operator. A unary operator generally provides a scalar context to its
argument, while a list operator may provide either scalar and list
contexts for its arguments. If it does both, the scalar arguments will
-be first, and the list argument will follow. (Note that there can only
-ever be one list argument.) For instance, splice() has three scalar
+be first, and the list argument will follow. (Note that there can ever
+be only one list argument.) For instance, splice() has three scalar
arguments followed by a list.
In the syntax descriptions that follow, list operators that expect a
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
-parens.) If you use the parens, the simple (but occasionally
+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
function, and precedence doesn't matter. Otherwise it's a list
operator or unary operator, and precedence does matter. And whitespace
=over 8
-=item
-
-I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
+=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
=back
dbmclose, dbmopen
-
=back
=head2 Alphabetical Listing of Perl Functions
-
=over 8
=item -X FILEHANDLE
-C Same for inode change time.
The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
-C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
+C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
uids and gids of the user. There may be other reasons you can't actually
read, write or execute the file. Also note that, for the superuser,
-C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
+C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
1 if any execute bit is set in the mode. Scripts run by the superuser may
-thus need to do a stat() in order to determine the actual mode of the
+thus need to do a stat() to determine the actual mode of the
file, or temporarily set the uid to something else.
Example:
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 stat() or lstat() operators) are given the
-special filehandle consisting of a solitary underline, then the stat
+If any of the file tests (or either the stat() or 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
that lstat() and C<-l> will leave values in the stat structure for the
restart system calls on some systems. Using eval/die always works.
eval {
- local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
+ local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
+ alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
+ alarm 0;
};
- die if $@ and $@ ne "alarm\n"; # propagate errors
+ die if $@ && $@ ne "alarm\n"; # propagate errors
if ($@) {
# timed out
}
Returns the arctangent of Y/X in the range -PI to PI.
+For the tangent operation, you may use the POSIX::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
=item bless REF
-This function tells the referenced object (passed as REF) that it is now
+This function tells the thingy referenced by REF that it is now
an object in the CLASSNAME package--or the current package if no CLASSNAME
is specified, which is often the case. It returns the reference for
-convenience, since a bless() is often the last thing in a constructor.
+convenience, because a bless() is often the last thing in a constructor.
Always use the two-argument version if the function doing the blessing
might be inherited by a derived class. See L<perlobj> for more about the
blessing (and blessings) of objects.
=item caller
Returns the context of the current subroutine call. In a scalar context,
-returns TRUE if there is a caller, that is, if we're in a subroutine or
-eval() or require(), and FALSE otherwise. In a list context, returns
+returns the caller's package name if there is a caller, that is, if
+we're in a subroutine or eval() or require(), and the undefined value
+otherwise. In a 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, $wantargs) = caller($i);
+ ($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 $evaltext is undefined. (Note
+also that C<L<use>> statement creates a C<L<require>> frame inside
+an C<L<eval EXPR>>) frame.
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable @DB::args to be the
This is a slightly safer version of chop (see below). It removes any
line ending that corresponds to the current value of C<$/> (also known as
-$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
-of characters removed. It's often used to remove the newline from the
-end of an input record when you're worried that the final record may be
-missing its newline. When in paragraph mode (C<$/ = "">), it removes all
-trailing newlines from the string. If VARIABLE is omitted, it chomps
-$_. Example:
+$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
+number of characters removed from all its arguments. It's often used to
+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. If
+VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
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
begin with a "/" by your process and all of its children. (It doesn't
-change your current working directory is unaffected.) For security
+change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is
omitted, does chroot to $_.
Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor. You don't have to close FILEHANDLE if you are immediately
-going to do another open() on it, since open() will close it for you. (See
+going to do another open() on it, because open() will close it for you. (See
open().) However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by open() does not. Also,
closing a pipe will wait for the process executing on the pipe to
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
takes cosine of $_.
