PERLFUNC(1) PERLFUNC(1)
NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an expression. They
fall into two major categories: list operators and named unary operators.
These differ in their precedence relationship with a following comma.
(See the precedence table in the perlop manpage.) List operators take
more than one argument, while unary operators can never take more than
one argument. Thus, a comma terminates the argument of a unary operator,
but merely separates the arguments of a list operator. A unary operator
generally provides a scalar context to its argument, while a list
operator may provide either scalar and list contexts for its arguments.
If it does both, the scalar arguments will be first, and the list
argument will follow. (Note that there can ever be only one list
argument.) For instance, splice() has three scalar arguments followed by
a list.
In the syntax descriptions that follow, list operators that expect a list
(and provide list context for the elements of the list) are shown with
LIST as an argument. Such a list may consist of any combination of
scalar arguments or list values; the list values will be included in the
list as if each individual element were interpolated at that point in the
list, forming a longer single-dimensional list value. Elements of the
LIST should be separated by commas.
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use the parentheses, the simple (but occasionally
surprising) rule is this: It LOOKS like a function, therefore it IS a
function, and precedence doesn't matter. Otherwise it's a list operator
or unary operator, and precedence does matter. And whitespace between
the function and left parenthesis doesn't count--so you need to be
careful sometimes:
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 -w switch it can warn you about this. For
example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
For functions that can be used in either a scalar or list context,
nonabortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.
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Remember the following rule:
THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!
Each operator and function decides which sort of value it would be most
appropriate to return in a scalar context. Some operators return the
length of the list that would have been returned in a list context. Some
operators return the first value in the list. Some operators return the
last value in the list. Some operators return a count of successful
operations. In general, they do what you want, unless you want
consistency.
Perl Functions by Category
Here are Perl's functions (including things that look like functions,
like some of the keywords and named operators) arranged by category.
Some functions appear in more than one place.
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///
Regular expressions and pattern matching
m//, pos, quotemeta, s///, split, study
Numeric functions
abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand
Functions for real @ARRAYs
pop, push, shift, splice, unshift
Functions for list data
grep, join, map, qw/STRING/, reverse, sort, unpack
Functions for real %HASHes
delete, each, exists, keys, values
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, sysseek, syswrite, tell, telldir,
truncate, warn, write
Functions for fixed length data or records
pack, read, syscall, sysread, syswrite, unpack, vec
Functions for filehandles, files, or directories
-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, lstat,
mkdir, open, opendir, readlink, rename, rmdir, stat, symlink, umask,
unlink, utime
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Keywords related to the control flow of your perl program
caller, continue, die, do, dump, eval, exit, goto, last, next, redo,
return, sub, wantarray
Keywords related to scoping
caller, import, local, my, package, use
Miscellaneous functions
defined, dump, eval, formline, local, my, reset, scalar, undef,
wantarray
Functions for processes and process groups
alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe,
qx/STRING/, setpgrp, setpriority, sleep, system, times, wait,
waitpid
Keywords related to perl modules
do, import, no, package, require, use
Keywords related to classes and object-orientedness
bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
Low-level socket functions
accept, bind, connect, getpeername, getsockname, getsockopt, listen,
recv, send, setsockopt, shutdown, socket, socketpair
System V interprocess communication functions
msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl,
shmget, shmread, shmwrite
Fetching user and group info
endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid,
getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent
Fetching network info
endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent,
getnetbyaddr, getnetbyname, getnetent, getprotobyname,
getprotobynumber, getprotoent, getservbyname, getservbyport,
getservent, sethostent, setnetent, setprotoent, setservent
Time-related functions
gmtime, localtime, time, times
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
* - sub was a keyword in perl4, but in perl5 it is an operator which
can be used in expressions.
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Functions obsoleted in perl5
dbmclose, dbmopen
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-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 tests the associated file to see if something is
true about it. If the argument is omitted, tests $_, except for
-t, which tests STDIN. Unless otherwise documented, it returns 1
for TRUE and '' for FALSE, or the undefined value if the file
doesn't exist. Despite the funny names, precedence is the same
as any other named unary operator, and the argument may be
parenthesized like any other unary operator. The operator may be
any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has nonzero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO).
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is a text file.
-B File is a binary file (opposite of -T).
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-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
The interpretation of the file permission operators -r, -R, -w,
-W, -x, and -X is based solely on the mode of the file and the
uids and gids of the user. There may be other reasons you can't
actually read, write or execute the file. Also note that, for
the superuser, -r, -R, -w, and -W always return 1, and -x and -X
return 1 if any execute bit is set in the mode. Scripts run by
the superuser may thus need to do a stat() to determine the
actual mode of the file, or temporarily set the uid to something
else.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
...
}
Note that -s/a/b/ does not do a negated substitution. Saying
-exp($foo) still works as expected, however--only single letters
following a minus are interpreted as file tests.
The -T and -B switches work as follows. The first block or so of
the file is examined for odd characters such as strange control
codes or characters with the high bit set. If too many odd
characters (>30%) are found, it's a -B file, otherwise it's a -T
file. Also, any file containing null in the first block is
considered a binary file. If -T or -B is used on a filehandle,
the current stdio buffer is examined rather than the first block.
Both -T and -B return TRUE on a null file, or a file at EOF when
testing a filehandle. Because you have to read a file to do the
-T test, on most occasions you want to use a -f against the file
first, as in 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 structure of the previous file
test (or stat operator) is used, saving a system call. (This
doesn't work with -t, and you need to remember that lstat() and
-l will leave values in the stat structure for the symbolic link,
not the real file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
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stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted,
uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system
call does. Returns the packed address if it succeeded, FALSE
otherwise. See example in the section on Sockets: Client/Server
Communication in the perlipc manpage.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed. If SECONDS is not
specified, the value stored in $_ is used. (On some machines,
unfortunately, the elapsed time may be up to one second less than
you specified because of how seconds are counted.) Only one
timer may be counting at once. Each call disables the previous
timer, and an argument of 0 may be supplied to cancel the
previous timer without starting a new one. The returned value is
the amount of time remaining 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 the select() entry elsewhere in this
document . 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 fail with $! set to EINTR because Perl sets up
signal handlers to restart system calls on some systems. Using
eval/die always works.
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eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
die if $@ && $@ ne "alarm\n"; # propagate errors
if ($@) {
# timed out
}
else {
# didn't
}
atan2 Y,X
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]) }
bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call
does. Returns TRUE if it succeeded, FALSE otherwise. NAME
should be a packed address of the appropriate type for the
socket. See the examples in the section on Sockets:
Client/Server Communication in the perlipc manpage.
binmode FILEHANDLE
Arranges for the file to be read or written in "binary" mode in
operating systems that distinguish between binary and text files.
Files that are not in binary mode have CR LF sequences translated
to LF on input and LF translated to CR LF on output. Binmode has
no effect under Unix; in MS-DOS and similarly archaic systems, it
may be imperative--otherwise your MS-DOS-damaged C library may
mangle your file. The key distinction between systems that need
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 binmode.
The rest need it. If FILEHANDLE is an expression, the value is
taken as the name of the filehandle.
bless REF,CLASSNAME
bless REF
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
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thing in a constructor. Always use the two-argument version if
the function doing the blessing might be inherited by a derived
class. See the perlobj manpage for more about the blessing (and
blessings) of objects.
caller EXPR
caller Returns the context of the current subroutine call. In a scalar
context, 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, $line) = caller;
With EXPR, it returns some extra information that the debugger
uses to print a stack trace. The value of EXPR indicates how
many call frames to go back before the current one.
