between the function and left parenthesis doesn't count--so you need to
be careful sometimes:
- print 1+2+3; # Prints 6.
- print(1+2) + 3; # Prints 3.
- print (1+2)+3; # Also prints 3!
- print +(1+2)+3; # Prints 6.
- print ((1+2)+3); # Prints 6.
+ print 1+2+4; # Prints 7.
+ print(1+2) + 4; # Prints 3.
+ print (1+2)+4; # Also prints 3!
+ print +(1+2)+4; # Prints 7.
+ print ((1+2)+4); # Prints 7.
If you run Perl with the B<-w> switch it can warn you about this. For
example, the third line above produces:
Useless use of integer addition in void context at - line 1.
For functions that can be used in either a scalar or list context,
-non-abortive failure is generally indicated in a scalar context by
+nonabortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.
binmode, close, closedir, dbmclose, dbmopen, die, eof,
fileno, flock, format, getc, print, printf, read, readdir,
-rewinddir, seek, seekdir, select, syscall, sysread,
+rewinddir, seek, seekdir, select, syscall, sysread, sysseek,
syswrite, tell, telldir, truncate, warn, write
=item Functions for fixed length data or records
caller, continue, die, do, dump, eval, exit, goto, last,
next, redo, return, sub, wantarray
-=item Keywords related to scoping
+=item Keywords related to scoping
caller, import, local, my, package, use
-e File exists.
-z File has zero size.
- -s File has non-zero size (returns size).
+ -s File has nonzero size (returns size).
-f File is a plain file.
-d File is a directory.
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
-file, or a file at EOF when testing a filehandle. Because you have to
+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>.
=item abs VALUE
-=item abs
+=item abs
Returns the absolute value of its argument.
If VALUE is omitted, uses $_.
=item alarm SECONDS
-=item alarm
+=item alarm
Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed. If SECONDS is not specified,
on the previous timer.
For delays of finer granularity than one second, you may use Perl's
-syscall() interface to access setitimer(2) if your system supports it,
-or else see L</select()> below. It is not advised to intermix alarm()
+syscall() interface to access setitimer(2) if your system supports it,
+or else see L</select()>. It is usually a mistake to intermix alarm()
and sleep() calls.
If you want to use alarm() to time out a system call you need to use an
Arranges for the file to be read or written in "binary" mode in operating
systems that distinguish between binary and text files. Files that are
not in binary mode have CR LF sequences translated to LF on input and LF
-translated to CR LF on output. Binmode has no effect under Unix; in DOS
+translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
and similarly archaic systems, it may be imperative--otherwise your
-DOS-damaged C library may mangle your file. The key distinction between
+MS-DOS-damaged C library may mangle your file. The key distinction between
systems that need binmode and those that don't is their text file
formats. Systems like Unix and Plan9 that delimit lines with a single
character, and that encode that character in C as '\n', do not need
print a stack trace. The value of EXPR indicates how many call frames
to go back before the current one.
- ($package, $filename, $line, $subroutine,
+ ($package, $filename, $line, $subroutine,
$hasargs, $wantarray, $evaltext, $is_require) = caller($i);
Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
-call, but C<L<eval>>. In such a case additional elements $evaltext and
-$is_require are set: $is_require is true if the frame is created by
-C<L<require>> or C<L<use>> statement, $evaltext contains the text of
-C<L<eval EXPR>> statement. In particular, for C<L<eval BLOCK>>
-statement $filename is C<"(eval)">, but $evaltext is undefined. (Note
-also that C<L<use>> statement creates a C<L<require>> frame inside
-an C<L<eval EXPR>>) frame.
+call, but an C<eval>. In such a case additional elements $evaltext and
+$is_require are set: $is_require is true if the frame is created by a
+C<require> or C<use> statement, $evaltext contains the text of the
+C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
+$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.
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable @DB::args to be the
-arguments with which that subroutine was invoked.
+arguments with which the subroutine was invoked.
=item chdir EXPR
list must be the numerical mode, which should probably be an octal
number, and which definitely should I<not> a string of octal digits:
C<0644> is okay, C<'0644'> is not. Returns the number of files
-successfully changed. See also L<oct>, if all you have is a string.
