+
=head1 NAME
perlfunc - Perl builtin functions
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.
=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
=item Functions for SCALARs or strings
-chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
-oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
-sprintf, substr, tr///, uc, ucfirst, y///
+C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
+C<length>, C<oct>, C<ord>, C<pack>, C<q>/STRING/, C<qq>/STRING/, C<reverse>,
+C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y>///
=item Regular expressions and pattern matching
-m//, pos, quotemeta, s///, split, study
+C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>
=item Numeric functions
-abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
-srand
+C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
+C<sin>, C<sqrt>, C<srand>
=item Functions for real @ARRAYs
-pop, push, shift, splice, unshift
+C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
=item Functions for list data
-grep, join, map, qw/STRING/, reverse, sort, unpack
+C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>
=item Functions for real %HASHes
-delete, each, exists, keys, values
+C<delete>, C<each>, C<exists>, C<keys>, C<values>
=item Input and output functions
-binmode, close, closedir, dbmclose, dbmopen, die, eof,
-fileno, flock, format, getc, print, printf, read, readdir,
-rewinddir, seek, seekdir, select, syscall, sysread,
-syswrite, tell, telldir, truncate, warn, write
+C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
+C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
+C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
+C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
+C<warn>, C<write>
=item Functions for fixed length data or records
-pack, read, syscall, sysread, syswrite, unpack, vec
+C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
=item Functions for filehandles, files, or directories
-I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
-lstat, mkdir, open, opendir, readlink, rename, rmdir,
-stat, symlink, umask, unlink, utime
+C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
+C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
+C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
=item Keywords related to the control flow of your perl program
-caller, continue, die, do, dump, eval, exit, goto, last,
-next, redo, return, sub, wantarray
+C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
+C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
-=item Keywords related to scoping
+=item Keywords related to scoping
-caller, import, local, my, package, use
+C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
=item Miscellaneous functions
-defined, dump, eval, formline, local, my, reset, scalar,
-undef, wantarray
+C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
+C<scalar>, C<undef>, C<wantarray>
=item Functions for processes and process groups
-alarm, exec, fork, getpgrp, getppid, getpriority, kill,
-pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
-times, wait, waitpid
+C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
+C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
+C<times>, C<wait>, C<waitpid>
=item Keywords related to perl modules
-do, import, no, package, require, use
+C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
=item Keywords related to classes and object-orientedness
-bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
+C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
+C<untie>, C<use>
=item Low-level socket functions
-accept, bind, connect, getpeername, getsockname,
-getsockopt, listen, recv, send, setsockopt, shutdown,
-socket, socketpair
+C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
+C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
+C<socket>, C<socketpair>
=item System V interprocess communication functions
-msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
-shmctl, shmget, shmread, shmwrite
+C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
+C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
=item Fetching user and group info
-endgrent, endhostent, endnetent, endpwent, getgrent,
-getgrgid, getgrnam, getlogin, getpwent, getpwnam,
-getpwuid, setgrent, setpwent
+C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
+C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
+C<getpwuid>, C<setgrent>, C<setpwent>
=item Fetching network info
-endprotoent, endservent, gethostbyaddr, gethostbyname,
-gethostent, getnetbyaddr, getnetbyname, getnetent,
-getprotobyname, getprotobynumber, getprotoent,
-getservbyname, getservbyport, getservent, sethostent,
-setnetent, setprotoent, setservent
+C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
+C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
+C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
+C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
+C<setnetent>, C<setprotoent>, C<setservent>
=item Time-related functions
-gmtime, localtime, time, times
+C<gmtime>, C<localtime>, C<time>, C<times>
=item Functions new in perl5
-abs, bless, chomp, chr, exists, formline, glob, import, lc,
-lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
-ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
+C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
+C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
+C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
+C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
* - C<sub> was a keyword in perl4, but in perl5 it is an
operator which can be used in expressions.
=item Functions obsoleted in perl5
-dbmclose, dbmopen
-
+C<dbmclose>, C<dbmopen>
=back
=head2 Alphabetical Listing of Perl Functions
-
=over 8
-=item -X FILEHANDLE
+=item I<-X> FILEHANDLE
-=item -X EXPR
+=item I<-X> EXPR
-=item -X
+=item I<-X>
A file test, where X is one of the letters listed below. This unary
operator takes one argument, either a filename or a filehandle, and
-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>.
-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
=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
-eval/die pair. You can't rely on the alarm causing the system call to
+eval/die pair. You can't rely on the alarm causing the system call to
fail with $! set to EINTR because Perl sets up signal handlers to
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, $size;
alarm 0;
Returns the arctangent of Y/X in the range -PI to PI.
+For the tangent operation, you may use the POSIX::tan()
+function, or use the familiar relation:
+
+ sub tan { sin($_[0]) / cos($_[0]) }
+
=item bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call
Arranges for the file to be read or written in "binary" mode in operating
systems that distinguish between binary and text files. Files that are
not in binary mode have CR LF sequences translated to LF on input and LF
-translated to CR LF on output. Binmode has no effect under Unix; in DOS
+translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
and similarly archaic systems, it may be imperative--otherwise your
-DOS-damaged C library may mangle your file. The key distinction between
+MS-DOS-damaged C library may mangle your file. The key distinction between
systems that need binmode and those that don't is their text file
formats. Systems like Unix and Plan9 that delimit lines with a single
character, and that encode that character in C as '\n', do not need
=item bless REF
-This function tells the referenced object (passed as REF) that it is now
+This function tells the thingy referenced by REF that it is now
an object in the CLASSNAME package--or the current package if no CLASSNAME
is specified, which is often the case. It returns the reference for
convenience, because a bless() is often the last thing in a constructor.
