true>" for zero, or the actual
+return value otherwise. The ARG must consist of a vector of native
+short integers, which may be created with C<pack("s!",(0)x$nsem)>.
+See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
+documentation.
=item semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the semaphore id, or
-the undefined value if there is an error. See also C<IPC::SysV> and
-C<IPC::SysV::Semaphore> documentation.
+the undefined value if there is an error. See also
+L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
+documentation.
=item semop KEY,OPSTRING
such as signaling and waiting. OPSTRING must be a packed array of
semop structures. Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
-operations is implied by the length of OPSTRING. Returns TRUE if
-successful, or FALSE if there is an error. As an example, the
-following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
+operations is implied by the length of OPSTRING. Returns true if
+successful, or false if there is an error. As an example, the
+following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
-To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
-and C<IPC::SysV::Semaphore> documentation.
+To signal the semaphore, replace C<-1> with C<1>. See also
+L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
+documentation.
=item send SOCKET,MSG,FLAGS,TO
Sends a message on a socket. Takes the same flags as the system call
of the same name. On unconnected sockets you must specify a
-destination to send TO, in which case it does a C C<sendto()>. Returns
+destination to send TO, in which case it does a C C<sendto>. Returns
the number of characters sent, or the undefined value if there is an
error. The C system call sendmsg(2) is currently unimplemented.
See L<perlipc/"UDP: Message Passing"> for examples.
Sets the current process group for the specified PID, C<0> for the current
process. Will produce a fatal error if used on a machine that doesn't
-implement setpgrp(2). If the arguments are omitted, it defaults to
-C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
-arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
+implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
+it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
+accept any arguments, so only C<setpgrp(0,0)> is portable. See also
+C<POSIX::setsid()>.
=item setpriority WHICH,WHO,PRIORITY
array, returns the undefined value. If ARRAY is omitted, shifts the
C<@_> array within the lexical scope of subroutines and formats, and the
C<@ARGV> array at file scopes or within the lexical scopes established by
-the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
-See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
-same thing to the left end of an array that C<pop()> and C<push()> do to the
+the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
+constructs.
+
+See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
+same thing to the left end of an array that C<pop> and C<push> do to the
right end.
=item shmctl ID,CMD,ARG
then ARG must be a variable which will hold the returned C<shmid_ds>
structure. Returns like ioctl: the undefined value for error, "C<0> but
true" for zero, or the actual return value otherwise.
-See also C<IPC::SysV> documentation.
+See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
=item shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory
segment id, or the undefined value if there is an error.
-See also C<IPC::SysV> documentation.
+See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
=item shmread ID,VAR,POS,SIZE
detaching from it. When reading, VAR must be a variable that will
hold the data read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
-SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
-See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
-from CPAN.
+SIZE bytes. Return true if successful, or false if there is an error.
+shmread() taints the variable. See also L<perlipc/"SysV IPC">,
+C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
=item shutdown SOCKET,HOW
This is useful with sockets when you want to tell the other
side you're done writing but not done reading, or vice versa.
-It's also a more insistent form of close because it also
-disables the filedescriptor in any forked copies in other
+It's also a more insistent form of close because it also
+disables the file descriptor in any forked copies in other
processes.
=item sin EXPR
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
returns sine of C<$_>.
-For the inverse sine operation, you may use the C<POSIX::asin()>
+For the inverse sine operation, you may use the C<Math::Trig::asin>
function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted if the process receives a signal such as C<SIGALRM>.
Returns the number of seconds actually slept. You probably cannot
-mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
-using C<alarm()>.
+mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
+using C<alarm>.
On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds. Most modern systems
busy multitasking system.
For delays of finer granularity than one second, you may use Perl's
-C<syscall()> interface to access setitimer(2) if your system supports it,
-or else see L</select> above.
+C<syscall> interface to access setitimer(2) if your system supports
+it, or else see L</select> above. The Time::HiRes module (from CPAN,
+and starting from Perl 5.8 part of the standard distribution) may also
+help.
+
+See also the POSIX module's C<pause> function.
+
+=item sockatmark SOCKET
+
+Returns true if the socket is positioned at the out-of-band mark
+(also known as the urgent data mark), false otherwise. Use right
+after reading from the socket.
+
+Not available directly, one has to import the function from
+the IO::Socket extension
+
+ use IO::Socket 'sockatmark';
-See also the POSIX module's C<sigpause()> function.
+Even this doesn't guarantee that sockatmark() really is available,
+though, because sockatmark() is a relatively recent addition to
+the family of socket functions. If it is unavailable, attempt to
+use it will fail
+
+ IO::Socket::atmark not implemented on this architecture ...
+
+See also L<IO::Socket>.
=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle
-SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
-system call of the same name. You should "C<use Socket;>" first to get
-the proper definitions imported. See the examples in L<perlipc/"Sockets: Client/Server Communication">.
+SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
+the system call of the same name. You should C<use Socket> first
+to get the proper definitions imported. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
+
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptor, as determined by the
+value of $^F. See L<perlvar/$^F>.
=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the
specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
for the system call of the same name. If unimplemented, yields a fatal
-error. Returns TRUE if successful.
+error. Returns true if successful.
+
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptors, as determined by the value
+of $^F. See L<perlvar/$^F>.
-Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
+Some systems defined C<pipe> in terms of C<socketpair>, in which a call
to C<pipe(Rdr, Wtr)> is essentially:
use Socket;
=item sort LIST
Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
-is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
+is omitted, C<sort>s in standard string comparison order. If SUBNAME is
specified, it gives the name of a subroutine that returns an integer
less than, equal to, or greater than C<0>, depending on how the elements
-of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
+of the list are to be ordered. (The C<< <=> >> and C<cmp>
operators are extremely useful in such routines.) SUBNAME may be a
scalar variable name (unsubscripted), in which case the value provides
the name of (or a reference to) the actual subroutine to use. In place
of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.
-In the interests of efficiency the normal calling code for subroutines is
-bypassed, with the following effects: the subroutine may not be a
-recursive subroutine, and the two elements to be compared are passed into
-the subroutine not via C<@_> but as the package global variables C<$a> and
-C<$b> (see example below). They are passed by reference, so don't
-modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
+If the subroutine's prototype is C<($$)>, the elements to be compared
+are passed by reference in C<@_>, as for a normal subroutine. This is
+slower than unprototyped subroutines, where the elements to be
+compared are passed into the subroutine
+as the package global variables $a and $b (see example below). Note that
+in the latter case, it is usually counter-productive to declare $a and
+$b as lexicals.
+
+In either case, the subroutine may not be recursive. The values to be
+compared are always passed by reference, so don't modify them.