+successfully changed. See also L</oct>, if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
-Here's an example that looks up non-numeric uids in the passwd file:
+Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chop($user = <STDIN>);
@ary = <${pattern}>; # expand filenames
chown $uid, $gid, @ary;
-On most systems, you are not allowed to change the ownership of the
+On most systems, you are not allowed to change the ownership of the
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.
=item chr NUMBER
-=item chr
+=item chr
Returns the character represented by that NUMBER in the character set.
-For example, C<chr(65)> is "A" in ASCII. For the reverse, use L<ord>.
+For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
If NUMBER is omitted, uses $_.
=item chroot FILENAME
-=item chroot
+=item chroot
This function works as the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
die "Sorry...\n";
} else {
print "ok\n";
- }
+ }
-Of course, typing in your own password to whomever asks you
+Of course, typing in your own password to whomever asks you
for it is unwise.
=item dbmclose HASH
=item defined EXPR
-=item defined
+=item defined
Returns a Boolean value telling whether EXPR has a value other than
the undefined value C<undef>. If EXPR is not present, C<$_> will be
avoided.
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
+not whether the key exists in the hash. Use L</exists> for the latter
purpose.
Examples:
plan to use them again, because it saves time when you load them up
again to have memory already ready to be filled.
-This counter-intuitive behaviour of defined() on aggregates may be
+This counterintuitive behaviour of defined() on aggregates may be
changed, fixed, or broken in a future release of Perl.
-See also L<undef>, L<exists>, L<ref>.
+See also L</undef>, L</exists>, L</ref>.
=item delete EXPR
Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
-C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
+C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
is 0, exits with 255. Inside an eval(), the error message is stuffed into
C<$@>, and the eval() is terminated with the undefined value; this makes
die() the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
- chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
+ chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
current filename for error messages, and searches all the B<-I>
libraries if the file isn't in the current directory (see also the @INC
array in L<perlvar/Predefined Names>). It's the same, however, in that it does
-re-parse the file every time you call it, so you probably don't want to
+reparse the file every time you call it, so you probably don't want to
do this inside a loop.
Note that inclusion of library modules is better done with the
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators return undef when they run out of data.
+input operators return undef when they run out of data.
=item eval EXPR
recompiling each time. The error, if any, is still returned in C<$@>.
Examples:
- # make divide-by-zero non-fatal
+ # make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
print $@ if $@; # prints "bar barfs here"
}
-With an eval(), you should be especially careful to remember what's
+With an eval(), you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
does nothing but return the value of C<$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 that
+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.
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST. (This always forces interpretation of the
-LIST as a multi-valued list, even if there is only a single scalar in
+LIST as a multivalued list, even if there is only a single scalar in
the list.) Example:
$shell = '/bin/csh';
exit 0 if $ans =~ /^[Xx]/;
See also die(). If EXPR is omitted, exits with 0 status. The only
-univerally portable values for EXPR are 0 for success and 1 for error;
+universally portable values for EXPR are 0 for success and 1 for error;
all other values are subject to unpredictable interpretation depending
on the environment in which the Perl program is running.
=item exp EXPR
-=item exp
+=item exp
-Returns I<e> (the natural logarithm base) to the power of EXPR.
+Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.
=item fcntl FILEHANDLE,FUNCTION,SCALAR
=item flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
-success, FALSE on failure. Will produce a fatal error if used on a
-machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
-flock() is Perl's portable file locking interface, although it will lock
-only entire files, not records.
+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). flock()
+is Perl's portable file locking interface, although it locks only entire
+files, not records.
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 you pull them in with an explicit
-request to the Fcntl module. The names can be requested as a group with
-the :flock tag (or they can be requested individually, of course).
-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 flock() will return immediately rather than
-blocking waiting for the lock (check the return status to see if you got
-it).
+you can use the symbolic names if 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 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 mis-coordination, Perl flushes FILEHANDLE
+before (un)locking it.
Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent. These
$SIG{CHLD} = sub { wait };
-There's also the double-fork trick (error checking on
+There's also the double-fork trick (error checking on
fork() returns omitted);
unless ($pid = fork) {
Declare a picture format with use by the write() function. For
example:
- format Something =
+ format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
See L<perlform> for many details and examples.