+For the inverse cosine operation, you may use the POSIX::acos()
+function, or use this relation:
+
+ sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
+
=item crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) function in the C library
print "ok\n";
}
-Of course, typing in your own password to whoever asks you
+Of course, typing in your own password to whomever asks you
for it is unwise.
=item dbmclose ASSOC_ARRAY
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 umask()).
-If your system only supports the older DBM functions, you may perform only
+If your system supports only the older DBM functions, you may perform only
one dbmopen() in your program. In older versions of Perl, if your system
had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
falls back to sdbm(3).
matched something that happened to be 0 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 only use defined() when you're questioning the integrity
+you should use defined() only when you're questioning the integrity
of what you're trying to do. At other times, a simple comparison to
0 or "" is what you want.
+Another surprise is that using 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 undef() on these, however, does clear their memory and then report
+them as not defined anymore, but you shoudln'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.
+
+This counter-intuitive behaviour of defined() on aggregates may be
+changed, fixed, or broken in a future release of Perl.
+
=item delete EXPR
-Deletes the specified value from its hash array. Returns the deleted
-value, or the undefined value if nothing was deleted. Deleting from
-C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
-file deletes the entry from the DBM file. (But deleting from a tie()d
-hash doesn't necessarily return anything.)
+Deletes the specified key(s) and their associated values from a hash
+array. 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 an array tied to a DBM file
+deletes the entry from the DBM file. (But deleting from a tie()d hash
+doesn't necessarily return anything.)
The following deletes all the values of an associative array:
- foreach $key (keys %ARRAY) {
- delete $ARRAY{$key};
+ foreach $key (keys %HASH) {
+ delete $HASH{$key};
}
-(But it would be faster to use the undef() command.) Note that the
-EXPR can be arbitrarily complicated as long as the final operation is
-a hash key lookup:
+And so does this:
+
+ delete @HASH{keys %HASH}
+
+(But both of these are slower than the undef() command.) Note that the
+EXPR can be arbitrarily complicated as long as the final operation is a
+hash element lookup or hash slice:
delete $ref->[$x][$y]{$key};
+ delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
=item die LIST
Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
-C<($? E<gt>E<gt> 8)> (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.
+C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
+is 0, exits with 255. Inside an eval(), the error message is stuffed into
+C<$@>, and the eval() is terminated with the undefined value; this makes
+die() the way to raise an exception.
Equivalent examples:
See also exit() and warn().
+You can arrange for a callback to be called just before the die() does
+its deed, by setting the C<$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 die() again. See L<perlvar> for details on
+setting C<%SIG> entries, and eval() for some examples.
+
=item do BLOCK
Not really a function. Returns the value of the last command in the
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
-reparse the file every time you call it, so you probably don't want to
+re-parse 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
When called in a list context, returns a 2-element array consisting
of the key and value for the next element of an associative array,
so that you can iterate over it. When called in a scalar context,
-returns the key only for the next element in the associative array.
+returns the key for only the next element in the associative array.
Entries are returned in an apparently random order. When the array is
entirely read, a null array is returned in list context (which when
assigned produces a FALSE (0) value), and C<undef> is returned in a
iterating again. The iterator can be reset only by reading all the
elements from the array. You should not add elements to an array while
you're iterating over it. There is a single iterator for each
-associative array, shared by all each(), keys() and values() function
+associative array, shared by all each(), keys(), and values() function
calls in the program. The following prints out your environment like
the printenv(1) program, only in a different order:
An C<eof> without an argument uses the last file read as argument.
Empty parentheses () may be used to indicate
-the pseudofile formed of the files listed on the command line, i.e.
+the pseudo file formed of the files listed on the command line, i.e.,
C<eof()> is reasonable to use inside a 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:
EXPR is parsed and executed as if it were a little Perl program. It
is executed in the context of the current Perl program, so that any
-variable settings, subroutine or format definitions remain afterwards.
+variable settings or subroutine and format definitions remain afterwards.