You also cannot exit out of the sort block or subroutine using any of the
-loop control operators described in L<perlsyn> or with C<goto()>.
+loop control operators described in L<perlsyn> or with C<goto>.
When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
current collation locale. See L<perllocale>.
+Perl does B<not> guarantee that sort is stable. (A I<stable> sort
+preserves the input order of elements that compare equal.) 5.7 and
+5.8 happen to use a stable mergesort, but 5.6 and earlier used quicksort,
+which is not stable. Do not assume that future perls will continue to
+use a stable sort.
+
Examples:
# sort lexically
# sort numerically descending
@articles = sort {$b <=> $a} @files;
+ # this sorts the %age hash by value instead of key
+ # using an in-line function
+ @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
+
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
- # this sorts the %age hash by value instead of key
- # using an in-line function
- @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
-
- sub backwards { $b cmp $a; }
- @harry = ('dog','cat','x','Cain','Abel');
- @george = ('gone','chased','yz','Punished','Axed');
+ sub backwards { $b cmp $a }
+ @harry = qw(dog cat x Cain Abel);
+ @george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
} 0..$#old
];
- # same thing using a Schwartzian Transform (no temps)
+ # same thing, but without any temps
@new = map { $_->[0] }
- sort { $b->[1] <=> $a->[1]
- ||
- $a->[2] cmp $b->[2]
- } map { [$_, /=(\d+)/, uc($_)] } @old;
+ sort { $b->[1] <=> $a->[1]
+ ||
+ $a->[2] cmp $b->[2]
+ } map { [$_, /=(\d+)/, uc($_)] } @old;
-If you're using strict, you I<MUST NOT> declare C<$a>
-and C<$b> as lexicals. They are package globals. That means
-if you're in the C<main> package, it's
+ # using a prototype allows you to use any comparison subroutine
+ # as a sort subroutine (including other package's subroutines)
+ package other;
+ sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
- @articles = sort {$main::b <=> $main::a} @files;
+ package main;
+ @new = sort other::backwards @old;
-or just
+If you're using strict, you I<must not> declare $a
+and $b as lexicals. They are package globals. That means
+if you're in the C<main> package and type
- @articles = sort {$::b <=> $::a} @files;
+ @articles = sort {$b <=> $a} @files;
-but if you're in the C<FooPack> package, it's
+then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
+but if you're in the C<FooPack> package, it's the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
=item splice ARRAY,OFFSET
+=item splice ARRAY
+
Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any. In list context,
returns the elements removed from the array. In scalar context,
returns the last element removed, or C<undef> if no elements are
removed. The array grows or shrinks as necessary.
-If OFFSET is negative then it start that far from the end of the array.
+If OFFSET is negative then it starts that far from the end of the array.
If LENGTH is omitted, removes everything from OFFSET onward.
-If LENGTH is negative, leave that many elements off the end of the array.
+If LENGTH is negative, leaves that many elements off the end of the array.
+If both OFFSET and LENGTH are omitted, removes everything.
+
The following equivalences hold (assuming C<$[ == 0>):
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
=item split
-Splits a string into an array of strings, and returns it. By default,
+Splits a string into a list of strings and returns that list. By default,
empty leading fields are preserved, and empty trailing ones are deleted.
-If not in list context, returns the number of fields found and splits into
-the C<@_> array. (In list context, you can force the split into C<@_> by
-using C<??> as the pattern delimiters, but it still returns the list
-value.) The use of implicit split to C<@_> is deprecated, however, because
-it clobbers your subroutine arguments.
+In scalar context, returns the number of fields found and splits into
+the C<@_> array. Use of split in scalar context is deprecated, however,
+because it clobbers your subroutine arguments.
If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace). Anything
matching PATTERN is taken to be a delimiter separating the fields. (Note
that the delimiter may be longer than one character.)
-If LIMIT is specified and positive, splits into no more than that
-many fields (though it may split into fewer). If LIMIT is unspecified
-or zero, trailing null fields are stripped (which potential users
-of C<pop()> would do well to remember). If LIMIT is negative, it is
-treated as if an arbitrarily large LIMIT had been specified.
+If LIMIT is specified and positive, it represents the maximum number
+of fields the EXPR will be split into, though the actual number of
+fields returned depends on the number of times PATTERN matches within
+EXPR. If LIMIT is unspecified or zero, trailing null fields are
+stripped (which potential users of C<pop> would do well to remember).
+If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
+had been specified. Note that splitting an EXPR that evaluates to the
+empty string always returns the empty list, regardless of the LIMIT
+specified.
A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
produces the output 'h:i:t:h:e:r:e'.
+Empty leading (or trailing) fields are produced when there positive width
+matches at the beginning (or end) of the string; a zero-width match at the
+beginning (or end) of the string does not produce an empty field. For
+example:
+
+ print join(':', split(/(?=\w)/, 'hi there!'));
+
+produces the output 'h:i :t:h:e:r:e!'.
+
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
default. In time critical applications it behooves you not to split
into more fields than you really need.
-If the PATTERN contains parentheses, additional array elements are
+If the PATTERN contains parentheses, additional list elements are
created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
(1, '-', 10, ',', 20)
-If you had the entire header of a normal Unix email message in C<$header>,
+If you had the entire header of a normal Unix email message in $header,
you could split it up into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
use C</$variable/o>.)
As a special case, specifying a PATTERN of space (C<' '>) will split on
-white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
+white space just as C<split> with no arguments does. Thus, C<split(' ')> can
be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
will give you as many null initial fields as there are leading spaces.
-A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
-whitespace produces a null first field. A C<split()> with no arguments
+A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
+whitespace produces a null first field. A C<split> with no arguments
really does a C<split(' ', $_)> internally.
+A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
+much use otherwise.
+
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
- ($login, $passwd, $uid, $gid,
+ chomp;
+ ($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
-(Note that C<$shell> above will still have a newline on it. See L</chop>,
-L</chomp>, and L</join>.)
=item sprintf FORMAT, LIST
-Returns a string formatted by the usual C<printf()> conventions of the
-C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
-on your system for an explanation of the general principles.
+Returns a string formatted by the usual C<printf> conventions of the C
+library function C<sprintf>. See below for more details
+and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
+the general principles.
+
+For example:
+
+ # Format number with up to 8 leading zeroes
+ $result = sprintf("%08d", $number);
-Perl does its own C<sprintf()> formatting -- it emulates the C
-function C<sprintf()>, but it doesn't use it (except for floating-point
+ # Round number to 3 digits after decimal point
+ $rounded = sprintf("%.3f", $number);
+
+Perl does its own C<sprintf> formatting--it emulates the C
+function C<sprintf>, but it doesn't use it (except for floating-point
numbers, and even then only the standard modifiers are allowed). As a
-result, any non-standard extensions in your local C<sprintf()> are not
+result, any non-standard extensions in your local C<sprintf> are not
available from Perl.