=item caller
Returns the context of the current subroutine call. In a scalar context,
-returns TRUE if there is a caller, that is, if we're in a subroutine or
-eval() or require(), and FALSE otherwise. In a list context, returns
+returns the caller's package name if there is a caller, that is, if
+we're in a subroutine or eval() or require(), and the undefined value
+otherwise. In a list context, returns
($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 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
Changes the permissions of a list of files. The first element of the
list must be the numerical mode, which should probably be an octal
-number. Returns the number of files successfully changed.
+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.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
+ $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
+ $mode = '0644'; chmod oct($mode), 'foo'; # this is better
+ $mode = 0644; chmod $mode, 'foo'; # this is best
=item chomp VARIABLE
=item chomp
-This is a slightly safer version of chop (see below). It removes any
+This is a slightly safer version of L</chop>. It removes any
line ending that corresponds to the current value of C<$/> (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
$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 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
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 $_.
Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
-descriptor. You don't have to close FILEHANDLE if you are immediately
-going to do another open() on it, because open() will close it for you. (See
+descriptor.
+
+You don't have to close FILEHANDLE if you are immediately going to do
+another open() on it, because open() will close it for you. (See
open().) However, an explicit close on an input file resets the line
-counter ($.), while the implicit close done by open() does not. Also,
-closing a pipe will wait for the process executing on the pipe to
-complete, in case you want to look at the output of the pipe
-afterwards. Closing a pipe explicitly also puts the status value of
-the command into C<$?>. Example:
+counter ($.), while the implicit close done by open() does not.
+
+If the file handle came from a piped open C<close> will additionally
+return FALSE if one of the other system calls involved fails or if the
+program exits with non-zero status. (If the only problem was that the
+program exited non-zero $! will be set to 0.) Also, closing a pipe will
+wait for the process executing on the pipe to complete, in case you
+want to look at the output of the pipe afterwards. Closing a pipe
+explicitly also puts the exit status value of the command into C<$?>.
+Example:
- open(OUTPUT, '|sort >foo'); # pipe to sort
+ open(OUTPUT, '|sort >foo') # pipe to sort
+ or die "Can't start sort: $!";
... # print stuff to output
- close OUTPUT; # wait for sort to finish
- open(INPUT, 'foo'); # get sort's results
+ close OUTPUT # wait for sort to finish
+ or warn $! ? "Error closing sort pipe: $!"
+ : "Exit status $? from sort";
+ open(INPUT, 'foo') # get sort's results
+ or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value gives the real filehandle name.
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
the password file for lousy passwords, amongst other things. Only the
guys wearing white hats should do this.
+Note that crypt is intended to be a one-way function, much like breaking
+eggs to make an omelette. There is no (known) corresponding decrypt
+function. As a result, this function isn't all that useful for
+cryptography. (For that, see your nearby CPAN mirror.)
+
Here's an example that makes sure that whoever runs this program knows
their own password:
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 whoever asks you
for it is unwise.
-=item dbmclose ASSOC_ARRAY
+=item dbmclose HASH
[This function has been superseded by the untie() function.]
-Breaks the binding between a DBM file and an associative array.
+Breaks the binding between a DBM file and a hash.
-=item dbmopen ASSOC,DBNAME,MODE
+=item dbmopen HASH,DBNAME,MODE
[This function has been superseded by the tie() function.]
-This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
-associative array. ASSOC is the name of the associative array. (Unlike
-normal open, the first argument is I<NOT> a filehandle, even though it
-looks like one). DBNAME is the name of the database (without the F<.dir>
-or F<.pag> extension if any). If the database does not exist, it is
-created with protection specified by MODE (as modified by the umask()).
-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).
-
-If you don't have write access to the DBM file, you can only read
-associative array variables, not set them. If you want to test whether
-you can write, either use file tests or try setting a dummy array entry
-inside an eval(), which will trap the error.
+This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
+hash. HASH is the name of the hash. (Unlike normal open, the first
+argument is I<NOT> a filehandle, even though it looks like one). DBNAME
+is the name of the database (without the F<.dir> or F<.pag> extension if
+any). If the database does not exist, it is created with protection
+specified by MODE (as modified by the umask()). 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).
+
+If you don't have write access to the DBM file, you can only read hash
+variables, not set them. If you want to test whether you can write,
+either use file tests or try setting a dummy hash entry inside an eval(),
+which will trap the error.
Note that functions such as keys() and values() may return huge array
values when used on large DBM files. You may prefer to use the each()
=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
+checked.
+
+Many operations return C<undef> to indicate failure, end of file,
+system error, uninitialized variable, and other exceptional
+conditions. This function allows you to distinguish C<undef> from
+other values. (A simple Boolean test will not distinguish among
+C<undef>, zero, the empty string, and "0", which are all equally
+false.) Note that since C<undef> is a valid scalar, its presence
+doesn't I<necessarily> indicate an exceptional condition: pop()
+returns C<undef> when its argument is an empty array, I<or> when the
+element to return happens to be C<undef>.