($package, $filename, $line, $subroutine,
$hasargs, $wantarray, $evaltext, $is_require) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine
call, but an eval. In such a case additional elements $evaltext
and $is_require are set: $is_require is true if the frame is
created by a require or use statement, $evaltext contains the
text of the eval EXPR statement. In particular, for a eval BLOCK
statement, $filename is "(eval)", but $evaltext is undefined.
(Note also that each use statement creates a require frame inside
an eval EXPR) frame.
Furthermore, when called from within the DB package, caller
returns more detailed information: it sets the list variable
@DB::args to be the arguments with which the subroutine was
invoked.
chdir EXPR
Changes the working directory to EXPR, if possible. If EXPR is
omitted, changes to home directory. Returns TRUE upon success,
FALSE otherwise. See example under die().
chmod LIST
Changes the permissions of a list of files. The first element of
the list must be the numerical mode, which should probably be an
octal number, and which definitely should not a string of octal
digits: 0644 is okay, '0644' is not. Returns the number of
files successfully changed. See also the oct entry elsewhere in
this document if all you have is a string.
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$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
chomp VARIABLE
chomp LIST
chomp This is a slightly safer version of the chop entry elsewhere in
this document . It removes any line ending that corresponds to
the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in
the English module). It returns the total number of characters
removed from all its arguments. It's often used to remove the
newline from the end of an input record when you're worried that
the final record may be missing its newline. When in paragraph
mode ($/ = ""), it removes all trailing newlines from the string.
If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
...
}
You can actually chomp anything that's an lvalue, including an
assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total
number of characters removed is returned.
chop VARIABLE
chop LIST
chop Chops off the last character of a string and returns the
character chopped. It's used primarily to remove the newline
from the end of an input record, but is much more efficient than
s/\n// because it neither scans nor copies the string. If
VARIABLE is omitted, chops $_. Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
...
}
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You can actually chop anything that's an lvalue, including an
assignment:
chop($cwd = `pwd`);
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only the value of
the last chop is returned.
Note that chop returns the last character. To return all but the
last character, use substr($string, 0, -1).
chown LIST
Changes the owner (and group) of a list of files. The first two
elements of the list must be the NUMERICAL uid and gid, in that
order. Returns the number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd
file:
print "User: ";
chop($user = <STDIN>);
print "Files: "
chop($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = <${pattern}>; # expand filenames
chown $uid, $gid, @ary;
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.
chr NUMBER
chr Returns the character represented by that NUMBER in the character
set. For example, chr(65) is "A" in ASCII. For the reverse, use
the ord entry elsewhere in this document .
If NUMBER is omitted, uses $_.
chroot FILENAME
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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,
which is unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omitted, does chroot
to $_.
close FILEHANDLE
Closes the file or pipe associated with the file handle,
returning TRUE only if stdio successfully flushes buffers and
closes the system file descriptor.
You don't have to close FILEHANDLE if you are immediately going
to do another open() on it, because open() will close it for you.
(See open().) However, an explicit close on an input file resets
the line counter ($.), while the implicit close done by open()
does not.
If the file handle came from a piped open 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 $?. Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
... # print stuff to output
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.
closedir DIRHANDLE
Closes a directory opened by opendir().
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect
system call does. Returns TRUE if it succeeded, FALSE otherwise.
NAME should be a packed address of the appropriate type for the
socket. See the examples in the section on Sockets:
Client/Server Communication in the perlipc manpage.
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continue BLOCK
Actually a flow control statement rather than a function. If
there is a continue BLOCK attached to a BLOCK (typically in a
while or foreach), it is always executed just before the
conditional is about to be evaluated again, just like the third
part of a for loop in C. Thus it can be used to increment a loop
variable, even when the loop has been continued via the next
statement (which is similar to the C continue statement).
cos EXPR
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] ) }
crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) function in the C
library (assuming that you actually have a version there that has
not been extirpated as a potential munition). This can prove
useful for checking 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:
$pwd = (getpwuid($<))[1];
$salt = substr($pwd, 0, 2);
system "stty -echo";
print "Password: ";
chop($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $salt) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it
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is unwise.
dbmclose HASH
[This function has been superseded by the untie() function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MODE
[This function has been superseded by the tie() function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB
file to a hash. HASH is the name of the hash. (Unlike normal
open, the first argument is NOT a filehandle, even though it
looks like one). DBNAME is the name of the database (without the
.dir or .pag extension if any). If the database does not exist,
it is created with protection specified by MODE (as modified by
the umask()). If your system supports only the older DBM
functions, you may perform only one dbmopen() in your program.
In older versions of Perl, if your system had neither DBM nor
ndbm, calling dbmopen() produced a fatal error; it now falls back
to sdbm(3).
If you don't have write access to the DBM file, you can only read
hash variables, not set them. If you want to test whether you
can write, either use file tests or try setting a dummy hash
entry inside an eval(), which will trap the error.
Note that functions such as keys() and values() may return huge
array values when used on large DBM files. You may prefer to use
the each() function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also the AnyDBM_File manpage for a more general description
of the pros and cons of the various dbm approaches, as well as
the DB_File manpage for a particularly rich implementation.
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other
than the undefined value undef. If EXPR is not present, $_ will
be checked.
Many operations return undef to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions. This function allows you to distinguish undef from
other values. (A simple Boolean test will not distinguish among
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undef, zero, the empty string, and "0", which are all equally
false.) Note that since undef is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition: pop()
returns undef when its argument is an empty array, or when the
element to return happens to be undef.
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 element, it tells you whether the value is
defined, not whether the key exists in the hash. Use the exists
entry elsewhere in this document for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
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/;
the pattern match succeeds, and $1 is defined, despite the fact
that it 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.
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
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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 the undef, exists, and ref entries elsewhere in this
document .
delete EXPR
Deletes the specified key(s) and their associated values from a
hash. For each key, returns the deleted value associated with
that key, or the undefined value if there was no such key.
Deleting from $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 a hash:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
And so does this:
delete @HASH{keys %HASH}
(But both of these are slower than the undef() command.) Note
that the EXPR can be arbitrarily complicated as long as the final
operation is a hash element lookup or hash slice:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
die LIST
Outside of an eval(), prints the value of LIST to STDERR and
exits with the current value of $! (errno). If $! is 0, exits
with the value of ($? >> 8) (backtick `command` status). If ($?
>> 8) is 0, exits with 255. Inside an eval(), the error message
is stuffed into $@, 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"
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 is supplied. Hint: sometimes appending ",
Page 15
PERLFUNC(1) PERLFUNC(1)
stopped" to your message will cause it to make better sense when
the string "at foo line 123" is appended. Suppose you are
running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
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 $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 the
section on $SIG{expr} in the perlvar manpage for details on
setting %SIG entries, and the section on eval BLOCK for some
examples.
Note that the $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 the section on $^S in the
perlvar manpage).
do BLOCK
Not really a function. Returns the value of the last command in
the sequence of commands indicated by BLOCK. When modified by a
loop modifier, executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers test the
conditional first.)
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See the perlsub manpage.