-Perl's C<sprintf()> permits the following universally-known conversions:
+Unlike C<printf>, C<sprintf> does not do what you probably mean when you
+pass it an array as your first argument. The array is given scalar context,
+and instead of using the 0th element of the array as the format, Perl will
+use the count of elements in the array as the format, which is almost never
+useful.
+
+Perl's C<sprintf> permits the following universally-known conversions:
%% a percent sign
%c a character with the given number
%b an unsigned integer, in binary
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
- into the next variable in the parameter list
+ into the next variable in the parameter list
Finally, for backward (and we do mean "backward") compatibility, Perl
permits these unnecessary but widely-supported conversions:
%O a synonym for %lo
%F a synonym for %f
+Note that the number of exponent digits in the scientific notation by
+C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
+exponent less than 100 is system-dependent: it may be three or less
+(zero-padded as necessary). In other words, 1.23 times ten to the
+99th may be either "1.23e99" or "1.23e099".
+
Perl permits the following universally-known flags between the C<%>
and the conversion letter:
for integer
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
+ If no flags, interpret integer as C type "int" or "unsigned"
+
+Perl supports parameter ordering, in other words, fetching the
+parameters in some explicitly specified "random" ordering as opposed
+to the default implicit sequential ordering. The syntax is, instead
+of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
+where the I<digits> is the wanted index, from one upwards. For example:
+
+ printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
+ printf "%*2\$d\n", 12, 3; # will print " 12\n"
-There is also one Perl-specific flag:
+Note that using the reordering syntax does not interfere with the usual
+implicit sequential fetching of the parameters:
- V interpret integer as Perl's standard integer type
+ printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
+ printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
+ printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
+ printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
+ printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
-Where a number would appear in the flags, an asterisk ("C<*>") may be
+There are also two Perl-specific flags:
+
+ V interpret integer as Perl's standard integer type
+ v interpret string as a vector of integers, output as
+ numbers separated either by dots, or by an arbitrary
+ string received from the argument list when the flag
+ is preceded by C<*>
+
+Where a number would appear in the flags, an asterisk (C<*>) may be
used instead, in which case Perl uses the next item in the parameter
list as the given number (that is, as the field width or precision).
-If a field width obtained through "C<*>" is negative, it has the same
-effect as the "C<->" flag: left-justification.
+If a field width obtained through C<*> is negative, it has the same
+effect as the C<-> flag: left-justification.
+
+The C<v> flag is useful for displaying ordinal values of characters
+in arbitrary strings:
+
+ printf "version is v%vd\n", $^V; # Perl's version
+ printf "address is %*vX\n", ":", $addr; # IPv6 address
+ printf "bits are %*vb\n", " ", $bits; # random bitstring
If C<use locale> is in effect, the character used for the decimal
point in formatted real numbers is affected by the LC_NUMERIC locale.
See L<perllocale>.
+If Perl understands "quads" (64-bit integers) (this requires
+either that the platform natively support quads or that Perl
+be specifically compiled to support quads), the characters
+
+ d u o x X b i D U O
+
+print quads, and they may optionally be preceded by
+
+ ll L q
+
+For example
+
+ %lld %16LX %qo
+
+You can find out whether your Perl supports quads via L<Config>:
+
+ use Config;
+ ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
+ print "quads\n";
+
+If Perl understands "long doubles" (this requires that the platform
+support long doubles), the flags
+
+ e f g E F G
+
+may optionally be preceded by
+
+ ll L
+
+For example
+
+ %llf %Lg
+
+You can find out whether your Perl supports long doubles via L<Config>:
+
+ use Config;
+ $Config{d_longdbl} eq 'define' && print "long doubles\n";
+
=item sqrt EXPR
=item sqrt
=item srand
-Sets the random number seed for the C<rand()> operator. If EXPR is
+Sets the random number seed for the C<rand> operator. If EXPR is
omitted, uses a semi-random value supplied by the kernel (if it supports
the F</dev/urandom> device) or based on the current time and process
ID, among other things. In versions of Perl prior to 5.004 the default
-seed was just the current C<time()>. This isn't a particularly good seed,
+seed was just the current C<time>. This isn't a particularly good seed,
so many old programs supply their own seed value (often C<time ^ $$> or
-C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
+C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
-In fact, it's usually not necessary to call C<srand()> at all, because if
+In fact, it's usually not necessary to call C<srand> at all, because if
it is not called explicitly, it is called implicitly at the first use of
-the C<rand()> operator. However, this was not the case in version of Perl
+the C<rand> operator. However, this was not the case in version of Perl
before 5.004, so if your script will run under older Perl versions, it
-should call C<srand()>.
+should call C<srand>.
Note that you need something much more random than the default seed for
cryptographic purposes. Checksumming the compressed output of one or more
If you're particularly concerned with this, see the C<Math::TrulyRandom>
module in CPAN.
-Do I<not> call C<srand()> multiple times in your program unless you know
+Do I<not> call C<srand> multiple times in your program unless you know
exactly what you're doing and why you're doing it. The point of the
-function is to "seed" the C<rand()> function so that C<rand()> can produce
+function is to "seed" the C<rand> function so that C<rand> can produce
a different sequence each time you run your program. Just do it once at the
-top of your program, or you I<won't> get random numbers out of C<rand()>!
+top of your program, or you I<won't> get random numbers out of C<rand>!
Frequently called programs (like CGI scripts) that simply use
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
- 8 atime last access time since the epoch
- 9 mtime last modify time since the epoch
- 10 ctime inode change time (NOT creation time!) since the epoch
+ 8 atime last access time in seconds since the epoch
+ 9 mtime last modify time in seconds since the epoch
+ 10 ctime inode change time (NOT creation time!) in seconds since the epoch
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
print "$file is executable NFS file\n";
}
-(This works on machines only for which the device number is negative under NFS.)
+(This works on machines only for which the device number is negative
+under NFS.)
Because the mode contains both the file type and its permissions, you
-should mask off the file type portion and (s)printf using a C<"%o">
+should mask off the file type portion and (s)printf using a C<"%o">
if you want to see the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
-
-In scalar context, C<stat()> returns a boolean value indicating success
+In scalar context, C<stat> returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle C<_>.
use File::stat;
$sb = stat($filename);
- printf "File is %s, size is %s, perm %04o, mtime %s\n",
+ printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
+You can import symbolic mode constants (C<S_IF*>) and functions
+(C<S_IS*>) from the Fcntl module:
+
+ use Fcntl ':mode';
+
+ $mode = (stat($filename))[2];
+
+ $user_rwx = ($mode & S_IRWXU) >> 6;
+ $group_read = ($mode & S_IRGRP) >> 3;
+ $other_execute = $mode & S_IXOTH;
+
+ printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
+
+ $is_setuid = $mode & S_ISUID;
+ $is_setgid = S_ISDIR($mode);
+
+You could write the last two using the C<-u> and C<-d> operators.