The value returned is the value of the last expression evaluated, or a
return statement may be used, just as with subroutines. The last
expression is evaluated in scalar or array context, depending on the
If there is a syntax error or runtime error, or a die() statement is
executed, an undefined value is returned by eval(), and C<$@> is set to the
error message. If there was no error, C<$@> is guaranteed to be a null
-string. If EXPR is omitted, evaluates $_. The final semicolon, if
-any, may be omitted from the expression.
+string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
+any, may be omitted from the expression. Beware that using 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 warn() and L<perlvar>.
-Note that, since eval() traps otherwise-fatal errors, it is useful for
+Note that, because eval() traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket() or symlink())
is implemented. It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
# a run-time error
eval '$answer ='; # sets $@
+When using the eval{} form as an exception trap in libraries, you may
+wish not to trigger any C<__DIE__> hooks that user code may have
+installed. You can use the C<local $SIG{__DIE__}> construct for this
+purpose, as shown in this example:
+
+ # a very private exception trap for divide-by-zero
+ eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
+
+This is especially significant, given that C<__DIE__> hooks can call
+die() again, which has the effect of changing their error messages:
+
+ # __DIE__ hooks may modify error messages
+ {
+ local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
+ eval { die "foo foofs here" };
+ print $@ if $@; # prints "bar barfs here"
+ }
+
With an eval(), you should be especially careful to remember what's
being looked at when:
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
-A hash element can only be TRUE if it's defined, and defined if
+A hash element can be TRUE only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
-See also die(). If EXPR is omitted, exits with 0 status.
+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;
+all other values are subject to unpredictable interpretation depending
+on the environment in which the Perl program is running.
+
+You shouldn't use exit() to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use die() instead,
+which can be trapped by an eval().
=item exp EXPR
=item flock FILEHANDLE,OPERATION
-Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
-OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
-fatal error if used on a machine that doesn't implement either flock(2) or
-fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
-is missing from your system. This makes flock() the portable file locking
-strategy, although it will only lock entire files, not records. Note also
-that some versions of flock() cannot lock things over the network; you
-would need to use the more system-specific fcntl() for that.
+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.
+
+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).
+
+Note that the emulation built with lockf(3) doesn't provide shared
+locks, and it requires that FILEHANDLE be open with write intent. These
+are the semantics that lockf(3) implements. Most (all?) systems
+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 flock() cannot lock things over the
+network; you would need to use the more system-specific 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
+perl.
Here's a mailbox appender for BSD systems.
- $LOCK_SH = 1;
- $LOCK_EX = 2;
- $LOCK_NB = 4;
- $LOCK_UN = 8;
+ use Fcntl ':flock'; # import LOCK_* constants
sub lock {
- flock(MBOX,$LOCK_EX);
+ flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
- flock(MBOX,$LOCK_UN);
+ flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
Does a fork(2) system call. Returns the child pid to the parent process
and 0 to the child process, or C<undef> if the fork is unsuccessful.
Note: unflushed buffers remain unflushed in both processes, which means
-you may need to set C<$|> ($AUTOFLUSH in English) or call the
-autoflush() FileHandle method to avoid duplicate output.
+you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
+method of IO::Handle to avoid duplicate output.
If you fork() without ever waiting on your children, you will accumulate
zombies:
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, the remote server (such as, say, httpd or rsh) won't think
+you're done. You should reopen those to /dev/null if it's any issue.
+
=item format
Declare a picture format with use by the write() function. For
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, since 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.
formline() always returns TRUE. See L<perlform> for other examples.
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
- system "stty", 'icanon', 'eol', '^@'; # ascii null
+ system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
-Determination of whether to whether $BSD_STYLE should be set
+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().
- $login = getlogin || (getpwuid($<))[0] || "Kilroy";
+ $login = getlogin || getpwuid($<) || "Kilroy";
Do not consider getlogin() for authentication: it is not as
secure as getpwuid().