Page 16
PERLFUNC(1) PERLFUNC(1)
do EXPR Uses the value of EXPR as a filename and executes the contents of
the file as a Perl script. Its primary use is to include
subroutines from a Perl subroutine library.
do 'stat.pl';
is just like
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 -I
libraries if the file isn't in the current directory (see also
the @INC array in the section on Predefined Names in the perlvar
manpage). It's the same, however, in that it does 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
use() and require() operators, which also do error checking and
raise an exception if there's a problem.
dump LABEL
This causes an immediate core dump. Primarily this is so that
you can use the undump program to turn your core dump into an
executable binary after having initialized all your variables at
the beginning of the program. When the new binary is executed it
will begin by executing a goto LABEL (with all the restrictions
that goto suffers). Think of it as a goto with an intervening
core dump and reincarnation. If LABEL is omitted, restarts the
program from the top. WARNING: any files opened at the time of
the dump will NOT be open any more when the program is
reincarnated, with possible resulting confusion on the part of
Perl. See also -u option in the perlrun manpage.
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun' => 1,
'Mon' => 2,
'Tue' => 3,
'Wed' => 4,
'Thu' => 5,
'Fri' => 6,
'Sat' => 7,
);
dump QUICKSTART if $ARGV[0] eq '-d';
Page 17
PERLFUNC(1) PERLFUNC(1)
QUICKSTART:
Getopt('f');
each HASH
When called in a list context, returns a 2-element array
consisting of the key and value for the next element of a hash,
so that you can iterate over it. When called in a scalar
context, returns the key for only the next element in the hash.
(Note: Keys may be "0" or "", which are logically false; you may
wish to avoid constructs like while ($k = each %foo) {} for this
reason.)
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 undef is
returned in a 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 keys HASH or 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";
}
See also keys() and values().
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file,
or if FILEHANDLE is not open. FILEHANDLE may be an expression
whose value gives the real filehandle name. (Note that this
function actually reads a character and then ungetc()s it, so it
is not very useful in an interactive context.) Do not read from
a terminal file (or call eof(FILEHANDLE) on it) after end-of-file
is reached. Filetypes such as terminals may lose the end-of-file
condition if you do.
An 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., eof() is
reasonable to use inside a while (<>) loop to detect the end of
only the last file. Use eof(ARGV) or eof without the parentheses
to test EACH file in a while (<>) loop. Examples:
Page 18
PERLFUNC(1) PERLFUNC(1)
# reset line numbering on each input file
while (<>) {
print "$.\t$_";
close(ARGV) if (eof); # Not eof().
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
close(ARGV); # close or break; is needed if we
# are reading from the terminal
}
print;
}
Practical hint: you almost never need to use eof in Perl, because
the input operators return undef when they run out of data.
eval EXPR
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 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.
If there is a syntax error or runtime error, or a die() statement
is executed, an undefined value is returned by eval(), and $@ is
set to the error message. If there was no error, $@ is
guaranteed to be a null string. If EXPR is omitted, evaluates
$_. The final semicolon, if any, may be omitted from the
expression. Beware that using eval() neither silences perl from
printing warnings to STDERR, nor does it stuff the text of
warning messages into $@. To do either of those, you have to use
the $SIG{__WARN__} facility. See warn() and the perlvar manpage.
Note that, because eval() traps otherwise-fatal errors, it is
useful for determining whether a particular feature (such as
socket() or symlink()) is implemented. It is also Perl's
exception trapping mechanism, where the die operator is used to
raise exceptions.
If the code to be executed doesn't vary, you may use the eval-
BLOCK form to trap run-time errors without incurring the penalty
of recompiling each time. The error, if any, is still returned
in $@. Examples:
Page 19
PERLFUNC(1) PERLFUNC(1)
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
When using the eval{} form as an exception trap in libraries, you
may wish not to trigger any __DIE__ hooks that user code may have
installed. You can use the 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 __DIE__ hooks can call
die() again, which has the effect of changing their error
messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo foofs here" };
print $@ if $@; # prints "bar barfs here"
}
With an eval(), you should be especially careful to remember
what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$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 '$x', which does nothing but return
the value of $x. (Case 4 is preferred for purely visual reasons,
but it also has the advantage of compiling at compile-time
instead of at run-time.) Case 5 is a place where normally you
WOULD like to use double quotes, except that in this particular
Page 20
PERLFUNC(1) PERLFUNC(1)
situation, you can just use symbolic references instead, as in
case 6.
exec LIST
The exec() function executes a system command 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 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, and if there are
any, the entire argument is passed to the system's command shell
for parsing (this is /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 $| to avoid lost
output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want
to lie 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 multivalued list, even if
there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
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 the section
on `STRING` in the perlop manpage for details.
exists EXPR
Returns TRUE if the specified hash key exists in its hash array,
even if the corresponding value is undefined.
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can be TRUE only if it's defined, and defined if
Page 21
PERLFUNC(1) PERLFUNC(1)
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the
final operation is a hash key lookup:
if (exists $ref->[$x][$y]{$key}) { ... }
exit EXPR
Evaluates EXPR and exits immediately with that value. (Actually,
it calls any defined END routines first, but the END routines may
not abort the exit. Likewise any object destructors that need to
be called are called before exit.) Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
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().
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR. If
EXPR is omitted, gives exp($_).
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct function definitions. Argument
processing and value return works just like ioctl() below. Note
that fcntl() will produce a fatal error if used on a machine that
doesn't implement fcntl(2). For example:
use Fcntl;
fcntl($filehandle, F_GETLK, $packed_return_buffer);
fileno FILEHANDLE
Returns the file descriptor for a filehandle. This is useful for
constructing bitmaps for select(). If FILEHANDLE is an
expression, the value is taken as the name of the filehandle.
Page 22
PERLFUNC(1) PERLFUNC(1)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns
TRUE for success, FALSE on failure. Produces a fatal error if
used on a 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 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 are the semantics that lockf(3) implements. Most
(all?) systems implement lockf(3) in terms of fcntl(2) locking,
though, so the differing semantics shouldn't bite too many
people.
Note also that some versions of flock() cannot lock things over
the network; you would need to use the more system-specific
fcntl() for that. If you like you can force Perl to ignore your
system's flock(2) function, and so provide its own fcntl(2)-based
emulation, by passing the switch -Ud_flock to the Configure
program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
Page 23
PERLFUNC(1) PERLFUNC(1)
lock();
print MBOX $msg,"\n\n";
unlock();
See also the DB_File manpage for other flock() examples.
fork Does a fork(2) system call. Returns the child pid to the parent
process and 0 to the child process, or undef if the fork is
unsuccessful. Note: unflushed buffers remain unflushed in both
processes, which means you may need to set $| ($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 fork()
returns omitted);
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "no exec";
# ... or ...
## (some_perl_code_here)
exit 0;
}
exit 0;
}
waitpid($pid,0);
See also the perlipc manpage 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.
format Declare a picture format with use by the write() function. For
example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
Page 24
PERLFUNC(1) PERLFUNC(1)
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See the perlform manpage for many details and examples.
formline PICTURE,LIST
This is an internal function used by formats, though you may call
it too. It formats (see the perlform manpage) a list of values
according to the contents of PICTURE, placing the output into the
format output accumulator, $^A (or $ACCUMULATOR in English).
Eventually, when a write() is done, the contents of $^A are
written to some filehandle, but you could also read $^A yourself
and then set $^A back to "". Note that a format typically does
one formline() per line of form, but the formline() function
itself doesn't care how many newlines are embedded in the
PICTURE. This means that the ~ and ~~ tokens will treat the
entire PICTURE as a single line. You may therefore need to use
multiple formlines to implement a single record format, just like
the format compiler.