+The commonly available S_IF* constants are
+
+ # Permissions: read, write, execute, for user, group, others.
+
+ S_IRWXU S_IRUSR S_IWUSR S_IXUSR
+ S_IRWXG S_IRGRP S_IWGRP S_IXGRP
+ S_IRWXO S_IROTH S_IWOTH S_IXOTH
+
+ # Setuid/Setgid/Stickiness.
+
+ S_ISUID S_ISGID S_ISVTX S_ISTXT
+
+ # File types. Not necessarily all are available on your system.
+
+ S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
+
+ # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
+
+ S_IREAD S_IWRITE S_IEXEC
+
+and the S_IF* functions are
+
+ S_IFMODE($mode) the part of $mode containing the permission bits
+ and the setuid/setgid/sticky bits
+
+ S_IFMT($mode) the part of $mode containing the file type
+ which can be bit-anded with e.g. S_IFREG
+ or with the following functions
+
+ # The operators -f, -d, -l, -b, -c, -p, and -s.
+
+ S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
+ S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
+
+ # No direct -X operator counterpart, but for the first one
+ # the -g operator is often equivalent. The ENFMT stands for
+ # record flocking enforcement, a platform-dependent feature.
+
+ S_ISENFMT($mode) S_ISWHT($mode)
+
+See your native chmod(2) and stat(2) documentation for more details
+about the S_* constants.
+
=item study SCALAR
=item study
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
-frequencies in the string to be searched -- you probably want to compare
+frequencies in the string to be searched--you probably want to compare
run times with and without it to see which runs faster. Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
-one C<study()> active at a time -- if you study a different scalar the first
-is "unstudied". (The way C<study()> works is this: a linked list of every
+one C<study> active at a time--if you study a different scalar the first
+is "unstudied". (The way C<study> works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the C<'k'> characters are. From each search string,
the rarest character is selected, based on some static frequency tables
print;
}
-In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
-will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
+In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
+will be looked at, because C<f> is rarer than C<o>. In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
first place.
Note that if you have to look for strings that you don't know till
-runtime, you can build an entire loop as a string and C<eval()> that to
+runtime, you can build an entire loop as a string and C<eval> that to
avoid recompiling all your patterns all the time. Together with
undefining C<$/> to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1). The following
=item sub NAME BLOCK
This is subroutine definition, not a real function I<per se>. With just a
-NAME (and possibly prototypes), it's just a forward declaration. Without
-a NAME, it's an anonymous function declaration, and does actually return a
-value: the CODE ref of the closure you just created. See L<perlsub> and
-L<perlref> for details.
+NAME (and possibly prototypes or attributes), it's just a forward declaration.
+Without a NAME, it's an anonymous function declaration, and does actually
+return a value: the CODE ref of the closure you just created. See L<perlsub>
+and L<perlref> for details.
-=item substr EXPR,OFFSET,LEN,REPLACEMENT
+=item substr EXPR,OFFSET,LENGTH,REPLACEMENT
-=item substr EXPR,OFFSET,LEN
+=item substr EXPR,OFFSET,LENGTH
=item substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at
offset C<0>, or whatever you've set C<$[> to (but don't do that).
If OFFSET is negative (or more precisely, less than C<$[>), starts
-that far from the end of the string. If LEN is omitted, returns
-everything to the end of the string. If LEN is negative, leaves that
+that far from the end of the string. If LENGTH is omitted, returns
+everything to the end of the string. If LENGTH is negative, leaves that
many characters off the end of the string.
-If you specify a substring that is partly outside the string, the part
-within the string is returned. If the substring is totally outside
-the string a warning is produced.
-
You can use the substr() function as an lvalue, in which case EXPR
-must itself be an lvalue. If you assign something shorter than LEN,
-the string will shrink, and if you assign something longer than LEN,
+must itself be an lvalue. If you assign something shorter than LENGTH,
+the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it. To keep the string the same
-length you may need to pad or chop your value using C<sprintf()>.
+length you may need to pad or chop your value using C<sprintf>.
+
+If OFFSET and LENGTH specify a substring that is partly outside the
+string, only the part within the string is returned. If the substring
+is beyond either end of the string, substr() returns the undefined
+value and produces a warning. When used as an lvalue, specifying a
+substring that is entirely outside the string is a fatal error.
+Here's an example showing the behavior for boundary cases:
+
+ my $name = 'fred';
+ substr($name, 4) = 'dy'; # $name is now 'freddy'
+ my $null = substr $name, 6, 2; # returns '' (no warning)
+ my $oops = substr $name, 7; # returns undef, with warning
+ substr($name, 7) = 'gap'; # fatal error
An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
an int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string. You can't use a
-string literal (or other read-only string) as an argument to C<syscall()>
+string literal (or other read-only string) as an argument to C<syscall>
because Perl has to assume that any string pointer might be written
through. If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add C<0> to them to force them to look
-like numbers. This emulates the C<syswrite()> function (or vice versa):
+like numbers. This emulates the C<syswrite> function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
which in practice should usually suffice.
Syscall returns whatever value returned by the system call it calls.
-If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
+If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
Note that some system calls can legitimately return C<-1>. The proper
way to handle such calls is to assign C<$!=0;> before the call and
check the value of C<$!> if syscall returns C<-1>.
There's a problem with C<syscall(&SYS_pipe)>: it returns the file
number of the read end of the pipe it creates. There is no way
-to retrieve the file number of the other end. You can avoid this
-problem by using C<pipe()> instead.
+to retrieve the file number of the other end. You can avoid this
+problem by using C<pipe> instead.
=item sysopen FILEHANDLE,FILENAME,MODE
Opens the file whose filename is given by FILENAME, and associates it
with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
the name of the real filehandle wanted. This function calls the
-underlying operating system's C<open()> function with the parameters
+underlying operating system's C<open> function with the parameters
FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module C<Fcntl>.
+See the documentation of your operating system's C<open> to see which
+values and flag bits are available. You may combine several flags
+using the C<|>-operator.
+
+Some of the most common values are C<O_RDONLY> for opening the file in
+read-only mode, C<O_WRONLY> for opening the file in write-only mode,
+and C<O_RDWR> for opening the file in read-write mode, and.
+
For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only, and two
means read/write. We know that these values do I<not> work under
OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
use them in new code.