=item glob EXPR
+=item glob
+
Returns the value of EXPR with filename expansions such as a shell
would do. This is the internal function implementing the E<lt>*.*E<gt>
operator, except it's easier to use.
+If EXPR is omitted, $_ is used.
=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 timezone.
+with the time localized for the standard Greenwich time zone.
Typically used as follows:
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
+In a scalar context, prints out 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.
+
=item goto LABEL
=item goto EXPR
The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop. It
-also can't be used to go into a construct that is optimized away. It
-can be used to go almost anywhere else within the dynamic scope,
+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 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 last or die. The author of Perl has never felt the
need to use this form of goto (in Perl, that is--C is another matter).
@foo = grep {!/^#/} @bar; # weed out comments
-Note that, since $_ is a reference into the list value, it can be used
+Note that, because $_ 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.
Returns an lowercased version of EXPR. This is the internal function
implementing the \L escape in double-quoted strings.
-Should respect any POSIX setlocale() settings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
If EXPR is omitted, uses $_.
Returns the value of EXPR with the first character lowercased. This is
the internal function implementing the \l escape in double-quoted strings.
-Should respect any POSIX setlocale() settings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
If EXPR is omitted, uses $_.
=item local EXPR
A local modifies the listed variables to be local to the enclosing block,
-subroutine, C<eval{}> or C<do>. If more than one value is listed, the
-list must be placed in parens. See L<perlsub/"Temporary Values via
+subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
+list must be placed in parentheses. See L<perlsub/"Temporary Values via
local()"> for details.
But you really probably want to be using my() instead, because local() isn't
=item localtime EXPR
Converts a time as returned by the time function to a 9-element array
-with the time analyzed for the local timezone. Typically used as
+with the time analyzed for the local time zone. Typically used as
follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
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. If EXPR is omitted, does localtime(time).
+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)").
-In a scalar context, prints out the ctime(3) value:
+In a scalar context, returns the ctime(3) value:
- $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
+ $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
-Also see the F<timelocal.pl> library, and the strftime(3) function available
+Also see the Time::Local module, and the strftime(3) function available
via the POSIX module.
=item log EXPR
A "my" declares the listed variables to be local (lexically) to the
enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
-more than one value is listed, the list must be placed in parens. See
+more than one value is listed, the list must be placed in parentheses. See
L<perlsub/"Private Variables via my()"> for details.
=item next LABEL
=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 variable of
-the same name as the FILEHANDLE contains the filename. If the filename
-begins with "E<lt>" or nothing, the file is opened for input. If the filename
-begins with "E<gt>", the file is opened for output. If the filename begins
-with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
-front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
-access to the file; thus '+E<lt>' is usually preferred for read/write
-updates--the '+E<gt>' mode would clobber the file first. These correspond to
-the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
-
-If the filename begins with "|", the filename is interpreted
-as a command to which output is to be piped, and if the filename ends with
-a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
-for more examples of this. as command which pipes input to us. (You may
-not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
-L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
+FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
+name of the real filehandle wanted. 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
+to open.)
+
+If the filename begins with '<' or nothing, the file is opened for input.
+If the filename begins with '>', the file is truncated and opened for
+output. If the filename begins with '>>', the file is opened for
+appending. You can put a '+' in front of the '>' or '<' to indicate that
+you want both read and write access to the file; thus '+<' is almost
+always preferred for read/write updates--the '+>' mode would clobber the
+file first. The prefix and the filename may be separated with spaces.
+These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
+'w+', 'a', and 'a+'.
+
+If the filename begins with "|", the filename is interpreted as a command
+to which output is to be piped, and if the filename ends with a "|", the
+filename is interpreted See L<perlipc/"Using open() for IPC"> for more
+examples of this. as command which pipes input to us. (You may not have
+a raw 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.)
Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
non-zero upon success, the undefined value otherwise. If the open
with "E<gt>&", in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) which is to be
duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
-+E<gt>E<gt> and +E<lt>. The
++E<gt>E<gt>, and +E<lt>. 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.)
open(FILEHANDLE, "<&=$fd")
-If you open a pipe on the command "-", i.e. either "|-" or "-|", then
+If you open a pipe on the command "-", i.e., either "|-" or "-|", then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and 0 within the child
process. (Use C<defined($pid)> to determine whether the open was successful.)
unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.
-Using the FileHandle constructor from the FileHandle package,
+Using the constructor from the IO::Handle package (or one of its
+subclasses, such as IO::File or IO::Socket),
you can generate anonymous filehandles which have the scope of whatever
variables hold references to them, and automatically close whenever
and however you leave that scope:
- use FileHandle;
+ use IO::File;
...
sub read_myfile_munged {
my $ALL = shift;
- my $handle = new FileHandle;
+ my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
}
The filename that is passed to open will have leading and trailing
-whitespace deleted. In order to open a file with arbitrary weird
+whitespace deleted. To open a file with arbitrary weird
characters in it, it's necessary to protect any leading and trailing
whitespace thusly:
you should use the sysopen() function. This is another way to
protect your filenames from interpretation. For example:
- use FileHandle;
+ use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
or die "sysopen $path: $!";
HANDLE->autoflush(1);
=item opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by readdir(), telldir(),
-seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
+seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
=item ord EXPR
@ Null fill to absolute position.
Each letter may optionally be followed by a number which gives a repeat
-count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
+count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
pack function will gobble up that many values from the LIST. A * for the
repeat count means to use however many items are left. The "a" and "A"
types gobble just one value, but pack it as a string of length count,
both use IEEE floating point arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec). 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.
+float and thence back to double again will lose precision (i.e.,
C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
Examples:
of the package declaration is from the declaration itself through the end of
the enclosing block (the same scope as the local() operator). All further
unqualified dynamic identifiers will be in this namespace. A package
-statement only affects dynamic variables--including those you've used
+statement affects only dynamic variables--including those you've used
local() on--but I<not> lexical variables created with my(). Typically it
would be the first declaration in a file to be included by the C<require>
or C<use> operator. You can switch into a package in more than one place;
-it merely influences which symbol table is used by the compiler for the
+it influences merely which symbol table is used by the compiler for the
rest of that block. You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a double
colon: C<$Package::Variable>. If the package name is null, the C<main>
stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.
-See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
+See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.
=item pop ARRAY
+=item pop
+
Pops and returns the last value of the array, shortening the array by
1. Has a similar effect to
Returns the offset of where the last C<m//g> search left off for the variable
is in question ($_ is used when the variable is not specified). May be
-modified to change that offset.
+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 FILEHANDLE LIST
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 + or put parens around the arguments.) If FILEHANDLE is
+interpose a + 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 $_ to
STDOUT. To set the default output channel to something other than
evaluated in a 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 + or
-put parens around all the arguments.
+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:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
-=item printf FILEHANDLE LIST
+=item printf FILEHANDLE FORMAT, LIST
+
+=item printf FORMAT, LIST
-=item printf LIST
+Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
+of the list will be interpreted as the 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>.
-Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
-of the list will be interpreted as the printf format.
+Don't fall into the trap of using a printf() when a simple
+print() would do. The print() is more efficient, and less
+error prone.
=item prototype FUNCTION
Returns the prototype of a function as a string (or C<undef> if the
-function has no prototype). FUNCTION is a reference to the the
-function whose prototype you want to retrieve.
+function has no prototype). FUNCTION is a reference to, or the name of,
+the function whose prototype you want to retrieve.
=item push ARRAY,LIST
=item quotemeta
-Returns the value of EXPR with with all regular expression
-metacharacters backslashed. This is the internal function implementing
+Returns the value of EXPR with 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.)