Be careful if you put double quotes around the picture, because
an "@" character may be taken to mean the beginning of an array
name. formline() always returns TRUE. See the perlform manpage
for other examples.
getc FILEHANDLE
getc Returns the next character from the input file attached to
FILEHANDLE, or a null string at end of file. If FILEHANDLE is
omitted, reads from STDIN. This is not particularly efficient.
It cannot be used to get unbuffered single-characters, however.
For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Page 25
PERLFUNC(1) PERLFUNC(1)
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 Term::ReadKey
module from your nearest CPAN site; details on CPAN can be found
on the CPAN entry in the perlmod manpage.
getlogin
Returns the current login from /etc/utmp, if any. If null, use
getpwuid().
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider getlogin() for authentication: it is not as
secure as getpwuid().
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET
connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID. Use a
PID of 0 to get the current process group for the current
process. Will raise an exception if used on a machine that
doesn't implement getpgrp(2). If PID is omitted, returns process
group of current process. Note that the POSIX version of
getpgrp() does not accept a PID argument, so only PID==0 is truly
portable.
getppid Returns the process id of the parent process.
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a
user. (See the getpriority(2) manpage.) Will raise a fatal
exception if used on a machine that doesn't implement
getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
Page 26
PERLFUNC(1) PERLFUNC(1)
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
Page 27
PERLFUNC(1) PERLFUNC(1)
endprotoent
endservent
These routines perform the same functions as their counterparts
in the system library. Within a list context, the return values
from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless the function
was a lookup by name, in which case you get the other thing,
whatever it is. (If the entry doesn't exist you get the
undefined value.) For example:
$uid = getpwnam
$name = getpwuid
$name = getpwent
$gid = getgrnam
$name = getgrgid
$name = getgrent
etc.
The $members value returned by getgr*() is a space separated list
of the login names of the members of the group.
For the gethost*() functions, if the h_errno variable is
supported in C, it will be returned to you via $? if the function
call fails. The @addrs value returned by a successful call is a
list of the raw addresses returned by the corresponding system
library call. In the Internet domain, each address is four bytes
long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET
connection.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
Page 28
PERLFUNC(1) PERLFUNC(1)
getsockopt SOCKET,LEVEL,OPTNAME
Returns the socket option requested, or undefined if there is an
error.
glob EXPR
glob Returns the value of EXPR with filename expansions such as a
shell would do. This is the internal function implementing the
<*.c> operator, but you can use it directly. If EXPR is omitted,
$_ is used. The <*.c> operator is discussed in more detail in
the section on I/O Operators in the perlop manpage.
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. 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 with sunday as day 0. Also, $year is
the number of years since 1900, not simply the last two digits of
the year.
If EXPR is omitted, does 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.
goto LABEL
goto EXPR
goto &NAME
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, 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).
Page 29
PERLFUNC(1) PERLFUNC(1)
The goto-EXPR form expects a label name, whose scope will be
resolved dynamically. This allows for computed gotos per
FORTRAN, but isn't necessarily recommended if you're optimizing
for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The goto-&NAME form is highly magical, and substitutes a call to
the named subroutine for the currently running subroutine. This
is used by AUTOLOAD subroutines that wish to load another
subroutine and then pretend that the other subroutine had been
called in the first place (except that any modifications to @_ in
the current subroutine are propagated to the other subroutine.)
After the goto, not even caller() will be able to tell that this
routine was called first.
grep BLOCK LIST
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 context, returns the number of times the expression was
TRUE.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
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. Similarly, grep returns aliases into the
original list, much like the way that the section on Foreach
Loops's index variable aliases the list elements. That is,
modifying an element of a list returned by grep (for example, in
a foreach, map or another grep) actually modifies the element in
the original list.
See also the map entry elsewhere in this document for an array
composed of the results of the BLOCK or EXPR.
hex EXPR
Page 30
PERLFUNC(1) PERLFUNC(1)
hex Interprets EXPR as a hex string and returns the corresponding
value. (To convert strings that might start with either 0 or 0x
see the oct entry elsewhere in this document .) If EXPR is
omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
import 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 the use() entry
elsewhere in this document the perlmod manpage, and the Exporter
manpage.
index STR,SUBSTR,POSITION
index STR,SUBSTR
Returns the position of the first occurrence of SUBSTR in STR at
or after POSITION. If POSITION is omitted, starts searching from
the beginning of the string. The return value is based at 0 (or
whatever you've set the $[ variable to--but don't do that). If
the substring is not found, returns one less than the base,
ordinarily -1.
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses
$_.
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably have to say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
first to get the correct function definitions. If ioctl.ph
doesn't exist or doesn't have the correct definitions you'll have
to roll your own, based on your C header files such as
<sys/ioctl.h>. (There is a Perl script called h2ph that comes
with the Perl kit which 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
passed rather than a pointer to the string value. To guarantee
this to be TRUE, add a 0 to the scalar before using it.) The
pack() and unpack() functions are useful for manipulating the
values of structures used by ioctl(). The following example sets
the erase character to DEL.
Page 31
PERLFUNC(1) PERLFUNC(1)
require 'ioctl.ph';
$getp = &TIOCGETP;
die "NO TIOCGETP" if $@ || !$getp;
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$getp,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,&TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns TRUE on success and FALSE on failure, yet you
can still easily determine the actual value returned by the
operating system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
join EXPR,LIST
Joins the separate strings of LIST into a single string with
fields separated by the value of EXPR, and returns the string.
Example:
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See the split entry in the perlfunc manpage.
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.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
Page 32
PERLFUNC(1) PERLFUNC(1)
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
To sort an array by value, you'll need to use a 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 keys allows you to increase the number of hash
buckets allocated for the given hash. This can gain you a
measure of efficiency if you know the hash is going to get big.
(This is similar to pre-extending an array by assigning a larger
number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it.
These buckets will be retained even if you do %hash = (), use
undef %hash if you want to free the storage while %hash is still
in scope. You can't shrink the number of buckets allocated for
the hash using keys in this way (but you needn't worry about
doing this by accident, as trying has no effect).
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 processes
successfully signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
Unlike in the shell, in Perl if the SIGNAL is negative, it kills
process groups instead of processes. (On System V, a negative
PROCESS number will also kill process groups, but that's not
portable.) That means you usually want to use positive not
negative signals. You may also use a signal name in quotes. See
the section on Signals in the perlipc manpage for details.
last LABEL
last The last command is like the break statement in C (as used in
loops); it immediately exits the loop in question. If the LABEL
is omitted, the command refers to the innermost enclosing loop.
The continue block, if any, is not executed:
Page 33
PERLFUNC(1) PERLFUNC(1)
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
lc EXPR
lc Returns an lowercased version of EXPR. This is the internal
function implementing the \L escape in double-quoted strings.
Respects current LC_CTYPE locale if use locale in force. See the
perllocale manpage.
If EXPR is omitted, uses $_.
lcfirst EXPR
lcfirst Returns the value of EXPR with the first character lowercased.
This is the internal function implementing the \l escape in
double-quoted strings. Respects current LC_CTYPE locale if use
locale in force. See the perllocale manpage.