-If the file named by FILENAME does not exist and the C<open()> call creates
+If the file named by FILENAME does not exist and the C<open> call creates
it (typically because MODE includes the C<O_CREAT> flag), then the value of
PERMS specifies the permissions of the newly created file. If you omit
-the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
+the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
These permission values need to be in octal, and are modified by your
process's current C<umask>.
-You should seldom if ever use C<0644> as argument to C<sysopen()>, because
+In many systems the C<O_EXCL> flag is available for opening files in
+exclusive mode. This is B<not> locking: exclusiveness means here that
+if the file already exists, sysopen() fails. The C<O_EXCL> wins
+C<O_TRUNC>.
+
+Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
+
+You should seldom if ever use C<0644> as argument to C<sysopen>, because
that takes away the user's option to have a more permissive umask.
Better to omit it. See the perlfunc(1) entry on C<umask> for more
on this.
+Note that C<sysopen> depends on the fdopen() C library function.
+On many UNIX systems, fdopen() is known to fail when file descriptors
+exceed a certain value, typically 255. If you need more file
+descriptors than that, consider rebuilding Perl to use the C<sfio>
+library, or perhaps using the POSIX::open() function.
+
See L<perlopentut> for a kinder, gentler explanation of opening files.
=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the system call read(2). It bypasses stdio,
-so mixing this with other kinds of reads, C<print()>, C<write()>,
-C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
+so mixing this with other kinds of reads, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> can cause confusion because stdio
usually buffers data. Returns the number of bytes actually read, C<0>
at end of file, or undef if there was an error. SCALAR will be grown or
shrunk so that the last byte actually read is the last byte of the
There is no syseof() function, which is ok, since eof() doesn't work
very well on device files (like ttys) anyway. Use sysread() and check
-ofr a return value for 0 to decide whether you're done.
+for a return value for 0 to decide whether you're done.
=item sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position using the system call lseek(2). It
-bypasses stdio, so mixing this with reads (other than C<sysread()>),
-C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
-confusion. FILEHANDLE may be an expression whose value gives the name
-of the filehandle. The values for WHENCE are C<0> to set the new
-position to POSITION, C<1> to set the it to the current position plus
-POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
-For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
-C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
+bypasses stdio, so mixing this with reads (other than C<sysread>),
+C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
+FILEHANDLE may be an expression whose value gives the name of the
+filehandle. The values for WHENCE are C<0> to set the new position to
+POSITION, C<1> to set the it to the current position plus POSITION,
+and C<2> to set it to EOF plus POSITION (typically negative). For
+WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
+C<SEEK_END> (start of the file, current position, end of the file)
+from the Fcntl module.
Returns the new position, or the undefined value on failure. A position
-of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
-TRUE on success and FALSE on failure, yet you can still easily determine
+of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
+true on success and false on failure, yet you can still easily determine
the new position.
=item system LIST
=item system PROGRAM LIST
-Does exactly the same thing as "C<exec LIST>", except that a fork is done
-first, and the parent process waits for the child process to complete.
-Note that argument processing varies depending on the number of
-arguments. If there is more than one argument in LIST, or if LIST is
-an array with more than one value, starts the program given by the
-first element of the list with arguments given by the rest of the list.
-If there is only one scalar argument, the argument is
-checked for shell metacharacters, and if there are any, the entire
-argument is passed to the system's command shell for parsing (this is
-C</bin/sh -c> on Unix platforms, but varies on other platforms). If
-there are no shell metacharacters in the argument, it is split into
-words and passed directly to C<execvp()>, which is more efficient.
+Does exactly the same thing as C<exec LIST>, except that a fork is
+done first, and the parent process waits for the child process to
+complete. Note that argument processing varies depending on the
+number of arguments. If there is more than one argument in LIST,
+or if LIST is an array with more than one value, starts the program
+given by the first element of the list with arguments given by the
+rest of the list. If there is only one scalar argument, the argument
+is checked for shell metacharacters, and if there are any, the
+entire argument is passed to the system's command shell for parsing
+(this is C</bin/sh -c> on Unix platforms, but varies on other
+platforms). If there are no shell metacharacters in the argument,
+it is split into words and passed directly to C<execvp>, which is
+more efficient.
+
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before any operation that may do a fork, but this may not be
+supported on some platforms (see L<perlport>). To be safe, you may need
+to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
+of C<IO::Handle> on any open handles.
The return value is the exit status of the program as
-returned by the C<wait()> call. To get the actual exit value divide by
-256. See also L</exec>. This is I<NOT> what you want to use to capture
+returned by the C<wait> call. To get the actual exit value divide by
+256. See also L</exec>. This is I<not> what you want to use to capture
the output from a command, for that you should use merely backticks or
-C<qx//>, as described in L<perlop/"`STRING`">.
+C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
+indicates a failure to start the program (inspect $! for the reason).
-Like C<exec()>, C<system()> allows you to lie to a program about its name if
-you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
+Like C<exec>, C<system> allows you to lie to a program about its name if
+you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
-Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
+Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
program they're running doesn't actually interrupt your program.
@args = ("command", "arg1", "arg2");
=item syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the
-specified FILEHANDLE, using the system call write(2). If LENGTH is
-not specified, writes whole SCALAR. It bypasses
-stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
-C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
-because stdio usually buffers data. Returns the number of bytes
-actually written, or C<undef> if there was an error. If the LENGTH is
-greater than the available data in the SCALAR after the OFFSET, only as
-much data as is available will be written.
+specified FILEHANDLE, using the system call write(2). If LENGTH
+is not specified, writes whole SCALAR. It bypasses stdio, so mixing
+this with reads (other than C<sysread())>, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> may cause confusion because stdio
+usually buffers data. Returns the number of bytes actually written,
+or C<undef> if there was an error. If the LENGTH is greater than
+the available data in the SCALAR after the OFFSET, only as much
+data as is available will be written.
An OFFSET may be specified to write the data from some part of the
string other than the beginning. A negative OFFSET specifies writing
=item tell
-Returns the current position for FILEHANDLE. FILEHANDLE may be an
-expression whose value gives the name of the actual filehandle. If
-FILEHANDLE is omitted, assumes the file last read.
+Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
+may be an expression whose value gives the name of the actual filehandle.
+If FILEHANDLE is omitted, assumes the file last read.
+
+The return value of tell() for the standard streams like the STDIN
+depends on the operating system: it may return -1 or something else.
+tell() on pipes, fifos, and sockets usually returns -1.
-There is no C<systell()> function. Use C<sysseek(FH, 0, 1)> for that.
+There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
=item telldir DIRHANDLE
-Returns the current position of the C<readdir()> routines on DIRHANDLE.