-Returns a boolean value saying whether EXPR has a real value
-or not. If EXPR is not present, $_ will be checked. Many operations
-return the undefined value under exceptional conditions, such as end of
-file, uninitialized variable, system error and such. This function
-allows you to distinguish between an undefined
-null scalar and a defined null scalar with operations that might return
-a real null string, such as referencing elements of an array. You may
-also check to see if arrays or subroutines exist. Use of defined on
-predefined variables is not guaranteed to produce intuitive results.
+You may also use defined() to check whether a subroutine exists. On
+the other hand, use of defined() upon aggregates (hashes and arrays)
+is not guaranteed to produce intuitive results, and should probably be
+avoided.
-When used on a hash array element, it tells you whether the value
-is defined, not whether the key exists in the hash. Use exists() for that.
+When used on a hash element, it tells you whether the value is defined,
+not whether the key exists in the hash. Use L</exists> for the latter
+purpose.
Examples:
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
- eval '@foo = ()' if defined(@foo);
- die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
+ $debugging = 0 unless defined $debugging;
-See also undef().
-
-Note: many folks tend to overuse defined(), and then are surprised to
-discover that the number 0 and the null string are, in fact, defined
-concepts. For example, if you say
+Note: Many folks tend to overuse defined(), and then are surprised to
+discover that the number 0 and "" (the zero-length string) are, in fact,
+defined values. For example, if you say
"ab" =~ /a(.*)b/;
matched "nothing". But it didn't really match nothing--rather, it
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 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.
+it's an admission that it couldn't give you an honest answer. So 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.
+
+Currently, 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 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>.
=item delete EXPR
-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 specified key(s) and their associated values from a hash.
+For each key, returns the deleted value associated with that key, or
+the undefined value if there was no such key. Deleting from C<$ENV{}>
+modifies the environment. Deleting from a hash tied to a DBM file
deletes the entry from the DBM file. (But deleting from a tie()d hash
doesn't necessarily return anything.)
-The following deletes all the values of an associative array:
+The following deletes all the values of a hash:
foreach $key (keys %HASH) {
delete $HASH{$key};
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)> 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)> (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
See also exit() and warn().
+If LIST is empty and $@ already contains a value (typically from a
+previous eval) that value is reused after appending "\t...propagated".
+This is useful for propagating exceptions:
+
+ eval { ... };
+ die unless $@ =~ /Expected exception/;
+
+If $@ is empty then the string "Died" is used.
+
+You can arrange for a callback to be called just before the die() does
+its deed, by setting the C<$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/$SIG{expr}> for details on
+setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
+
+Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
+blocks/strings. If one wants the hook to do nothing in such
+situations, put
+
+ die @_ if $^S;
+
+as the first line of the handler (see L<perlvar/$^S>).
+
=item do BLOCK
Not really a function. Returns the value of the last command in the
is just like
- eval `cat stat.pl`;
+ scalar eval `cat stat.pl`;
except that it's more efficient, more concise, keeps track of 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
-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
QUICKSTART:
Getopt('f');
-=item each ASSOC_ARRAY
+=item each HASH
+
+When called in a list context, returns a 2-element array consisting of the
+key and value for the next element of a hash, so that you can iterate over
+it. When called in a scalar context, returns the key for only the next
+element in the hash. (Note: Keys may be "0" or "", which are logically
+false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
+for this reason.)
-When called in a list context, returns a 2-element array consisting
-of the key and value for the next element of an associative array,
-so that you can iterate over it. When called in a scalar context,
-returns the key for only the next element in the associative array.
-Entries are returned in an apparently random order. When the array is
+Entries are returned in an apparently random order. When the hash is
entirely read, a null array is returned in list context (which when
assigned produces a FALSE (0) value), and C<undef> is returned in a
-scalar context. The next call to each() after that will start
-iterating again. The iterator can be reset only by reading all the
-elements from the array. You should not add elements to an array while
-you're iterating over it. There is a single iterator for each
-associative array, shared by all each(), keys(), and values() function
-calls in the program. The following prints out your environment like
-the printenv(1) program, only in a different order:
+scalar context. The next call to each() after that will start iterating
+again. There is a single iterator for each hash, shared by all each(),
+keys(), and values() function calls in the program; it can be reset by
+reading all the elements from the hash, or by evaluating C<keys HASH> or
+C<values HASH>. If you add or delete elements of a hash while you're
+iterating over it, you may get entries skipped or duplicated, so don't.
+
+The following prints out your environment like the printenv(1) program,
+only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
as terminals may lose the end-of-file condition if you do.
An C<eof> without an argument uses the last file read as argument.
-Empty parentheses () may be used to indicate
-the pseudo file formed of the files listed on the command line, i.e.,
-C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
-of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
-test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
+Empty parentheses () may be used to indicate the pseudo file formed of
+the files listed on the command line, i.e., C<eof()> is reasonable to
+use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
+last file. Use C<eof(ARGV)> or eof without the parentheses to test
+I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
# reset line numbering on each input file
while (<>) {
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators return undef when they run out of data.
+input operators return undef when they run out of data.
=item eval EXPR
=item eval BLOCK
-EXPR is parsed and executed as if it were a little Perl program. It
-is executed in the context of the current Perl program, so that any
+In the first form, the return value of EXPR is parsed and executed as if it
+were a little Perl program. The value of the expression (which is itself
+determined within a scalar context) is first parsed, and if there are no
+errors, executed in the context of the current Perl program, so that any
variable settings or subroutine and format definitions remain afterwards.