+This is the internal function implementing
the \Q escape in double-quoted strings.
If EXPR is omitted, uses $_.
a scalar context or a null list in a list context.
If you're planning to filetest the return values out of a readdir(), you'd
-better prepend the directory in question. Otherwise, since we didn't
+better prepend the directory in question. Otherwise, because we didn't
chdir() there, it would have been testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
=item rename OLDNAME,NEWNAME
Changes the name of a file. Returns 1 for success, 0 otherwise. Will
-not work across filesystem boundaries.
+not work across file system boundaries.
=item require EXPR
expression is interpreted as a list of single characters (hyphens
allowed for ranges). All variables and arrays beginning with one of
those letters are reset to their pristine state. If the expression is
-omitted, one-match searches (?pattern?) are reset to match again. Only
-resets variables or searches in the current package. Always returns
+omitted, one-match searches (?pattern?) are reset to match again. Resets
+only variables or searches in the current package. Always returns
1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
-Resetting "A-Z" is not recommended since you'll wipe out your
-ARGV and ENV arrays. Only resets package variables--lexical variables
+Resetting "A-Z" is not recommended because you'll wipe out your
+ARGV and ENV arrays. Resets only package variables--lexical variables
are unaffected, but they clean themselves up on scope exit anyway,
so you'll probably want to use them instead. See L</my>.
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. Hopefully.
+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:
Some programmers may prefer to think of filehandles as objects with
methods, preferring to write the last example as:
- use FileHandle;
+ use IO::Handle;
STDERR->autoflush(1);
=item select RBITS,WBITS,EBITS,TIMEOUT
-This calls the select(2) system call with the bitmasks specified, which
+This calls the select(2) system call with the bit masks specified, which
can be constructed using fileno() and vec(), along these lines:
$rin = $win = $ein = '';
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
-Most systems do not both to return anything useful in $timeleft, so
+Most systems do not bother to return anything useful in $timeleft, so
calling select() in a scalar context just returns $nfound.
-Any of the bitmasks can also be undef. The timeout, if specified, is
+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 the $timeleft. If not, they always return
$timeleft equal to the supplied $timeout.
-You can effect a 250-millisecond sleep this way:
+You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Sets the current process group for the specified PID, 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 ommitted, it defaults to
+implement setpgrp(2). If the arguments are omitted, it defaults to
0,0. Note that the POSIX version of setpgrp() does not accept any
arguments, so only setpgrp 0,0 is portable.
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
returns sine of $_.
+For the inverse sine operation, you may use the POSIX::sin()
+function, or use this relation:
+
+ sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
+
=item sleep EXPR
=item sleep
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted by sending the process a SIGALRM. Returns the
number of seconds actually slept. You probably cannot mix alarm() and
-sleep() calls, since sleep() is often implemented using alarm().
+sleep() calls, because sleep() is often implemented using 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
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.
+
=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
+SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
system call of the same name. You should "use Socket;" first to get
the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
=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
+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.
$b (see example below). They are passed by reference, so don't
modify $a and $b. And don't try to declare them as lexicals either.
+You also cannot exit out of the sort block or subroutine using any of the
+loop control operators described in L<perlsyn> or with goto().
+
+When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
+current collation locale. See L<perllocale>.
+
Examples:
# sort lexically
@sortedclass = sort byage @class;
# this sorts the %age associative arrays by value
- # instead of key using an inline function
+ # instead of key using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
sub backwards { $b cmp $a; }
replaces them with the elements of LIST, if any. Returns the elements
removed from the array. The array grows or shrinks as necessary. If
LENGTH is omitted, removes everything from OFFSET onward. The
-following equivalencies hold (assuming C<$[ == 0>):
+following equivalences hold (assuming C<$[ == 0>):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
produces the output 'h:i:t:h:e:r:e'.
-The LIMIT parameter can be used to partially split a line
+The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
(Note that $shell above will still have a newline on it. See L</chop>,
L</chomp>, and L</join>.)