If EXPR is omitted, uses $_.
length EXPR
length Returns the length in characters of the value of EXPR. If EXPR
is omitted, returns length of $_.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns 1 for
success, 0 otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call does. Returns
TRUE if it succeeded, FALSE otherwise. See example in the
section on Sockets: Client/Server Communication in the perlipc
manpage.
local EXPR
A local modifies the listed variables to be local to the
enclosing block, subroutine, eval{}, or do. If more than one
value is listed, the list must be placed in parentheses. See the
section on Temporary Values via local() in the perlsub manpage
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 the
section on Private Variables via my() in the perlsub manpage for
details.
Page 34
PERLFUNC(1) PERLFUNC(1)
localtime EXPR
Converts a time as returned by the time function to a 9-element
array 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 with sunday as day 0. Also, $year is
the number of years since 1900, that is, $year is 123 in year
2023.
If EXPR is omitted, uses the current time (localtime(time)).
In a scalar context, returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent, see the perllocale
manpage, but instead a Perl builtin. Also see the Time::Local
module, and the strftime(3) and mktime(3) function available via
the POSIX module.
log EXPR
log Returns logarithm (base e) of EXPR. If EXPR is omitted, returns
log of $_.
lstat FILEHANDLE
lstat EXPR
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 unimplemented on your system, a normal stat()
is done.
If EXPR is omitted, stats $_.
m// The match operator. See the perlop manpage.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally
setting $_ to each element) and returns the list value composed
of the results of each such evaluation. Evaluates BLOCK or EXPR
in a list context, so each element of LIST may produce zero, one,
or more elements in the returned value.
Page 35
PERLFUNC(1) PERLFUNC(1)
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters.
And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$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 the grep entry elsewhere in this
document for an array composed of those items of the original
list for which the BLOCK or EXPR evaluates to true.
mkdir FILENAME,MODE
Creates the directory specified by FILENAME, with permissions
specified by MODE (as modified by umask). If it succeeds it
returns 1, otherwise it returns 0 and sets $! (errno).
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT,
then ARG must be a variable which will hold the returned msqid_ds
structure. Returns like ioctl: the undefined value for error, "0
but true" for zero, or the actual return value otherwise.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message
queue id, or the undefined value if there is an error.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to
the message queue ID. MSG must begin with the long integer
message type, which may be created with pack("l", $type).
Returns TRUE if successful, or FALSE if there is an error.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE. Note that if a message is received, the message type will
be the first thing in VAR, and the maximum length of VAR is SIZE
plus the size of the message type. Returns TRUE if successful,
or FALSE if there is an error.
Page 36
PERLFUNC(1) PERLFUNC(1)
my EXPR A "my" declares the listed variables to be local (lexically) to
the enclosing block, subroutine, eval, or do/require/use'd file.
If more than one value is listed, the list must be placed in
parentheses. See the section on Private Variables via my() in
the perlsub manpage for details.
next LABEL
next The next command is like the continue statement in C; it starts
the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
Note that if there were a continue block on the above, it would
get executed even on discarded lines. If the LABEL is omitted,
the command refers to the innermost enclosing loop.
no Module LIST
See the "use" function, which "no" is the opposite of.
oct EXPR
oct Interprets EXPR as an octal string and returns the corresponding
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 $_. 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.)
open FILEHANDLE,EXPR
open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates it
with FILEHANDLE. If FILEHANDLE is an expression, its value is
used as the name of the real filehandle wanted. If EXPR is
omitted, the scalar variable of the same name as the FILEHANDLE
contains the filename. (Note that lexical variables--those
declared with my--will not work for this purpose; so if you're
using my, specify EXPR in your call to open.)
If the filename begins with '<' or nothing, the file is opened
for input. If the filename begins with '>', the file is
truncated and opened for output. If the filename begins with
Page 37
PERLFUNC(1) PERLFUNC(1)
'>>', the file is opened for appending. You can put a '+' in
front of the '>' or '<' to indicate that you want both read and
write access to the file; thus '+<' is almost always preferred
for read/write updates--the '+>' mode would clobber the file
first. The prefix and the filename may be separated with spaces.
These various prefixes correspond to the fopen(3) modes of 'r',
'r+', 'w', 'w+', 'a', and 'a+'.
If the filename begins with "|", the filename is interpreted as a
command to which output is to be piped, and if the filename ends
with a "|", the filename is interpreted See the section on Using
open() for IPC in the perlipc manpage for more examples of this.
as command which pipes input to us. (You may not have a raw
open() to a command that pipes both in and out, but see the
IPC::Open2 manpage, the IPC::Open3 manpage, and the section on
Bidirectional Communication in the perlipc manpage for
alternatives.)
Opening '-' opens STDIN and opening '>-' opens STDOUT. Open
returns nonzero upon success, the undefined value otherwise. If
the open involved a pipe, the return value happens to be the pid
of the subprocess.
If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern
operating systems don't care), then you should check out the
binmode entry elsewhere in this document for tips for dealing
with this. 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 binmode.
The rest need it.
When opening a file, it's usually a bad idea to continue normal
execution if the request failed, so open is frequently used in
connection with die. Even if 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;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
Page 38
PERLFUNC(1) PERLFUNC(1)
open(DBASE, '+<dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, "caesar <$article |") # decrypt article
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR
beginning with ">&", in which case the rest of the string is
interpreted as the name of a filehandle (or file descriptor, if
numeric) which is to be duped and opened. You may use & after >,
>>, <, +>, +>>, and +<. The mode you specify should match the
mode of the original filehandle. (Duping a filehandle does not
take into account any existing contents of stdio buffers.) Here
is a script that saves, redirects, and restores STDOUT and
STDERR:
#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
Page 39
PERLFUNC(1) PERLFUNC(1)
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify "<&=N", where N is a number, then Perl will do an
equivalent of C's fdopen() of that file descriptor; this is more
parsimonious of file descriptors. For example:
open(FILEHANDLE, "<&=$fd")
If you open a pipe on the command "-", i.e., either "|-" or "-|",
then there is an implicit fork done, and the return value of open
is the pid of the child within the parent process, and 0 within
the child process. (Use defined($pid) to determine whether the
open was successful.) The filehandle behaves normally for the
parent, but i/o to that filehandle is piped from/to the
STDOUT/STDIN of the child process. In the child process the
filehandle isn't opened--i/o happens from/to 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. The
following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
See the section on Safe Pipe Opens in the perlipc manpage for
more examples of this.
NOTE: On any operation which may do a fork, unflushed buffers
remain unflushed in both processes, which means you may need to
set $| 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 $?.
Using the constructor from the IO::Handle package (or one of its
subclasses, such as IO::File or IO::Socket), you can generate
anonymous filehandles which have the scope of whatever variables
hold references to them, and automatically close whenever and
Page 40
PERLFUNC(1) PERLFUNC(1)
however you leave that scope:
use IO::File;
...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
The filename that is passed to open will have leading and
trailing whitespace deleted. To open a file with arbitrary weird
characters in it, it's necessary to protect any leading and
trailing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
If you want a "real" C open() (see the open(2) manpage on your
system), then you should use the sysopen() function. This is
another way to protect your filenames from interpretation. For
example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
or die "sysopen $path: $!";
HANDLE->autoflush(1);
HANDLE->print("stuff $$\n");
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
See the seek() entry elsewhere in this document for some details
about mixing reading and writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by readdir(),
telldir(), seekdir(), rewinddir(), and closedir(). Returns TRUE
if successful. DIRHANDLEs have their own namespace separate from
FILEHANDLEs.
ord EXPR
ord Returns the numeric ascii value of the first character of EXPR.