-The value returned is the value of the last expression evaluated, or a
-return statement may be used, just as with subroutines. The last
-expression is evaluated in scalar or array context, depending on the
-context of the eval.
+Note that the value is parsed every time the eval executes. If EXPR is
+omitted, evaluates C<$_>. This form is typically used to delay parsing
+and subsequent execution of the text of EXPR until run time.
+
+In the second form, the code within the BLOCK is parsed only once--at the
+same time the code surrounding the eval itself was parsed--and executed
+within the context of the current Perl program. This form is typically
+used to trap exceptions more efficiently than the first (see below), while
+also providing the benefit of checking the code within BLOCK at compile
+time.
+
+The final semicolon, if any, may be omitted from the value of EXPR or within
+the BLOCK.
+
+In both forms, the value returned is the value of the last expression
+evaluated inside the mini-program, or a return statement may be used, just
+as with subroutines. The expression providing the return value is evaluated
+in void, scalar or array context, depending on the context of the eval itself.
+See L</wantarray> for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a 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 $_. The final semicolon, if
-any, may be omitted from the expression.
+string. Beware that using eval() neither silences perl from printing
+warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
+To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
+L</warn> and L<perlvar>.
Note that, because eval() traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket() or symlink())
recompiling each time. The error, if any, is still returned in C<$@>.
Examples:
- # make divide-by-zero non-fatal
+ # make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
# a run-time error
eval '$answer ='; # sets $@
-With an eval(), you should be especially careful to remember what's
+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 lives here" };
+ print $@ if $@; # prints "bar lives here"
+ }
+
+With an eval(), you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
eval "\$$x++" # CASE 5
$$x++; # CASE 6
-Cases 1 and 2 above behave identically: they run the code contained in the
-variable $x. (Although case 2 has misleading double quotes making the
-reader wonder what else might be happening (nothing is).) Cases 3 and 4
-likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
-nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
-is a place where normally you I<WOULD> like to use double quotes, except
-that in that particular situation, you can just use symbolic references
-instead, as in case 6.
+Cases 1 and 2 above behave identically: they run the code contained in
+the variable $x. (Although case 2 has misleading double quotes making
+the reader wonder what else might be happening (nothing is).) Cases 3
+and 4 likewise behave in the same way: they run the code '$x', which
+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 this
+particular situation, you can just use symbolic references instead, as
+in case 6.
=item exec LIST
-The exec() function executes a system command I<AND NEVER RETURNS>,
-unless the command does not exist and is executed directly instead of
-via C</bin/sh -c> (see below). Use system() instead of exec() if you
-want it to return.
+The exec() function executes a system command I<AND NEVER RETURNS> -
+use system() instead of exec() if you want it to return. It fails and
+returns FALSE only if the command does not exist I<and> it is executed
+directly instead of via your system's command shell (see below).
If there is more than one argument in LIST, or if LIST is an array with
more than one value, calls execvp(3) with the arguments in LIST. If
there is only one scalar argument, the argument is checked for shell
-metacharacters. If there are any, the entire argument is passed to
-C</bin/sh -c> for parsing. If there are none, the argument is split
-into words and passed directly to execvp(), which is more efficient.
-Note: exec() and system() do not flush your output buffer, so you may
-need to set C<$|> to avoid lost output. Examples:
+metacharacters, and if there are any, the entire argument is passed to
+the system's command shell for parsing (this is C</bin/sh -c> on Unix
+platforms, but varies on other platforms). If there are no shell
+metacharacters in the argument, it is split into words and passed
+directly to execvp(), which is more efficient. Note: exec() and
+system() do not flush your output buffer, so you may need to set C<$|>
+to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
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';
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
+When the arguments get executed via the system shell, results will
+be subject to its quirks and capabilities. See L<perlop/"`STRING`">
+for details.
+
=item exists EXPR
Returns TRUE if the specified hash key exists in its hash array, even
$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
+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.
+
+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 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
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:
$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) {
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
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 to 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($<))[0] || "Kilroy";
+ $login = getlogin || getpwuid($<) || "Kilroy";
Do not consider getlogin() for authentication: it is not as
secure as getpwuid().
=item glob EXPR
-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.
+=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, 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. If EXPR is omitted, does C<gmtime(time())>.
+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, returns the ctime(3) value:
+
+ $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
+
+Also see the timegm() function provided by the Time::Local module,
+and the strftime(3) function available via the POSIX module.
=item goto LABEL
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).
=item grep EXPR,LIST
+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.
+
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to TRUE. In a scalar
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.
+array. Similarly, grep returns aliases into the original list,
+much like the way that L<Foreach Loops>'s index variable aliases the list
+elements. That is, modifying an element of a list returned by grep
+(for example, in a C<foreach>, C<map> or another C<grep>)
+actually modifies the element in the original list.
+
+See also L</map> for an array composed of the results of the BLOCK or EXPR.
=item hex EXPR
-=item hex
+=item hex
-Interprets EXPR as a hex string and returns the corresponding decimal
-value. (To convert strings that might start with 0 or 0x see
-oct().) If EXPR is omitted, uses $_.
+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 $_.