-=item sprintf FORMAT,LIST
+=item sprintf FORMAT, LIST
Returns a string formatted by the usual printf conventions of the C
language. See L<sprintf(3)> or L<printf(3)> on your system for details.
(The * character for an indirectly specified length is not
supported, but you can get the same effect by interpolating a variable
-into the pattern.) Some C libraries' implementations of sprintf() can
+into the pattern.) 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>.
+Some C libraries' implementations of sprintf() can
dump core when fed ludicrous arguments.
=item sqrt EXPR
=item srand EXPR
Sets the random number seed for the C<rand> operator. If EXPR is omitted,
-uses a semirandom value based on the current time and process ID, among
-other things. Of course, you'd need something much more random than that for
-cryptographic purposes, since it's easy to guess the current time.
-Checksumming the compressed output of rapidly changing operating system
-status programs is the usual method. Examples are posted regularly to
-the comp.security.unix newsgroup.
+uses a semi-random value 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 time(). This isn't a particularly good seed, so many
+old programs supply their own seed value (often C<time ^ $$> or C<time ^
+($$ + ($$ << 15))>), but that isn't necessary any more.
+
+You need something much more random than the default seed for
+cryptographic purposes, though. Checksumming the compressed output of
+one or more rapidly changing operating system status programs is the
+usual method. For example:
+
+ srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
+
+If you're particularly concerned with this, see the Math::TrulyRandom
+module in CPAN.
+
+Do I<not> call 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 rand() function so that 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 rand()!
+
+Frequently called programs (like CGI scripts) that simply use
+
+ time ^ $$
+
+for a seed can fall prey to the mathematical property that
+
+ a^b == (a+1)^(b+1)
+
+one-third of the time. So don't do that.
=item stat FILEHANDLE
mode file mode (type and permissions)
nlink number of (hard) links to the file
uid numeric user ID of file's owner
- gid numer group 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 type!) since the epoch
- blksize preferred blocksize for file system I/O
+ 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
(The epoch was at 00:00 January 1, 1970 GMT.)
print "$file is executable NFS file\n";
}
-(This only works on machines for which the device number is negative under NFS.)
+(This works on machines only for which the device number is negative under NFS.)
=item study SCALAR
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
-runtimes with and without it to see which runs faster. Those loops
+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
@ARGV = @files;
undef $/;
eval $search; # this screams
- $/ = "\n"; # put back to normal input delim
+ $/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
-Note that Perl only supports passing of up to 14 arguments to your system call,
+Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.
=item sysopen FILEHANDLE,FILENAME,MODE
file. If PERMS is omitted, the default value is 0666, which allows
read and write for all. This default is reasonable: see C<umask>.
+The IO::File module provides a more object-oriented approach, if you're
+into that kind of thing.
+
=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
=item sysread FILEHANDLE,SCALAR,LENGTH
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 to the length actually read.
-In the case of growing the new data area will be padded with "\0" bytes.
-An OFFSET may be specified to place the read data at some other
-place than the beginning of the string. A negative OFFSET means
-placing the read data at that many bytes counting backwards from the end
-of the string.
+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
+placement at that many bytes counting backwards from the end of the
+string. A positive OFFSET greater than the length of SCALAR results
+in the string being padded to the required size with "\0" bytes before
+the result of the read is appended.
=item system LIST
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 merely use backticks, as
-described in L<perlop/"`STRING`">.
+the output from a command, for that you should use merely back-ticks or
+qx//, as described in L<perlop/"`STRING`">.
+
+Because system() and back-ticks 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: $?"
+
+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.
+
+ $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 "signal $rc\n"
+ }
+ $ok = ($rc != 0);
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
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. An OFFSET may be specified to write the
-data from some other place than the beginning of the string.
-A negative OFFSET means starting the writing from that many bytes
-counting backwards from the end of the string.