-=item formline PICTURE, LIST
+=item formline PICTURE,LIST
This is an internal function used by C<format>s, though you may call it
too. It formats (see L<perlform>) a list of values according to the
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
- system "stty", '-icanon', 'eol', "\001";
+ system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
}
print "\n";
-Determination of whether $BSD_STYLE should be set
-is left as an exercise to the reader.
+Determination of whether $BSD_STYLE should be set
+is left as an exercise to the reader.
The POSIX::getattr() function can do this more portably on systems
alleging POSIX compliance.
See also the C<Term::ReadKey> module from your nearest CPAN site;
-details on CPAN can be found on L<perlmod/CPAN>.
+details on CPAN can be found on L<perlmod/CPAN>.
=item getlogin
Returns the current login from F</etc/utmp>, if any. If null, use
-getpwuid().
+getpwuid().
$login = getlogin || getpwuid($<) || "Kilroy";
=item glob
-Returns the value of EXPR with filename expansions such as a shell
-would do. This is the internal function implementing the
-C<E<lt>*.cE<gt>> operator, except it's easier to use. If EXPR is
-omitted, $_ is used. The E<lt>E<gt> operator is discussed in more
-detail in L<perlop/"I/O Operators">.
+Returns the value of EXPR with filename expansions such as a shell 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, $_ is used.
+The C<E<lt>*.cE<gt>> operator is discussed in more detail in
+L<perlop/"I/O Operators">.
=item gmtime EXPR
Converts a time as returned by the time function to a 9-element array
-with the time localized for the standard Greenwich time zone.
+with the time localized for the standard Greenwich time zone.
Typically used as follows:
-
+ # 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
-the range 0..6. Also, $year is the number of years since 1900, I<not>
-simply the last two digits of the year.
+the range 0..6 with sunday as day 0. Also, $year is the number of
+years since 1900, I<not> simply the last two digits of the year.
If EXPR is omitted, does C<gmtime(time())>.
-In a scalar context, prints out the ctime(3) value:
+In a scalar context, returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
-Also see the F<timegm.pl> library, and the strftime(3) function available
-via the POSIX module.
+Also see the timegm() function provided by the Time::Local module,
+and the strftime(3) function available via the POSIX module.
=item goto LABEL
=item grep EXPR,LIST
-This is similar in spirit to, but not the same as, L<grep(1)>
+This is similar in spirit to, but not the same as, grep(1)
and its relatives. In particular, it is not limited to using
regular expressions.
=item hex EXPR
-=item hex
+=item hex
-Interprets EXPR as a hex string and returns the corresponding
+Interprets EXPR as a hex string and returns the corresponding
value. (To convert strings that might start with either 0 or 0x
-see L<oct>.) If EXPR is omitted, uses $_.
+see L</oct>.) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
=item import
-There is no built-in import() function. It is merely an ordinary
+There is no builtin import() function. It is merely an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module. The use() function calls the import() method
-for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
+for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
=item index STR,SUBSTR,POSITION
=item int EXPR
-=item int
+=item int
Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
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>>.
(There is a Perl script called B<h2ph> that comes with the Perl kit which
-may help you in this, but it's non-trivial.) SCALAR will be read and/or
+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 ioctl call. (If SCALAR
has no string value but does have a numeric value, that value will be
=item join EXPR,LIST
-Joins the separate strings of LIST or ARRAY into a single string with
+Joins the separate strings of LIST into a single string with
fields separated by the value of EXPR, and returns the string.
Example:
print $key, '=', $ENV{$key}, "\n";
}
-To sort an array by value, you'll need to use a C<sort{}> function.
+To sort an array 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)) {
=item kill 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
+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.
$cnt = kill 1, $child1, $child2;
=item lc EXPR
-=item lc
+=item lc
Returns an lowercased version of EXPR. This is the internal function
-implementing the \L escape in double-quoted strings.
+implementing the \L escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
If EXPR is omitted, uses $_.
=item lcfirst EXPR
-=item lcfirst
+=item lcfirst
Returns the value of EXPR with the first character lowercased. This is
the internal function implementing the \l escape in double-quoted strings.