+
+ 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:
See L<perlfunc/split>.
-=item keys ASSOC_ARRAY
+=item keys HASH
+
+Returns a normal array consisting of all the keys of the named hash. (In
+a scalar context, returns the number of keys.) The keys are returned in
+an apparently random order, but it is the same order as either the
+values() or each() function produces (given that the hash has not been
+modified). As a side effect, it resets HASH's iterator.
-Returns a normal array consisting of all the keys of the named
-associative array. (In a scalar context, returns the number of keys.)
-The keys are returned in an apparently random order, but it is the same
-order as either the values() or each() function produces (given that
-the associative array has not been modified). Here is yet another way
-to print your environment:
+Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
print $key, '=', $ENV{$key}, "\n";
}
-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:
+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)) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue C<keys> allows you to increase the number of hash buckets
-allocated for the given associative array. This can gain you a measure
-of efficiency if you know the hash is going to get big. (This is
-similar to pre-extending an array by assigning a larger number to
-$#array.) If you say
+allocated for the given hash. This can gain you a measure of efficiency if
+you know the hash is going to get big. (This is similar to pre-extending
+an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
=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 $_.
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 parentheses. See L<perlsub/"Temporary Values via
-local()"> for details.
+local()"> for details, including issues with tied arrays and hashes.
But you really probably want to be using my() instead, because local() isn't
what most people think of as "local"). See L<perlsub/"Private Variables
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. If EXPR is omitted, does 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.
-In a scalar context, prints out the ctime(3) value:
+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 F<timelocal.pl> library, and the strftime(3) function available
-via the POSIX module.
+This scalar value is B<not> locale dependent, see L<perllocale>,
+but instead a Perl builtin.
+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
$hash{getkey($_)} = $_;
}
+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. See also L</grep> for an array composed of those items of the
+original list for which the BLOCK or EXPR evaluates to true.
+
=item mkdir FILENAME,MODE
Creates the directory specified by FILENAME, with permissions specified
=item oct EXPR
-=item oct
+=item oct
Interprets EXPR as an octal string and returns the corresponding
-decimal value. (If EXPR happens to start off with 0x, interprets it as
+value. (If EXPR happens to start off with 0x, interprets it as
a hex string instead.) The following will handle decimal, octal, and
hex in the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
-If EXPR is omitted, uses $_.
+If EXPR is omitted, uses $_. This function is commonly used when
+a string such as "644" needs to be converted into a file mode, for
+example. (Although perl will automatically convert strings into
+numbers as needed, this automatic conversion assumes base 10.)
=item open FILEHANDLE,EXPR
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
Plan9 that delimit lines with a single character, and that encode that
character in C as '\n', do not need C<binmode>. The rest need it.
+When opening a file, it's usually a bad idea to continue normal execution
+if the request failed, so C<open> is frequently used in connection with
+C<die>. Even if C<die> won't do what you want (say, in a CGI script,
+where you want to make a nicely formatted error message (but there are
+modules which can help with that problem)) you should always check
+the return value from opening a file. The infrequent exception is when
+working with an unopened filehandle is actually what you want to do.
+
Examples:
$ARTICLE = 100;
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
+ # if the open fails, output is discarded
- open(DBASE, '+<dbase.mine'); # open for update
+ open(DBASE, '+<dbase.mine') # open for update
+ or die "Can't open 'dbase.mine' for update: $!";
- open(ARTICLE, "caesar <$article |"); # decrypt article
+ open(ARTICLE, "caesar <$article |") # decrypt article
+ or die "Can't start caesar: $!";
- open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
+ open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
+ or die "Can't start sort: $!";
# process argument list of files along with any includes
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]'");
See L<perlipc/"Safe Pipe Opens"> for more examples of this.
-Explicitly closing any piped filehandle causes the parent process to
-wait for the child to finish, and returns the status value in C<$?>.
-Note: on any operation which may do a fork, unflushed buffers remain
+NOTE: On any operation which may do a fork, unflushed buffers remain
unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.
+Closing any piped filehandle causes the parent process to wait for the
+child to finish, and returns the status value in C<$?>.
+
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
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 ord EXPR
-=item ord
+=item ord
Returns the numeric ascii value of the first character of EXPR. If
-EXPR is omitted, uses $_.
+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.
padding with nulls or spaces as necessary. (When unpacking, "A" strips
trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
fields pack a string that many bits long. The "h" and "H" fields pack a
-string that many nybbles long. The "P" packs a pointer to a structure of
-the size indicated by the length. Real numbers (floats and doubles) are
+string that many nybbles long. The "p" type packs a pointer to a null-
+terminated string. You are responsible for ensuring the string is not a
+temporary value (which can potentially get deallocated before you get
+around to using the packed result). The "P" packs a pointer to a structure
+of the size indicated by the length. A NULL pointer is created if the
+corresponding value for "p" or "P" is C<undef>.
+Real numbers (floats and doubles) are
in the native machine format only; due to the multiplicity of floating
formats around, and the lack of a standard "network" representation, no
facility for interchange has been made. This means that packed floating
=item pop ARRAY
+=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
-modified to change that offset.
+is in question ($_ is used when the variable is not specified). May be
+modified to change that offset. Such modification will also influence
+the C<\G> zero-width assertion in regular expressions. See L<perlre> and
+L<perlop>.
=item print FILEHANDLE LIST
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale. See L<perllocale>.