If EXPR is omitted, uses $_. For the reverse, see the chr entry
elsewhere in this document .
Page 41
PERLFUNC(1) PERLFUNC(1)
pack TEMPLATE,LIST
Takes an array or list of values and packs it into a binary
structure, returning the string containing the structure. The
TEMPLATE is a sequence of characters that give the order and type
of values, as follows:
A An ascii string, will be space padded.
a An ascii string, will be null padded.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
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" (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.
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
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.
Page 42
PERLFUNC(1) PERLFUNC(1)
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
Each letter may optionally be followed by a number which gives a
repeat count. With all types except "a", "A", "b", "B", "h",
"H", and "P" the pack function will gobble up that many values
from the LIST. A * for the repeat count means to use however
many items are left. The "a" and "A" types gobble just one
value, but pack it as a string of length count, 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" 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 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 point data
written on one machine may not be readable on another - even if
both use IEEE floating point arithmetic (as the endian-ness of
the memory representation is not part of the IEEE spec). Note
that Perl uses doubles internally for all numeric calculation,
and converting from double into float and thence back to double
again will lose precision (i.e., unpack("f", pack("f", $foo))
will not in general equal $foo).
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
Page 43
PERLFUNC(1) PERLFUNC(1)
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in the unpack
function.
package NAMESPACE
Declares the compilation unit as being in the given namespace.
The scope of the package declaration is from the declaration
itself through the end of the enclosing block (the same scope as
the local() operator). All further unqualified dynamic
identifiers will be in this namespace. A package statement
affects only dynamic variables--including those you've used
local() on--but not lexical variables created with my().
Typically it would be the first declaration in a file to be
included by the require or use operator. You can switch into a
package in more than one place; it influences merely which symbol
table is used by the compiler for the rest of that block. You
can refer to variables and filehandles in other packages by
prefixing the identifier with the package name and a double
colon: $Package::Variable. If the package name is null, the
main package as assumed. That is, $::sail is equivalent to
$main::sail.
See the section on Packages in the perlmod manpage for more
information about packages, modules, and classes. See the
perlsub manpage for other scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system
call. Note that if you set up a loop of piped processes,
deadlock can occur unless you are very careful. In addition,
note that Perl's pipes use stdio buffering, so you may need to
set $| to flush your WRITEHANDLE after each command, depending on
the application.
See the IPC::Open2 manpage, the IPC::Open3 manpage, and the
section on Bidirectional Communication in the perlipc manpage for
examples of such things.
pop ARRAY
pop Pops and returns the last value of the array, shortening the
array by 1. Has a similar effect to
Page 44
PERLFUNC(1) PERLFUNC(1)
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined
value. If ARRAY is omitted, pops the @ARGV array in the main
program, and the @_ array in subroutines, just like shift().
pos SCALAR
pos Returns the offset of where the last 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. Such
modification will also influence the \G zero-width assertion in
regular expressions. See the perlre manpage and the perlop
manpage.
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of strings. Returns
TRUE if successful. FILEHANDLE may be a scalar variable name, in
which case the variable contains the name of or a reference to
the filehandle, thus introducing one level of indirection.
(NOTE: If FILEHANDLE is a variable and the next token is a term,
it may be misinterpreted as an operator unless you interpose a +
or put parentheses around the arguments.) If FILEHANDLE is
omitted, prints by default to standard output (or to the last
selected output channel--see the select entry elsewhere in this
document ). If LIST is also omitted, prints $_ to STDOUT. To
set the default output channel to something other than STDOUT use
the select operation. Note that, because print takes a LIST,
anything in the LIST is evaluated in a list context, and any
subroutine that you call will have one or more of its expressions
evaluated in a list context. Also be careful not to follow the
print keyword with a left parenthesis unless you want the
corresponding right parenthesis to terminate the arguments to the
print--interpose a + or put parentheses around all the arguments.
Note that if you're storing FILEHANDLES in an array or other
expression, you will have to use a block returning its value
instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to print FILEHANDLE sprintf(FORMAT, LIST). The first
argument of the list will be interpreted as the printf format.
If use locale is in effect, the character used for the decimal
Page 45
PERLFUNC(1) PERLFUNC(1)
point in formatted real numbers is affected by the LC_NUMERIC
locale. See the perllocale manpage.
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.
prototype FUNCTION
Returns the prototype of a function as a string (or undef if the
function has no prototype). FUNCTION is a reference to, or the
name of, the function whose prototype you want to retrieve.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST onto the
end of ARRAY. The length of ARRAY increases by the length of
LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of elements in the
array.
q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See the perlop manpage.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-alphanumeric characters
backslashed. (That is, all characters not matching /[A-Za-z_0-
9]/ will be preceded by a backslash in the returned string,
regardless of any locale settings.) This is the internal
function implementing the \Q escape in double-quoted strings.
If EXPR is omitted, uses $_.
rand EXPR
rand 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
Page 46
PERLFUNC(1) PERLFUNC(1)
are too large or too small, then your version of Perl was
probably compiled with the wrong number of RANDBITS.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from
the specified FILEHANDLE. Returns the number of bytes actually
read, or undef if there was an error. SCALAR will be grown or
shrunk to the length actually read. An OFFSET may be specified
to place the read data at some other place than the beginning of
the string. This call is actually implemented in terms of
stdio's fread call. To get a true read system call, see
sysread().
readdir DIRHANDLE
Returns the next directory entry for a directory opened by
opendir(). If used in a list context, returns all the rest of
the entries in the directory. If there are no more entries,
returns an undefined value in a scalar context or a null list in
a list context.
If you're planning to filetest the return values out of a
readdir(), you'd better prepend the directory in question.
Otherwise, because we didn't chdir() there, it would have been
testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
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 <EXPR> operator, but you can
use it directly. The <EXPR> operator is discussed in more detail
in the section on I/O Operators in the perlop manpage.
readlink EXPR
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 $! (errno).
If EXPR is omitted, uses $_.
readpipe EXPR
EXPR is interpolated and then executed as a system command. The
collected standard output of the command is returned. In scalar
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PERLFUNC(1) PERLFUNC(1)
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 qx/EXPR/ operator, but you can
use it directly. The qx/EXPR/ operator is discussed in more
detail in the section on I/O Operators in the perlop manpage.
recv SOCKET,SCALAR,LEN,FLAGS
Receives a message on a socket. Attempts to receive LENGTH bytes
of data into variable SCALAR from the specified SOCKET
filehandle. Actually does a C 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. See the section on UDP: Message Passing in the
perlipc manpage for examples.
redo LABEL
redo The redo command restarts the loop block without evaluating the
conditional again. The continue block, if any, is not executed.
If the LABEL is omitted, the command refers to the innermost
enclosing loop. This command is normally used by programs that
want to lie to themselves about what was just input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo LINE;
}
}
}
print;
}
ref EXPR
ref Returns a TRUE value if EXPR is a reference, FALSE otherwise. If
EXPR is not specified, $_ will be used. The value returned
depends on the type of thing the reference is a reference to.
Builtin types include:
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PERLFUNC(1) PERLFUNC(1)
REF
SCALAR
ARRAY
HASH
CODE
GLOB
If the referenced object has been blessed into a package, then
that package name is returned instead. You can think of ref() as
a typeof() operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
if (!ref ($r) {
print "r is not a reference at all.\n";
}
See also the perlref manpage.
rename OLDNAME,NEWNAME
Changes the name of a file. Returns 1 for success, 0 otherwise.