=item length EXPR
-=item length
+=item length
Returns the length in characters of the value of EXPR. If EXPR is
omitted, returns length of $_.
with the time analyzed for the local time zone. Typically used as
follows:
+ # 0 1 2 3 4 5 6 7 8
($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 $mon has the range 0..11 and $wday has
-the range 0..6 and $year is year-1900, that is, $year is 123 in year
-2023. If EXPR is omitted, uses the current time ("localtime(time)").
+the range 0..6 with sunday as day 0. Also, $year is the number of
+years since 1900, that is, $year is 123 in year 2023.
+
+If EXPR is omitted, uses the current time (C<localtime(time)>).
In a scalar context, returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
-Also see the Time::Local module, and the strftime(3) function available
-via the POSIX module.
+Also see the Time::Local module, and the strftime(3) and mktime(3)
+function available via the POSIX module.
=item log EXPR
-=item log
+=item log
Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
of $_.
=item lstat EXPR
-=item lstat
+=item lstat
Does the same thing as the stat() function, but stats a symbolic link
instead of the file the symbolic link points to. If symbolic links are
=item oct EXPR
-=item oct
+=item oct
Interprets EXPR as an octal string and returns the corresponding
value. (If EXPR happens to start off with 0x, interprets it as
for alternatives.)
Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
-non-zero upon success, the undefined value otherwise. If the open
+nonzero upon success, the undefined value otherwise. If the open
involved a pipe, the return value happens to be the pid of the
-subprocess.
+subprocess.
If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
the new STDOUT or STDIN. Typically this is used like the normal
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.
+don't want to have to scan shell commands for metacharacters.
The following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
=item ord EXPR
-=item ord
+=item ord
Returns the numeric ascii value of the first character of EXPR. If
-EXPR is omitted, uses $_. For the reverse, see L<chr>.
+EXPR is omitted, uses $_. For the reverse, see L</chr>.
=item pack TEMPLATE,LIST
c A signed char value.
C An unsigned char value.
+
s A signed short value.
S An unsigned short value.
+ (This 'short' is _exactly_ 16 bits, which may differ from
+ what a local C compiler calls 'short'.)
+
i A signed integer value.
I An unsigned integer value.
+ (This 'integer' is _at_least_ 32 bits wide. Its exact size
+ depends on what a local C compiler calls 'int', and may
+ even be larger than the 'long' described in the next item.)
+
l A signed long value.
L An unsigned long value.
+ (This 'long' is _exactly_ 32 bits, which may differ from
+ what a local C compiler calls 'long'.)
- n A short in "network" order.
- N A long in "network" order.
+ n A short in "network" (big-endian) order.
+ N A long in "network" (big-endian) order.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
+ (These 'shorts' and 'longs' are _exactly_ 16 bits and
+ _exactly_ 32 bits, respectively.)
f A single-precision float in the native format.
d A double-precision float in the native format.
u A uuencoded string.
- w A BER compressed integer. Bytes give an unsigned integer base
- 128, most significant digit first, with as few digits as
- possible, and with the bit 8 of each byte except the last set
- to "1."
+ w A BER compressed integer. Its bytes represent an unsigned
+ integer in base 128, most significant digit first, with as few
+ digits as possible. Bit eight (the high bit) is set on each
+ byte except the last.
x A null byte.
X Back up a byte.
=item pop ARRAY
-=item pop
+=item pop
Pops and returns the last value of the array, shortening the array by
1. Has a similar effect to
=item pos SCALAR
-=item pos
+=item pos
Returns the offset of where the last C<m//g> search left off for the variable
is in question ($_ is used when the variable is not specified). May be
=item quotemeta EXPR
-=item quotemeta
+=item quotemeta
-Returns the value of EXPR with with all non-alphanumeric
+Returns the value of EXPR with all non-alphanumeric
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.)
=item rand
Returns a random fractional number between 0 and the value of EXPR.
-(EXPR should be positive.) If EXPR is omitted, returns a value between
+(EXPR should be positive.) If EXPR is omitted, returns a value between
0 and 1. Automatically calls srand() unless srand() has already been
called. See also srand().
=item readlink EXPR
-=item readlink
+=item readlink
Returns the value of a symbolic link, if symbolic links are
implemented. If not, gives a fatal error. If there is some system
Actually does a C recvfrom(), so that it can returns 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.
+as the system call of the same name.
See L<perlipc/"UDP: Message Passing"> for examples.