+Don't fall into the trap of using a 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, or the name of,
the function whose prototype you want to retrieve.
+If FUNCTION is a string starting with C<CORE::>, the rest is taken as
+a name for Perl builtin. If builtin is not I<overridable> (such as
+C<qw>) or its arguments cannot be expressed by a prototype (such as
+C<system>) - in other words, the builtin does not behave like a Perl
+function - returns C<undef>. Otherwise, the string describing the
+equivalent prototype is returned.
+
=item push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST
=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
-0 and 1. This function produces repeatable sequences unless srand()
-is invoked. See also srand().
+Returns a random fractional number greater than or equal to 0 and less
+than the value of EXPR. (EXPR should be positive.) If EXPR is
+omitted, the value 1 is used. Automatically calls srand() unless
+srand() has already been called. See also srand().
-(Note: if your rand function consistently returns numbers that are too
+(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
-with the wrong number of RANDBITS. As a workaround, you can usually
-multiply EXPR by the correct power of 2 to get the range you want.
-This will make your script unportable, however. It's better to recompile
-if you can.)
+with the wrong number of RANDBITS.)
=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
+=item readline EXPR
+
+Reads from the file handle EXPR. In scalar context, a single line
+is read and returned. In list context, reads until end-of-file is
+reached and returns a list of lines (however you've defined lines
+with $/ or $INPUT_RECORD_SEPARATOR).
+This is the internal function implementing the C<E<lt>EXPRE<gt>>
+operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
+operator is discussed in more detail in L<perlop/"I/O Operators">.
+
=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
error, returns the undefined value and sets C<$!> (errno). If EXPR is
omitted, uses $_.
+=item readpipe EXPR
+
+EXPR is interpolated and then executed as a system command.
+The collected standard output of the command is returned.
+In scalar context, it comes back as a single (potentially
+multi-line) string. In list context, returns a list of lines
+(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
+This is the internal function implementing the C<qx/EXPR/>
+operator, but you can use it directly. The C<qx/EXPR/>
+operator is discussed in more detail in L<perlop/"I/O Operators">.
+
=item recv SOCKET,SCALAR,LEN,FLAGS
Receives a message on a socket. Attempts to receive LENGTH bytes of
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
+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
type of thing the reference is a reference to.
Builtin types include:
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 an associative array.\n";
- }
+ 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 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.)
+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.
+
+(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
In a list context, returns a list value consisting of the elements
-of LIST in the opposite order. In a scalar context, returns a string
-value consisting of the bytes of the first element of LIST in the
-opposite order.
+of LIST in the opposite order. In a scalar context, concatenates the
+elements of LIST, and returns a string value consisting of those bytes,
+but in the opposite order.
- print reverse <>; # line tac
+ print reverse <>; # line tac, last line first
- undef $/;
- print scalar reverse scalar <>; # byte tac
+ undef $/; # for efficiency of <>
+ print scalar reverse <>; # byte tac, last line tsrif
+
+This operator is also handy for inverting a hash, although there are some
+caveats. If a value is duplicated in the original hash, only one of those
+can be represented as a key in the inverted hash. Also, this has to
+unwind one hash and build a whole new one, which may take some time
+on a large hash.
+
+ %by_name = reverse %by_address; # Invert the hash
=item rewinddir DIRHANDLE
=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:
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
($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);
Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down. If there are no elements in the
array, returns the undefined value. If ARRAY is omitted, shifts the
-@ARGV array in the main program, and the @_ array in subroutines.
-(This is determined lexically.) See also unshift(), push(), and pop().
-Shift() and unshift() do the same thing to the left end of an array
-that push() and pop() do to the right end.
+@_ array within the lexical scope of subroutines and formats, and the
+@ARGV array at file scopes or within the lexical scopes established by
+the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
+See also unshift(), push(), and pop(). Shift() and unshift() do the
+same thing to the left end of an array that pop() and push() do to the
+right end.
=item shmctl ID,CMD,ARG
=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::asin()
+function, or use this relation:
+
+ sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
+
=item sleep EXPR
=item sleep
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.
=item sort LIST
-Sorts the LIST and returns the sorted list value. Nonexistent values
-of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
-in standard string comparison order. If SUBNAME is specified, it
-gives the name of a subroutine that returns an integer less than, equal
-to, or greater than 0, depending on how the elements of the array are
-to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
-routines.) SUBNAME may be a scalar variable name, in which case the
-value provides the name of the subroutine to use. In place of a
-SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
-subroutine.
+Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
+is omitted, sorts in standard string comparison order. If SUBNAME is
+specified, it gives the name of a subroutine that returns an integer
+less than, equal to, or greater than 0, depending on how the elements
+of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
+operators are extremely useful in such routines.) SUBNAME may be a
+scalar variable name, in which case the value provides the name of the
+subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
+an anonymous, in-line sort subroutine.
In the interests of efficiency the normal calling code for subroutines is
bypassed, with the following effects: the subroutine may not be a
$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>.