Will not work across file system boundaries.
require EXPR
require Demands some semantics specified by EXPR, or by $_ if EXPR is not
supplied. If EXPR is numeric, demands that the current version
of Perl ($] or $PERL_VERSION) be equal or greater than EXPR.
Otherwise, demands that a library file be included if it hasn't
already been included. The file is included via the do-FILE
mechanism, which is essentially just a variety of eval(). Has
semantics similar to the following subroutine:
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PERLFUNC(1) PERLFUNC(1)
sub require {
local($filename) = @_;
return 1 if $INC{$filename};
local($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
$result;
}
Note that the file will not be included twice under the same
specified name. The file must return TRUE as the last statement
to indicate successful execution of any initialization code, so
it's customary to end such a file with "1;" unless you're sure
it'll return TRUE otherwise. But it's better just to put the
"1;", in case you add more statements.
If EXPR is a bareword, the require assumes a ".pm" extension and
replaces "::" with "/" in the filename for you, 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 the use entry
elsewhere in this document and the perlmod manpage.
reset EXPR
reset Generally used in a continue block at the end of a loop to clear
variables and reset ?? searches so that they work again. The
expression is interpreted as a list of single characters (hyphens
allowed for ranges). All variables and arrays beginning with one
of those letters are reset to their pristine state. If the
expression is omitted, one-match searches (?pattern?) are reset
to match again. Resets only variables or searches in the current
package. Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Resetting "A-Z" is not recommended because you'll wipe out your
ARGV and ENV arrays. Resets only package variables--lexical
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PERLFUNC(1) PERLFUNC(1)
variables are unaffected, but they clean themselves up on scope
exit anyway, so you'll probably want to use them instead. See
the my entry elsewhere in this document .
return EXPR
return Returns from a subroutine, eval(), or do FILE with the value of
the given EXPR. Evaluation of EXPR may be in a list, scalar, or
void context, depending on how the return value will be used, and
the context may vary from one execution to the next (see
wantarray()). If no EXPR is given, returns an empty list in a
list context, an undefined value in a scalar context, or nothing
in a void context.
(Note that in the absence of a return, a subroutine, eval, or do
FILE will automatically return the value of the last expression
evaluated.)
reverse LIST
In a list context, returns a list value consisting of the
elements 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, last line first
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
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for
the readdir() routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index except that it returns the position of the
LAST occurrence of SUBSTR in STR. If POSITION is specified,
returns the last occurrence at or before that position.
rmdir FILENAME
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PERLFUNC(1) PERLFUNC(1)
rmdir Deletes the directory specified by FILENAME if it is empty. If
it succeeds it returns 1, otherwise it returns 0 and sets $!
(errno). If FILENAME is omitted, uses $_.
s/// The substitution operator. See the perlop manpage.
scalar EXPR
Forces EXPR to be interpreted in a scalar context and returns the
value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be
interpolated in a list context because it's in practice never
needed. If you really wanted to do so, however, you could use
the construction @{[ (some expression) ]}, but usually a simple
(some expression) suffices.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the fseek() call of stdio.
FILEHANDLE may be an expression whose value gives the name of the
filehandle. The values for WHENCE are 0 to set the new position
to POSITION, 1 to set it to the current position plus POSITION,
and 2 to set it to EOF plus POSITION (typically negative). For
WHENCE you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END
from either the IO::Seekable or the POSIX module. Returns 1 upon
success, 0 otherwise.
If you want to position file for sysread() or syswrite(), don't
use seek() -- buffering makes its effect on the file's system
position unpredictable and non-portable. Use sysseek() instead.
On some systems you have to do a seek whenever you switch between
reading and writing. Amongst other things, this may have the
effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR)
is useful for not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating tail -f. Once you
hit EOF on your read, and then sleep for a while, you might have
to stick in a seek() to reset things. The seek() doesn't change
the current position, but it does clear the end-of-file condition
on the handle, so that the next <FILE> makes Perl try again to
read something. We hope.
If that doesn't work (some stdios are particularly cantankerous),
then you may need something more like this:
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PERLFUNC(1) PERLFUNC(1)
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the readdir() routine on DIRHANDLE.
POS must be a value returned by telldir(). Has the same caveats
about possible directory compaction as the corresponding system
library routine.
select FILEHANDLE
select Returns the currently selected filehandle. Sets the current
default filehandle for output, if FILEHANDLE is supplied. This
has two effects: first, a write or a print without a filehandle
will default to this FILEHANDLE. Second, references to variables
related to output will refer to this output channel. For
example, if you have to set the top of form format for more than
one output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the
actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects
with methods, preferring to write the last example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit masks
specified, which can be constructed using fileno() and vec(),
along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
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PERLFUNC(1) PERLFUNC(1)
If you want to select on many filehandles you might wish to write
a subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in
$timeleft, so calling select() in a scalar context just returns
$nfound.
Any of the bit masks can also be undef. The timeout, if
specified, is in seconds, which may be fractional. Note: not all
implementations are capable of returning the $timeleft. If not,
they always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
WARNING: Do not attempt to mix buffered I/O (like read() or <FH>)
with select(). You have to use sysread() instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl. If CMD is &IPC_STAT or
&GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array. Returns like ioctl:
the undefined value for error, "0 but true" for zero, or the
actual return value otherwise.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the semaphore
id, or the undefined value if there is an error.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform semaphore
operations such as signaling and waiting. OPSTRING must be a
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PERLFUNC(1) PERLFUNC(1)
packed array of semop structures. Each semop structure can be
generated with pack("sss", $semnum, $semop, $semflag). The
number of semaphore operations is implied by the length of
OPSTRING. Returns TRUE if successful, or FALSE if there is an
error. As an example, the following code waits on semaphore
$semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Takes the same flags as the system
call of the same name. On unconnected sockets you must specify a
destination to send TO, in which case it does a C sendto().
Returns the number of characters sent, or the undefined value if
there is an error. See the section on UDP: Message Passing in
the perlipc manpage for examples.
setpgrp PID,PGRP
Sets the current process group for the specified PID, 0 for the
current process. Will produce a fatal error if used on a machine
that doesn't implement setpgrp(2). If the arguments are omitted,
it defaults to 0,0. Note that the POSIX version of setpgrp()
does not accept any arguments, so only setpgrp 0,0 is portable.
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a
user. (See setpriority(2).) Will produce a fatal error if used
on a machine that doesn't implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if there is
an error. OPTVAL may be specified as undef if you don't want to
pass an argument.
shift ARRAY
shift 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 pop() and push()
do to the right end.
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PERLFUNC(1) PERLFUNC(1)
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. If CMD is &IPC_STAT,
then ARG must be a variable which will hold the returned shmid_ds
structure. Returns like ioctl: the undefined value for error, "0
but true" for zero, or the actual return value otherwise.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared
memory segment id, or the undefined value if there is an error.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out,
and detaching from it. When reading, VAR must be a variable
which will hold the data read. When writing, if STRING is too
long, only SIZE bytes are used; if STRING is too short, nulls are
written to fill out SIZE bytes. Return TRUE if successful, or
FALSE if there is an error.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW,
which has the same interpretation as in the system call of the
same name.
sin EXPR
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])) }
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or forever if no
EXPR. May be interrupted by sending the process a SIGALRM.