=item redo LABEL
=item ref EXPR
-=item ref
+=item ref
Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
is not specified, $_ will be used. The value returned depends on the
CODE
GLOB
-If the referenced object has been blessed into a package, then that package
+If the referenced object has been blessed into a package, then that package
name is returned instead. You can think of ref() as a typeof() operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
- }
+ }
if (!ref ($r) {
print "r is not a reference at all.\n";
- }
+ }
See also L<perlref>.
otherwise. But it's better just to put the "C<1;>", in case you add more
statements.
-If EXPR is a bare word, the require assumes a "F<.pm>" extension and
+If EXPR is a bareword, the require assumes a "F<.pm>" extension and
replaces "F<::>" with "F</>" in the filename for you,
-to make it easy to load standard modules. This form of loading of
+to make it easy to load standard modules. This form of loading of
modules does not risk altering your namespace.
-For a yet-more-powerful import facility, see L</use> and
+For a yet-more-powerful import facility, see L</use> and
L<perlmod>.
=item reset EXPR
are unaffected, but they clean themselves up on scope exit anyway,
so you'll probably want to use them instead. See L</my>.
-=item return LIST
+=item return EXPR
+
+=item return
+
+Returns from a subroutine, eval(), or do FILE with the value of the
+given EXPR. Evaluation of EXPR may be in a list, scalar, or void
+context, depending on how the return value will be used, and the context
+may vary from one execution to the next (see wantarray()). If no EXPR
+is given, returns an empty list in a list context, an undefined value in
+a scalar context, or nothing in a void context.
-Returns from a subroutine or eval with the value specified. (Note that
-in the absence of a return a subroutine or eval() will automatically
-return the value of the last expression evaluated.)
+(Note that in the absence of a return, a subroutine, eval, or do FILE
+will automatically return the value of the last expression evaluated.)
=item reverse LIST
=item rmdir FILENAME
-=item rmdir
+=item rmdir
Deletes the directory specified by FILENAME if it is empty. If it
succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
=item scalar EXPR
Forces EXPR to be interpreted in a scalar context and returns the value
-of EXPR.
+of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
-There is no equivalent operator to force an expression to
+There is no equivalent operator to force an expression to
be interpolated in a list context because it's in practice never
needed. If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
=item seek FILEHANDLE,POSITION,WHENCE
-Randomly positions the file pointer for FILEHANDLE, just like the fseek()
-call of stdio. FILEHANDLE may be an expression whose value gives the name
-of the filehandle. The values for WHENCE are 0 to set the file pointer to
-POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
-plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
-this from POSIX module. Returns 1 upon success, 0 otherwise.
+Sets FILEHANDLE's position, just like the fseek() call of stdio.
+FILEHANDLE may be an expression whose value gives the name of the
+filehandle. The values for WHENCE are 0 to set the new position to
+POSITION, 1 to set it to the current position plus POSITION, and 2 to
+set it to EOF plus POSITION (typically negative). For WHENCE you may
+use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
+IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
+
+If you want to position file for sysread() or syswrite(), don't use
+seek() -- buffering makes its effect on the file's system position
+unpredictable and non-portable. Use sysseek() instead.
On some systems you have to do a seek whenever you switch between reading
and writing. Amongst other things, this may have the effect of calling
-stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
-the file pointer:
+stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
+the file position:
seek(TEST,0,1);
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. First the simple trick listed above to clear the
-filepointer. The 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.
+seek() to reset things. The 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.
If that doesn't work (some stdios are particularly cantankerous), then
you may need something more like this:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
-or to block until something becomes ready just do this
+or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
=item sin EXPR
-=item sin
+=item sin
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
returns sine of $_.
-For the inverse sine operation, you may use the POSIX::sin()
+For the inverse sine operation, you may use the POSIX::asin()
function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
always sleep the full amount.
For delays of finer granularity than one second, you may use Perl's
-syscall() interface to access setitimer(2) if your system supports it,
-or else see L</select()> below.
+syscall() interface to access setitimer(2) if your system supports it,
+or else see L</select()> below.
See also the POSIX module's sigpause() function.