@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;
# sort using explicit subroutine name
sub byage {
- $age{$a} <=> $age{$b}; # presuming integers
+ $age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
- # this sorts the %age associative arrays by value
- # instead of key using an in-line function
+ # this sorts the %age hash by value instead of key
+ # using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
sub backwards { $b cmp $a; }
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]
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace). Anything
matching PATTERN is taken to be a delimiter separating the fields. (Note
-that the delimiter may be longer than one character.) If LIMIT is
-specified and is not negative, splits into no more than that many fields
-(though it may split into fewer). If LIMIT is unspecified, trailing null
-fields are stripped (which potential users of pop() would do well to
-remember). If LIMIT is negative, it is treated as if an arbitrarily large
-LIMIT had been specified.
+that the delimiter may be longer than one character.)
+
+If LIMIT is specified and is not negative, splits into no more than
+that many fields (though it may split into fewer). If LIMIT is
+unspecified, trailing null fields are stripped (which potential users
+of pop() would do well to remember). If LIMIT is negative, it is
+treated as if an arbitrarily large LIMIT had been specified.
A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
(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
- %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
+ %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern C</PATTERN/> may be replaced with an expression to specify
patterns that vary at runtime. (To do runtime compilation only once,
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
-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.) 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.
+Returns a string formatted by the usual printf conventions of the
+C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
+on your system for an explanation of the general principles.
+
+Perl does all of its own sprintf() formatting -- it emulates the C
+function sprintf(), but it doesn't use it (except for floating-point
+numbers, and even then only the standard modifiers are allowed). As a
+result, any non-standard extensions in your local sprintf() are not
+available from Perl.
+
+Perl's sprintf() permits the following universally-known conversions:
+
+ %% a percent sign
+ %c a character with the given number
+ %s a string
+ %d a signed integer, in decimal
+ %u an unsigned integer, in decimal
+ %o an unsigned integer, in octal
+ %x an unsigned integer, in hexadecimal
+ %e a floating-point number, in scientific notation
+ %f a floating-point number, in fixed decimal notation
+ %g a floating-point number, in %e or %f notation
+
+In addition, Perl permits the following widely-supported conversions:
+
+ %X like %x, but using upper-case letters
+ %E like %e, but using an upper-case "E"
+ %G like %g, but with an upper-case "E" (if applicable)
+ %p a pointer (outputs the Perl value's address in hexadecimal)
+ %n special: *stores* the number of characters output so far
+ into the next variable in the parameter list
+
+Finally, for backward (and we do mean "backward") compatibility, Perl
+permits these unnecessary but widely-supported conversions:
+
+ %i a synonym for %d
+ %D a synonym for %ld
+ %U a synonym for %lu
+ %O a synonym for %lo
+ %F a synonym for %f
+
+Perl permits the following universally-known flags between the C<%>
+and the conversion letter:
+
+ space prefix positive number with a space
+ + prefix positive number with a plus sign
+ - left-justify within the field
+ 0 use zeros, not spaces, to right-justify
+ # prefix octal with "0", hex with "0x"
+ number minimum field width
+ .number "precision": digits after decimal point for floating-point,
+ max length for string, minimum length for integer
+ l interpret integer as C type "long" or "unsigned long"
+ h interpret integer as C type "short" or "unsigned short"
+
+There is also one Perl-specific flag:
+
+ V interpret integer as Perl's standard integer type
+
+Where a number would appear in the flags, an asterisk ("*") may be
+used instead, in which case Perl uses the next item in the parameter
+list as the given number (that is, as the field width or precision).
+If a field width obtained through "*" is negative, it has the same
+effect as the '-' flag: left-justification.
+
+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>.
=item sqrt EXPR
-=item sqrt
+=item sqrt
Return the square root of EXPR. If EXPR is omitted, returns square
root of $_.
=item srand EXPR
-Sets the random number seed for the C<rand> operator. If EXPR is omitted,
-uses a semi-random 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, because 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.
+=item srand
+
+Sets the random number seed for the C<rand> operator. If EXPR is
+omitted, 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.
+
+In fact, it's usually not necessary to call srand() at all, because if
+it is not called explicitly, it is called implicitly at the first use of
+the C<rand> operator. However, this was not the case in version of Perl
+before 5.004, so if your script will run under older Perl versions, it
+should call srand().
+
+Note that you need something much more random than the default seed for
+cryptographic purposes. 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
=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
+file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
stats $_. Returns a null list if the stat fails. Typically used as
follows:
$atime,$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 type!) 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,
This is subroutine definition, not a real function I<per se>. With just a
NAME (and possibly prototypes), it's just a forward declaration. Without
a NAME, it's an anonymous function declaration, and does actually return a
-value: the CODE ref of the closure you just created. See L<perlsub> and
+value: the CODE ref of the closure you just created.
See L<perlsub> and
L<perlref> for details.
=item substr EXPR,OFFSET,LEN
=item substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at
-offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
+offset 0, or whatever you've set C<$[> to (but don't do that).
+If OFFSET is negative (or more precisely, less than C<$[>), starts
that far from the end of the string. If LEN is omitted, returns
everything to the end of the string. If LEN is negative, leaves that
many characters off the end of the string.
+If you specify a substring which is partly outside the string, the part
+within the string is returned. If the substring is totally outside
+the string a warning is produced.
+
You can use the substr() function
as an lvalue, in which case EXPR must be an lvalue. If you assign
something shorter than LEN, the string will shrink, and if you assign
symbolic links, produces a fatal error at run time. To check for that,
use eval:
- $symlink_exists = (eval 'symlink("","");', $@ eq '');
+ $symlink_exists = (eval {symlink("","")};, $@ eq '');
=item syscall LIST
Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.