-Value may be given to C<seekdir()> to access a particular location in a
+Returns the current position of the C<readdir> routines on DIRHANDLE.
+Value may be given to C<seekdir> to access a particular location in a
directory. Has the same caveats about possible directory compaction as
the corresponding system library routine.
This function binds a variable to a package class that will provide the
implementation for the variable. VARIABLE is the name of the variable
to be enchanted. CLASSNAME is the name of a class implementing objects
-of correct type. Any additional arguments are passed to the "C<new()>"
+of correct type. Any additional arguments are passed to the C<new>
method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
or C<TIEHASH>). Typically these are arguments such as might be passed
-to the C<dbm_open()> function of C. The object returned by the "C<new()>"
-method is also returned by the C<tie()> function, which would be useful
+to the C<dbm_open()> function of C. The object returned by the C<new>
+method is also returned by the C<tie> function, which would be useful
if you want to access other methods in CLASSNAME.
-Note that functions such as C<keys()> and C<values()> may return huge lists
+Note that functions such as C<keys> and C<values> may return huge lists
when used on large objects, like DBM files. You may prefer to use the
-C<each()> function to iterate over such. Example:
+C<each> function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
FIRSTKEY this
NEXTKEY this, lastkey
DESTROY this
+ UNTIE this
A class implementing an ordinary array should have the following methods:
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
+ UNTIE this
A class implementing a file handle should have the following methods:
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
+ BINMODE this
+ EOF this
+ FILENO this
+ SEEK this, position, whence
+ TELL this
+ OPEN this, mode, LIST
CLOSE this
DESTROY this
+ UNTIE this
A class implementing a scalar should have the following methods:
FETCH this,
STORE this, value
DESTROY this
+ UNTIE this
Not all methods indicated above need be implemented. See L<perltie>,
L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
-Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
+Unlike C<dbmopen>, the C<tie> function will not use or require a module
for you--you need to do that explicitly yourself. See L<DB_File>
-or the F<Config> module for interesting C<tie()> implementations.
+or the F<Config> module for interesting C<tie> implementations.
For further details see L<perltie>, L<"tied VARIABLE">.
=item tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value
-that was originally returned by the C<tie()> call that bound the variable
+that was originally returned by the C<tie> call that bound the variable
to a package.) Returns the undefined value if VARIABLE isn't tied to a
package.
Returns the number of non-leap seconds since whatever time the system
considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
and 00:00:00 UTC, January 1, 1970 for most other systems).
-Suitable for feeding to C<gmtime()> and C<localtime()>.
+Suitable for feeding to C<gmtime> and C<localtime>.
+
+For measuring time in better granularity than one second,
+you may use either the Time::HiRes module from CPAN, or
+if you have gettimeofday(2), you may be able to use the
+C<syscall> interface of Perl, see L<perlfaq8> for details.
=item times
=item tr///
-The transliteration operator. Same as C<y///>. See L<perlop>.
+The transliteration operator. Same as C<y///>. See L<perlop>.
=item truncate FILEHANDLE,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Produces a fatal error if truncate isn't implemented
-on your system. Returns TRUE if successful, the undefined value
+on your system. Returns true if successful, the undefined value
otherwise.
=item uc EXPR
implementing the C<\U> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
-does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
+does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
If EXPR is omitted, uses C<$_>.
if your umask is C<0022> then the file will actually be created with
permissions C<0755>. If your C<umask> were C<0027> (group can't
write; others can't read, write, or execute), then passing
-C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
+C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
027> is C<0640>).
Here's some advice: supply a creation mode of C<0666> for regular
-files (in C<sysopen()>) and one of C<0777> for directories (in
-C<mkdir()>) and executable files. This gives users the freedom of
+files (in C<sysopen>) and one of C<0777> for directories (in
+C<mkdir>) and executable files. This gives users the freedom of
choice: if they want protected files, they might choose process umasks
of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
Programs should rarely if ever make policy decisions better left to
=item undef
Undefines the value of EXPR, which must be an lvalue. Use only on a
-scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
-(using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
+scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
+(using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that; see L<delete>.) Always returns the
undefined value. You can omit the EXPR, in which case nothing is
unlink @goners;
unlink <*.bak>;
-Note: C<unlink()> will not delete directories unless you are superuser and
+Note: C<unlink> will not delete directories unless you are superuser and
the B<-U> flag is supplied to Perl. Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
-filesystem. Use C<rmdir()> instead.
+filesystem. Use C<rmdir> instead.
If LIST is omitted, uses C<$_>.
=item unpack TEMPLATE,EXPR
-C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
-structure and expands it out into a list value, returning the array
-value. (In scalar context, it returns merely the first value
-produced.) The TEMPLATE has the same format as in the C<pack()> function.
+C<unpack> does the reverse of C<pack>: it takes a string
+and expands it out into a list of values.
+(In scalar context, it returns merely the first value produced.)
+
+The string is broken into chunks described by the TEMPLATE. Each chunk
+is converted separately to a value. Typically, either the string is a result
+of C<pack>, or the bytes of the string represent a C structure of some
+kind.
+
+The TEMPLATE has the same format as in the C<pack> function.
Here's a subroutine that does substring:
sub substr {
sub ordinal { unpack("c",$_[0]); } # same as ord()
-In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
-you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
-themselves. Default is a 16-bit checksum. For example, the following
+In addition to fields allowed in pack(), you may prefix a field with
+a %<number> to indicate that
+you want a <number>-bit checksum of the items instead of the items
+themselves. Default is a 16-bit checksum. Checksum is calculated by
+summing numeric values of expanded values (for string fields the sum of
+C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
+
+For example, the following
computes the same number as the System V sum program:
- while (<>) {
- $checksum += unpack("%32C*", $_);
- }
- $checksum %= 65535;
+ $checksum = do {
+ local $/; # slurp!
+ unpack("%32C*",<>) % 65535;
+ };
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
-See L</pack> for more examples.
+The C<p> and C<P> formats should be used with care. Since Perl
+has no way of checking whether the value passed to C<unpack()>
+corresponds to a valid memory location, passing a pointer value that's
+not known to be valid is likely to have disastrous consequences.
+
+If the repeat count of a field is larger than what the remainder of
+the input string allows, repeat count is decreased. If the input string
+is longer than one described by the TEMPLATE, the rest is ignored.
+
+See L</pack> for more examples and notes.
=item untie VARIABLE
-Breaks the binding between a variable and a package. (See C<tie()>.)
+Breaks the binding between a variable and a package. (See C<tie>.)
=item unshift ARRAY,LIST
-Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
+Does the opposite of a C<shift>. Or the opposite of a C<push>,
depending on how you look at it. Prepends list to the front of the
array, and returns the new number of elements in the array.
- unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
+ unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the
-prepended elements stay in the same order. Use C<reverse()> to do the
+prepended elements stay in the same order. Use C<reverse> to do the
reverse.
+=item use Module VERSION LIST
+
+=item use Module VERSION
+
=item use Module LIST
=item use Module
-=item use Module VERSION LIST
-
=item use VERSION
Imports some semantics into the current package from the named module,
except that Module I<must> be a bareword.
-If the first argument to C<use> is a number, it is treated as a version
-number instead of a module name. If the version of the Perl interpreter
-is less than VERSION, then an error message is printed and Perl exits
-immediately. This is often useful if you need to check the current
-Perl version before C<use>ing library modules that have changed in
-incompatible ways from older versions of Perl. (We try not to do
-this more than we have to.)
+VERSION, which can be specified as a literal of the form v5.6.1, demands
+that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
+as recent as that version. (For compatibility with older versions of Perl,
+a numeric literal will also be interpreted as VERSION.) If the version
+of the running Perl interpreter is less than VERSION, then an error
+message is printed and Perl exits immediately without attempting to
+parse the rest of the file. Compare with L</require>, which can do a
+similar check at run time.
+
+ use v5.6.1; # compile time version check
+ use 5.6.1; # ditto
+ use 5.005_03; # float version allowed for compatibility
-The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
+This is often useful if you need to check the current Perl version before
+C<use>ing library modules that have changed in incompatible ways from
+older versions of Perl. (We try not to do this more than we have to.)
+
+The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
C<require> makes sure the module is loaded into memory if it hasn't been
-yet. The C<import()> is not a builtin--it's just an ordinary static method
-call into the "C<Module>" package to tell the module to import the list of
+yet. The C<import> is not a builtin--it's just an ordinary static method
+call into the C<Module> package to tell the module to import the list of
features back into the current package. The module can implement its
-C<import()> method any way it likes, though most modules just choose to
-derive their C<import()> method via inheritance from the C<Exporter> class that
-is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
-method can be found then the error is currently silently ignored. This
-may change to a fatal error in a future version.
+C<import> method any way it likes, though most modules just choose to
+derive their C<import> method via inheritance from the C<Exporter> class that
+is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
+method can be found then the call is skipped.
-If you don't want your namespace altered, explicitly supply an empty list:
+If you do not want to call the package's C<import> method (for instance,
+to stop your namespace from being altered), explicitly supply the empty list:
use Module ();
If the VERSION argument is present between Module and LIST, then the
C<use> will call the VERSION method in class Module with the given
version as an argument. The default VERSION method, inherited from
-the Universal class, croaks if the given version is larger than the
-value of the variable C<$Module::VERSION>. (Note that there is not a
-comma after VERSION!)
+the UNIVERSAL class, croaks if the given version is larger than the
+value of the variable C<$Module::VERSION>.
+
+Again, there is a distinction between omitting LIST (C<import> called
+with no arguments) and an explicit empty LIST C<()> (C<import> not
+called). Note that there is no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way. Currently implemented pragmas are:
- use integer;
+ use constant;
use diagnostics;
- use sigtrap qw(SEGV BUS);
- use strict qw(subs vars refs);
- use subs qw(afunc blurfl);
+ use integer;
+ use sigtrap qw(SEGV BUS);
+ use strict qw(subs vars refs);
+ use subs qw(afunc blurfl);
+ use warnings qw(all);
-Some of these these pseudo-modules import semantics into the current
+Some of these pseudo-modules import semantics into the current
block scope (like C<strict> or C<integer>, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).
-There's a corresponding "C<no>" command that unimports meanings imported
-by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
+There's a corresponding C<no> command that unimports meanings imported
+by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
no integer;
no strict 'refs';
+ no warnings;
-If no C<unimport()> method can be found the call fails with a fatal error.
+If no C<unimport> method can be found the call fails with a fatal error.
-See L<perlmod> for a list of standard modules and pragmas.
+See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
+for the C<-M> and C<-m> command-line options to perl that give C<use>
+functionality from the command-line.
=item utime LIST
Changes the access and modification times on each file of a list of
files. The first two elements of the list must be the NUMERICAL access
and modification times, in that order. Returns the number of files
-successfully changed. The inode modification time of each file is set
-to the current time. This code has the same effect as the "C<touch>"
+successfully changed. The inode change time of each file is set
+to the current time. This code has the same effect as the C<touch>
command if the files already exist:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
+If the first two elements of the list are C<undef>, then the utime(2)
+function in the C library will be called with a null second argument.
+On most systems, this will set the file's access and modification
+times to the current time. (i.e. equivalent to the example above.)
+
+ utime undef, undef, @ARGV;
+
=item values HASH
Returns a list consisting of all the values of the named hash. (In a
scalar context, returns the number of values.) The values are
returned in an apparently random order. The actual random order is
subject to change in future versions of perl, but it is guaranteed to
-be the same order as either the C<keys()> or C<each()> function would
+be the same order as either the C<keys> or C<each> function would
produce on the same (unmodified) hash.
-Note that you cannot modify the values of a hash this way, because the
-returned list is just a copy. You need to use a hash slice for that,
-since it's lvaluable in a way that values() is not.
+Note that the values are not copied, which means modifying them will
+modify the contents of the hash:
- for (values %hash) { s/foo/bar/g } # FAILS!
- for (@hash{keys %hash}) { s/foo/bar/g } # ok
+ for (values %hash) { s/foo/bar/g } # modifies %hash values
+ for (@hash{keys %hash}) { s/foo/bar/g } # same
As a side effect, calling values() resets the HASH's internal iterator.
-See also C<keys()>, C<each()>, and C<sort()>.
+See also C<keys>, C<each>, and C<sort>.
=item vec EXPR,OFFSET,BITS
-Treats the string in EXPR as a vector of unsigned integers, and
-returns the value of the bit field specified by OFFSET. BITS specifies
-the number of bits that are reserved for each entry in the bit
-vector. This must be a power of two from 1 to 32. C<vec()> may also be
-assigned to, in which case parentheses are needed to give the expression
-the correct precedence as in
+Treats the string in EXPR as a bit vector made up of elements of
+width BITS, and returns the value of the element specified by OFFSET
+as an unsigned integer. BITS therefore specifies the number of bits
+that are reserved for each element in the bit vector. This must
+be a power of two from 1 to 32 (or 64, if your platform supports
+that).
+
+If BITS is 8, "elements" coincide with bytes of the input string.
+
+If BITS is 16 or more, bytes of the input string are grouped into chunks
+of size BITS/8, and each group is converted to a number as with
+pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
+for BITS==64). See L<"pack"> for details.