@articles = sort {$a cmp $b} @files;
# now case-insensitively
- @articles = sort { uc($a) cmp uc($b)} @files;
+ @articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
- # inefficiently sort by descending numeric compare using
- # the first integer after the first = sign, or the
+ # inefficiently sort by descending numeric compare using
+ # the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
- for (@old) {
+ for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
- }
+ }
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
(1, '-', 10, ',', 20)
-If you had the entire header of a normal Unix email message in $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:
$header =~ s/\n\s+/ /g; # fix continuation lines
open(passwd, '/etc/passwd');
while (<passwd>) {
- ($login, $passwd, $uid, $gid, $gcos,
+ ($login, $passwd, $uid, $gid, $gcos,
$home, $shell) = split(/:/);
...
}
-(Note that $shell above will still have a newline on it. See L</chop>,
+(Note that $shell above will still have a newline on it. See L</chop>,
L</chomp>, and L</join>.)
=item sprintf FORMAT, LIST
=item sqrt EXPR
-=item sqrt
+=item sqrt
Return the square root of EXPR. If EXPR is omitted, returns square
root of $_.
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 rand()!
-Frequently called programs (like CGI scripts) that simply use
+Frequently called programs (like CGI scripts) that simply use
time ^ $$
-for a seed can fall prey to the mathematical property that
+for a seed can fall prey to the mathematical property that
a^b == (a+1)^(b+1)
=item stat EXPR
-=item stat
+=item stat
Returns a 13-element array giving the status info for a file, either the
file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
-Not all fields are supported on all filesystem types. Here are the
+Not all fields are supported on all filesystem types. Here are the
meaning of the fields:
- dev device number of filesystem
- ino inode number
- mode file mode (type and permissions)
- nlink number of (hard) links to the file
- uid numeric user ID of file's owner
- gid numeric group ID of file's owner
- rdev the device identifier (special files only)
- size total size of file, in bytes
- atime last access time since the epoch
- mtime last modify time since the epoch
- ctime inode change time (NOT creation time!) since the epoch
- blksize preferred block size for file system I/O
- blocks actual number of blocks allocated
+ 0 dev device number of filesystem
+ 1 ino inode number
+ 2 mode file mode (type and permissions)
+ 3 nlink number of (hard) links to the file
+ 4 uid numeric user ID of file's owner
+ 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
+ 11 blksize preferred block size for file system I/O
+ 12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
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 study active at a time--if you study a different scalar the first
+one study active at a time -- if you study a different scalar the first
is "unstudied". (The way 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 'k' characters are. From each search string,
symbolic links, produces a fatal error at run time. To check for that,
use eval:
- $symlink_exists = (eval 'symlink("","");', $@ eq '');
+ $symlink_exists = (eval {symlink("","")};, $@ eq '');
=item syscall LIST
=item sysread FILEHANDLE,SCALAR,LENGTH
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 may cause confusion.
-Returns the number of bytes actually read, 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 scalar after the read.
+specified FILEHANDLE, using the system call read(2). It bypasses stdio,
+so mixing this with other kinds of reads, print(), write(), seek(), or
+tell() can cause confusion. Returns the number of bytes actually read,
+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 scalar after the
+read.
An OFFSET may be specified to place the read data at some place in the
string other than the beginning. A negative OFFSET specifies
in the string being padded to the required size with "\0" bytes before
the result of the read is appended.
+=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 sysread()),
+print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
+be an expression whose value gives the name of the filehandle. The
+values for WHENCE are 0 to set the new position to POSITION, 1 to set
+the it to the current position plus POSITION, and 2 to set it to EOF
+plus POSITION (typically negative). For WHENCE, you may use the
+constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
+or the POSIX module.
+
+Returns the new position, or the undefined value on failure. A position
+of zero is returned as the string "0 but true"; thus sysseek() returns
+TRUE on success and FALSE on failure, yet you can still easily determine
+the new position.
+
=item system LIST
Does exactly the same thing as "exec LIST" except that a fork is done
Note that argument processing varies depending on the number of
arguments. The return value is the exit status of the program as
returned by the 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 back-ticks or
+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
qx//, as described in L<perlop/"`STRING`">.
-Because system() and back-ticks block SIGINT and SIGQUIT, killing the
+Because system() and backticks block SIGINT and SIGQUIT, killing the
program they're running doesn't actually interrupt your program.
@args = ("command", "arg1", "arg2");
- system(@args) == 0
- or die "system @args failed: $?"