+Syscall returns whatever value returned by the system call it calls.
+If the system call fails, syscall returns -1 and sets C<$!> (errno).
+Note that some system calls can legitimately return -1. The proper
+way to handle such calls is to assign C<$!=0;> before the call and
+check the value of <$!> if syscall returns -1.
+
+There's a problem with C<syscall(&SYS_pipe)>: it returns the file
+number of the read end of the pipe it creates. There is no way
+to retrieve the file number of the other end. You can avoid this
+problem by using C<pipe> instead.
+
=item sysopen FILEHANDLE,FILENAME,MODE
=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
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
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.
+stdio, so mixing this with other kinds of reads, print(), write(),
+seek(), or tell() can cause confusion because stdio usually buffers
+data. 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, as
-described in L<perlop/"`STRING`">.
+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 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: $?"
+
+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 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 "core dump from ";
+ }
+ print "signal $rc\n"
+ }
+ $ok = ($rc != 0);
+
+When the arguments get executed via the system shell, results will
+be subject to its quirks and capabilities. See L<perlop/"`STRING`">
+for details.
=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 because stdio usually
+buffers data. Returns the number of bytes actually written, or undef
+if there was an error. If the LENGTH is greater than the available
+data in the SCALAR after the OFFSET, only as much data as is available
+will be written.
An OFFSET may be specified to write the data from some part of the
string other than the beginning. A negative OFFSET specifies writing
-from that many bytes counting backwards from the end of the string.
+that many bytes counting backwards from the end of the string. In the
+case the SCALAR is empty you can use OFFSET but only zero offset.
=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.
}
untie(%HIST);
-A class implementing an associative array should have the following
-methods:
+A class implementing a hash should have the following methods:
TIEHASH classname, LIST
DESTROY this
TIESCALAR classname, LIST
DESTROY this
- FETCH this,
+ FETCH this,
STORE this, value
Unlike dbmopen(), the tie() function will not use or require a module
for you--you need to do that explicitly yourself. See L<DB_File>
or the F<Config> module for interesting tie() implementations.
+For further details see L<perltie>, L<tied VARIABLE>.
+
=item tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value
=item tr///
-The translation operator. See L<perlop>.
+The translation operator. S
ame 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.
=item umask
-Sets the umask for the process and returns the old one. If EXPR is
-omitted, returns merely the current umask.
+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.
=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 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 nothing is undefined, but you still get an
-undefined value that you could, for instance, return from a
-subroutine. Examples:
+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
+nothing is undefined, but you still get an undefined value that you
+could, for instance, return from a subroutine, assign to a variable or
+pass as a parameter. Examples:
undef $foo;
- undef $bar{'blurfl'};
+ undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
- undef %assoc;
+ 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
features back into the current package. The module can implement its
import method any way it likes, though most modules just choose to
derive their import method via inheritance from the Exporter class that
-is defined in the Exporter module. See L<Exporter>. If no import
-method can be found then the error is currently silently ignored. This
+is defined in the Exporter module. See L<Exporter>. If no import
+method can be found then the error is currently silently ignored. This
may change to a fatal error in a future version.
If you don't want your namespace altered, explicitly supply an empty list:
$now = time;
utime $now, $now, @ARGV;
-=item values ASSOC_ARRAY
+=item values HASH
-Returns a normal array consisting of all the values of the named
-associative array. (In a scalar context, returns the number of
-values.) The values are returned in an apparently random order, but it
-is the same order as either the keys() or each() function would produce
-on the same array. See also keys(), each(), and sort().
+Returns a normal array consisting of all the values of the named hash.
+(In a scalar context, returns the number of values.) The values are
+returned in an apparently random order, but it is the same order as either
+the keys() or each() function would produce on the same hash. As a side
+effect, it resets HASH's iterator. See also keys(), each(), and sort().
=item vec EXPR,OFFSET,BITS
Treats the string in EXPR as a vector of unsigned integers, and
returns the value of the bit field specified by OFFSET. BITS specifies
the number of bits that are reserved for each entry in the bit
-vector. This must be a power of two from 1 to 32. vec() may also be
+vector. This must be a power of two from 1 to 32. vec() may also be
assigned to, in which case parentheses are needed to give the expression
the correct precedence as in
Returns TRUE if the context of the currently executing subroutine is
looking for a list value. Returns FALSE if the context is looking
-for a scalar.
+for a scalar. Returns the undefined value if the context is looking
+for no value (void context).
- return wantarray ? () : undef;
+ return unless defined wantarray; # don't bother doing more
+ my @a = complex_calculation();
+ return wantarray ? @a : "@a";
=item warn LIST
-Produces a message on STDERR just like die(), but doesn't exit or
-on an exception.
+Produces a message on STDERR just like die(), but doesn't exit or throw
+an exception.
+
+No message is printed if there is a C<$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 $foo!"; # does show up
+
+See L<perlvar> for details on setting C<%SIG> entries, and for more
+examples.
=item write FILEHANDLE
Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file. By default the format for
-a file is the one having the same name is the filehandle, but the
+a file is the one having the same name as the filehandle, but the
format for the current output channel (see the select() function) may be set
explicitly by assigning the name of the format to the C<$~> variable.
=item y///
-The translation operator. See L<perlop>.
+The translation operator. S
ame as tr///. See L<perlop>.
=back
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