+
+If bits is 4 or less, the string is broken into bytes, then the bits
+of each byte are broken into 8/BITS groups. Bits of a byte are
+numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
+C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
+breaking the single input byte C<chr(0x36)> into two groups gives a list
+C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
+
+C<vec> may also be assigned to, in which case parentheses are needed
+to give the expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
-Vectors created with C<vec()> can also be manipulated with the logical
-operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
-desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
+If the selected element is outside the string, the value 0 is returned.
+If an element off the end of the string is written to, Perl will first
+extend the string with sufficiently many zero bytes. It is an error
+to try to write off the beginning of the string (i.e. negative OFFSET).
+
+The string should not contain any character with the value > 255 (which
+can only happen if you're using UTF8 encoding). If it does, it will be
+treated as something which is not UTF8 encoded. When the C<vec> was
+assigned to, other parts of your program will also no longer consider the
+string to be UTF8 encoded. In other words, if you do have such characters
+in your string, vec() will operate on the actual byte string, and not the
+conceptual character string.
+
+Strings created with C<vec> can also be manipulated with the logical
+operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
+vector operation is desired when both operands are strings.
+See L<perlop/"Bitwise String Operators">.
The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
-The comments show the string after each step. Note that this code works
+The comments show the string after each step. Note that this code works
in the same way on big-endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
+
+ # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
+ print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
+
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
-To transform a bit vector into a string or array of 0's and 1's, use these:
+To transform a bit vector into a string or list of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the C<*>.
+Here is an example to illustrate how the bits actually fall in place:
+
+ #!/usr/bin/perl -wl
+
+ print <<'EOT';
+ 0 1 2 3
+ unpack("V",$_) 01234567890123456789012345678901
+ ------------------------------------------------------------------
+ EOT
+
+ for $w (0..3) {
+ $width = 2**$w;
+ for ($shift=0; $shift < $width; ++$shift) {
+ for ($off=0; $off < 32/$width; ++$off) {
+ $str = pack("B*", "0"x32);
+ $bits = (1<<$shift);
+ vec($str, $off, $width) = $bits;
+ $res = unpack("b*",$str);
+ $val = unpack("V", $str);
+ write;
+ }
+ }
+ }
+
+ format STDOUT =
+ vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
+ $off, $width, $bits, $val, $res
+ .
+ __END__
+
+Regardless of the machine architecture on which it is run, the above
+example should print the following table:
+
+ 0 1 2 3
+ unpack("V",$_) 01234567890123456789012345678901
+ ------------------------------------------------------------------
+ vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
+ vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
+ vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
+ vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
+ vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
+ vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
+ vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
+ vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
+ vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
+ vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
+ vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
+ vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
+ vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
+ vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
+ vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
+ vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
+ vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
+ vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
+ vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
+ vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
+ vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
+ vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
+ vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
+ vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
+ vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
+ vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
+ vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
+ vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
+ vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
+ vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
+ vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
+ vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
+ vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
+ vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
+ vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
+ vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
+ vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
+ vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
+ vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
+ vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
+ vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
+ vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
+ vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
+ vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
+ vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
+ vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
+ vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
+ vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
+ vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
+ vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
+ vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
+ vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
+ vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
+ vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
+ vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
+ vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
+ vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
+ vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
+ vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
+ vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
+ vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
+ vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
+ vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
+ vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
+ vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
+ vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
+ vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
+ vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
+ vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
+ vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
+ vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
+ vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
+ vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
+ vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
+ vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
+ vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
+ vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
+ vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
+ vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
+ vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
+ vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
+ vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
+ vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
+ vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
+ vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
+ vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
+ vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
+ vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
+ vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
+ vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
+ vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
+ vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
+ vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
+ vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
+ vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
+ vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
+ vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
+ vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
+ vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
+ vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
+ vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
+ vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
+ vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
+ vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
+ vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
+ vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
+ vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
+ vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
+ vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
+ vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
+ vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
+ vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
+ vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
+ vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
+ vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
+ vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
+ vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
+ vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
+ vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
+ vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
+ vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
+ vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
+ vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
+ vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
+
=item wait
Behaves like the wait(2) system call on your system: it waits for a child
process to terminate and returns the pid of the deceased process, or
-C<-1> if there are no child processes. The status is rketurned in C<$?>.
+C<-1> if there are no child processes. The status is returned in C<$?>.
Note that a return value of C<-1> could mean that child processes are
being automatically reaped, as described in L<perlipc>.
use POSIX ":sys_wait_h";
#...
- do {
+ do {
$kid = waitpid(-1,&WNOHANG);
} until $kid == -1;
=item wantarray
-Returns TRUE if the context of the currently executing subroutine is
-looking for a list value. Returns FALSE if the context is looking
+Returns true if the context of the currently executing subroutine is
+looking for a list value. Returns false if the context is looking
for a scalar. Returns the undefined value if the context is looking
for no value (void context).
my @a = complex_calculation();
return wantarray ? @a : "@a";
+This function should have been named wantlist() instead.
+
=item warn LIST
-Produces a message on STDERR just like C<die()>, but doesn't exit or throw
+Produces a message on STDERR just like C<die>, but doesn't exit or throw
an exception.
If LIST is empty and C<$@> already contains a value (typically from a
previous eval) that value is used after appending C<"\t...caught">
-to C<$@>. This is useful for staying almost, but not entirely similar to
-C<die()>.
+to C<$@>. This is useful for staying almost, but not entirely similar to
+C<die>.
If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
No message is printed if there is a C<$SIG{__WARN__}> handler
installed. It is the handler's responsibility to deal with the message
-as it sees fit (like, for instance, converting it into a C<die()>). Most
+as it sees fit (like, for instance, converting it into a C<die>). Most
handlers must therefore make arrangements to actually display the
-warnings that they are not prepared to deal with, by calling C<warn()>
+warnings that they are not prepared to deal with, by calling C<warn>
again in the handler. Note that this is quite safe and will not
produce an endless loop, since C<__WARN__> hooks are not called from
inside one.
You will find this behavior is slightly different from that of
C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
-instead call C<die()> again to change it).
+instead call C<die> again to change it).
Using a C<__WARN__> handler provides a powerful way to silence all
warnings (even the so-called mandatory ones). An example:
Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
using the format associated with that file. By default the format for
a file is the one having the same name as the filehandle, but the
-format for the current output channel (see the C<select()> function) may be set
+format for the current output channel (see the C<select> function) may be set
explicitly by assigning the name of the format to the C<$~> variable.
Top of form processing is handled automatically: if there is
If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
-C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
+C<select> operator. If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time. For more on formats, see L<perlform>.
-Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
+Note that write is I<not> the opposite of C<read>. Unfortunately.
=item y///