+ system(@args) == 0
+ or die "system @args failed: $?"
Here's a more elaborate example of analysing the return value from
-system() on a UNIX system to check for all possibilities, including for
-signals and coredumps.
+system() on a Unix system to check for all possibilities, including for
+signals and core dumps.
$rc = 0xffff & system @args;
printf "system(%s) returned %#04x: ", "@args", $rc;
if ($rc == 0) {
print "ran with normal exit\n";
- }
+ }
elsif ($rc == 0xff00) {
print "command failed: $!\n";
- }
+ }
elsif ($rc > 0x80) {
$rc >>= 8;
print "ran with non-zero exit status $rc\n";
- }
+ }
else {
print "ran with ";
if ($rc & 0x80) {
$rc &= ~0x80;
- print "coredump from ";
- }
+ print "core dump from ";
+ }
print "signal $rc\n"
- }
+ }
$ok = ($rc != 0);
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using the system call write(2). It bypasses
-stdio, so mixing this with prints may cause confusion. Returns the
-number of bytes actually written, or undef if there was an error.
-If the length is greater than the available data, only as much data as
-is available will be written.
+stdio, so mixing this with reads (other than sysread()), print(),
+write(), seek(), or tell() may cause confusion. Returns the number of
+bytes actually written, or undef if there was an error. If the length
+is greater than the available data, 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
-from that many bytes counting backwards from the end of the string.
+that many bytes counting backwards from the end of the string.
=item tell FILEHANDLE
=item tell
-Returns the current file position for FILEHANDLE. FILEHANDLE may be an
+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.
TIESCALAR classname, LIST
DESTROY this
- FETCH this,
+ FETCH this,
STORE this, value
Unlike dbmopen(), the tie() function will not use or require a module
=item tr///
-The translation operator. See L<perlop>.
+The translation operator. Same as y///. See L<perlop>.
=item truncate FILEHANDLE,LENGTH
=item uc EXPR
-=item uc
+=item uc
Returns an uppercased version of EXPR. This is the internal function
implementing the \U escape in double-quoted strings.
=item ucfirst EXPR
-=item ucfirst
+=item ucfirst
Returns the value of EXPR with the first character uppercased. This is
the internal function implementing the \u escape in double-quoted strings.
Sets the umask for the process to EXPR and returns the previous value.
If EXPR is omitted, merely returns the current umask. Remember that a
umask is a number, usually given in octal; it is I<not> a string of octal
-digits. See also L<oct>, if all you have is a string.
+digits. See also L</oct>, if all you have is a string.
=item undef EXPR
=item undef
-Undefines the value of EXPR, which must be an lvalue. Use on only a
-scalar value, an entire array or hash, or a subroutine name (using
+Undefines the value of EXPR, which must be an lvalue. Use only on a
+scalar value, an entire array, an entire hash, or a subroutine name (using
"&"). (Using undef() will probably not do what you expect on most
predefined variables or DBM list values, so don't do that.) Always
returns the undefined value. You can omit the EXPR, in which case
undef @ary;
undef %hash;
undef &mysub;
- return (wantarray ? () : undef) if $they_blew_it;
+ return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
=item unlink LIST
-=item unlink
+=item unlink
Deletes a list of files. Returns the number of files successfully
deleted.
BEGIN { require Module; import Module LIST; }
-except that Module I<must> be a bare word.
+except that Module I<must> be a bareword.
If the first argument to C<use> is a number, it is treated as a version
number instead of a module name. If the version of the Perl interpreter
Returns TRUE if the context of the currently executing subroutine is
looking for a list value. Returns FALSE if the context is looking
-for a scalar.
+for a scalar. Returns the undefined value if the context is looking
+for no value (void context).
- return wantarray ? () : undef;
+ return unless defined wantarray; # don't bother doing more
+ my @a = complex_calculation();
+ return wantarray ? @a : "@a";
=item warn LIST
Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file. By default the format for
-a file is the one having the same name is the filehandle, but the
+a file is the one having the same name as the filehandle, but the
format for the current output channel (see the select() function) may be set
explicitly by assigning the name of the format to the C<$~> variable.
=item y///
-The translation operator. See L<perlop>.
+The translation operator. Same as tr///. See L<perlop>.
=back
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