+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.
=item tell FILEHANDLE
Returns an uppercased version of EXPR. This is the internal function
implementing the \U escape in double-quoted strings.
-Should respect any POSIX setlocale() settings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
If EXPR is omitted, uses $_.
Returns the value of EXPR with the first character uppercased. This is
the internal function implementing the \u escape in double-quoted strings.
-Should respect any POSIX setlocale() settings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
If EXPR is omitted, uses $_.
=item umask
Sets the umask for the process and returns the old one. If EXPR is
-omitted, merely returns current umask.
+omitted, returns merely the current umask.
=item undef EXPR
=item undef
-Undefines the value of EXPR, which must be an lvalue. Use only on a
+Undefines the value of EXPR, which must be an lvalue. Use on only a
scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that.) Always returns the undefined value. You can omit
Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
-value. (In a scalar context, it merely returns the first value
+value. (In a scalar context, it returns merely the first value
produced.) The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:
BEGIN { require Module; }
If the VERSION argument is present between Module and LIST, then the
-C<use> will fail if the C<$VERSION> variable in package Module is
-less than VERSION.
+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 $Module::VERSION. (Note that there is not a
+comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way. Currently implemented pragmas are:
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
-These pseudomodules import semantics into the current block scope, unlike
+These pseudo-modules import semantics into the current block scope, unlike
ordinary modules, which import symbols into the current package (which are
effective through the end of the file).
There's a corresponding "no" command that unimports meanings imported
-by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
+by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
no integer;
no strict 'refs';
=item vec EXPR,OFFSET,BITS
Treats the string in EXPR as a vector of unsigned integers, and
-returns the value of the bitfield specified by OFFSET. BITS specifies
+returns the value of the bit field specified by OFFSET. BITS specifies
the number of bits that are reserved for each entry in the bit
vector. This must be a power of two from 1 to 32. vec() may also be
-assigned to, in which case parens are needed to give the expression
+assigned to, in which case parentheses are needed to give the expression
the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
Vectors created with vec() can also be manipulated with the logical
-operators |, & and ^, which will assume a bit vector operation is
+operators |, &, and ^, which will assume a bit vector operation is
desired when both operands are strings.
To transform a bit vector into a string or array of 0's and 1's, use these:
of the deceased process, or -1 if there is no such child process. The
status is returned in C<$?>. If you say
- use POSIX ":wait_h";
+ use POSIX ":sys_wait_h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-blocking wait
-is only available on machines supporting either the waitpid(2) or
+is available on machines supporting either the waitpid(2) or
wait4(2) system calls. However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere. (Perl emulates the system call
by remembering the status values of processes that have exited but have
=item warn LIST
-Produces a message on STDERR just like die(), but doesn't exit or
-on an exception.
+Produces a message on STDERR just like die(), but doesn't exit or throw
+an exception.
+
+No message is printed if there is a C<$SIG{__WARN__}> handler
+installed. It is the handler's responsibility to deal with the message
+as it sees fit (like, for instance, converting it into a die()). Most
+handlers must therefore make arrangements to actually display the
+warnings that they are not prepared to deal with, by calling warn()
+again in the handler. Note that this is quite safe and will not
+produce an endless loop, since C<__WARN__> hooks are not called from
+inside one.
+
+You will find this behavior is slightly different from that of
+C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
+instead call 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:
+
+ # wipe out *all* compile-time warnings
+ BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
+ my $foo = 10;
+ my $foo = 20; # no warning about duplicate my $foo,
+ # but hey, you asked for it!
+ # no compile-time or run-time warnings before here
+ $DOWARN = 1;
+
+ # run-time warnings enabled after here
+ warn "\$foo is alive and $foo!"; # does show up
+
+See L<perlvar> for details on setting C<%SIG> entries, and for more
+examples.
=item write FILEHANDLE
projects / p5sagit/p5-mst-13.2.git / blobdiff