Returns the number of seconds actually slept. You probably
cannot mix alarm() and sleep() calls, because sleep() is often
implemented using alarm().
On some older systems, it may sleep up to a full second less than
what you requested, depending on how it counts seconds. Most
modern systems 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 the select() entry elsewhere in this
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PERLFUNC(1) PERLFUNC(1)
document below.
See also the POSIX module's sigpause() function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to
filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the
same as for the system call of the same name. You should "use
Socket;" first to get the proper definitions imported. See the
example in the section on Sockets: Client/Server Communication in
the perlipc manpage.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of
the specified type. DOMAIN, TYPE, and PROTOCOL are specified the
same as for the system call of the same name. If unimplemented,
yields a fatal error. Returns TRUE if successful.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
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 <=> 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.
In the interests of efficiency the normal calling code for
subroutines is bypassed, with the following effects: the
subroutine may not be a recursive subroutine, and the two
elements to be compared are passed into the subroutine not via @_
but as the package global variables $a and $b (see example
below). They are passed by reference, so don't modify $a and $b.
And don't try to declare them as lexicals either.
You also cannot exit out of the sort block or subroutine using
any of the loop control operators described in the perlsyn
manpage or with goto().
When use locale is in effect, sort LIST sorts LIST according to
the current collation locale. See the perllocale manpage.
Examples:
Page 57
PERLFUNC(1) PERLFUNC(1)
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
sub backwards { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
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PERLFUNC(1) PERLFUNC(1)
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing using a Schwartzian Transform (no temps)
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
If you're using strict, you MUST NOT declare $a and $b as
lexicals. They are package globals. That means if you're in the
main package, it's
@articles = sort {$main::b <=> $main::a} @files;
or just
@articles = sort {$::b <=> $::a} @files;
but if you're in the FooPack package, it's
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns
inconsistent results (sometimes saying $x[1] is less than $x[2]
and sometimes saying the opposite, for example) the Perl
interpreter will probably crash and dump core. This is entirely
due to and dependent upon your system's qsort(3) library routine;
this routine often avoids sanity checks in the interest of speed.
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
Removes the elements designated by OFFSET and LENGTH from an
array, and replaces them with the elements of LIST, if any.
Returns the elements removed from the array. The array grows or
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PERLFUNC(1) PERLFUNC(1)
shrinks as necessary. If LENGTH is omitted, removes everything
from OFFSET onward. The following equivalences hold (assuming $[
== 0):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y);
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split Splits a string into an array of strings, and returns it.
If not in a list context, returns the number of fields found and
splits into the @_ array. (In a list context, you can force the
split into @_ by using ?? as the pattern delimiters, but it still
returns the array value.) The use of implicit split to @_ is
deprecated, however.
If EXPR is omitted, splits the $_ string. If PATTERN is also
omitted, splits on whitespace (after skipping any leading
whitespace). Anything matching PATTERN is taken to be a
delimiter separating the fields. (Note that the delimiter may be
longer than one character.)
If LIMIT is specified and is not negative, splits into no more
than that many fields (though it may split into fewer). If LIMIT
is unspecified, trailing null fields are stripped (which
potential users of pop() would do well to remember). If LIMIT is
negative, it is treated as if an arbitrarily large LIMIT had been
specified.
A pattern matching the null string (not to be confused with a
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PERLFUNC(1) PERLFUNC(1)
null pattern //, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into
separate characters at each point it matches that way. For
example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, Perl supplies a
LIMIT one larger than the number of variables in the list, to
avoid unnecessary work. For the list above LIMIT would have been
4 by default. In time critical applications it behooves you not
to split into more fields than you really need.
If the PATTERN contains parentheses, additional array elements
are created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
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*?):\s*/m, $header);
The pattern /PATTERN/ may be replaced with an expression to
specify patterns that vary at runtime. (To do runtime
compilation only once, use /$variable/o.)
As a special case, specifying a PATTERN of space (' ') will split
on white space just as split with no arguments does. Thus,
split(' ') can be used to emulate awk's default behavior, whereas
split(/ /) will give you as many null initial fields as there are
leading spaces. A split on /\s+/ is like a split(' ') except
that any leading whitespace produces a null first field. A split
with no arguments really does a split(' ', $_) internally.
Example:
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PERLFUNC(1) PERLFUNC(1)
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos,
$home, $shell) = split(/:/);
...
}
(Note that $shell above will still have a newline on it. See the
chop, chomp, and join entries elsewhere in this document .)
sprintf FORMAT, LIST
Returns a string formatted by the usual printf conventions of the
C library function sprintf(). See the sprintf(3) manpage or the
printf(3) manpage 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:
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PERLFUNC(1) PERLFUNC(1)
%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 %
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 use locale is in effect, the character used for the decimal
point in formatted real numbers is affected by the LC_NUMERIC
locale. See the perllocale manpage.
sqrt EXPR
sqrt Return the square root of EXPR. If EXPR is omitted, returns
square root of $_.
srand EXPR
srand Sets the random number seed for the 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 time ^ $$ or C<time ^ ($$ + ($$ << 15))>), but
that isn't necessary any more.
In fact, it's usually not necessary to call srand() at all,
because if it is not called explicitly, it is called implicitly
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PERLFUNC(1) PERLFUNC(1)
at the first use of the rand operator. However, this was not the
case in version of Perl before 5.004, so if your script will run
under older Perl versions, it should call srand().
Note that you need something much more random than the default
seed for cryptographic purposes. Checksumming the compressed
output of one or more rapidly changing operating system status
programs is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the
Math::TrulyRandom module in CPAN.
Do 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
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.
stat FILEHANDLE
stat EXPR
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 stats $_. Returns a null list if the stat fails.
Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are
the meaning of the fields:
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PERLFUNC(1) PERLFUNC(1)
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.)
If stat is passed the special filehandle consisting of an
underline, no stat is done, but the current contents of the stat
structure from the last stat or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is
negative under NFS.)
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified) in
anticipation of 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 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 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, the rarest character is selected, based on some static
frequency tables constructed from some C programs and English
text. Only those places that contain this "rarest" character are
examined.)
For example, here is a loop which inserts index producing entries
before any line containing a certain pattern:
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PERLFUNC(1) PERLFUNC(1)
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for /\bfoo\b/, only those locations in $_ that
contain "f" will be looked at, because "f" is rarer than "o". In
general, this is a big win except in pathological cases. The
only question is whether it saves you more time than it took to
build the linked list in the first place.
Note that if you have to look for strings that you don't know
till runtime, you can build an entire loop as a string and eval
that to avoid recompiling all your patterns all the time.
Together with undefining $/ to input entire files as one record,
this can be very fast, often faster than specialized programs
like fgrep(1). The following scans a list of files (@files) for
a list of words (@words), and prints out the names of those files
that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub BLOCK
sub NAME
sub NAME BLOCK
This is subroutine definition, not a real function 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 the perlsub manpage and the
perlref manpage for details.
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PERLFUNC(1) PERLFUNC(1)
substr EXPR,OFFSET,LEN
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character
is at offset 0, or whatever you've set $[ to (but don't do that).
If OFFSET is negative (or more precisely, less than $[), 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 something longer
than LEN, the string will grow to accommodate it. To keep the
string the same length you may need to pad or chop your value
using sprintf().
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise. On systems that don't
support symbolic links, produces a fatal error at run time. To
check for that, use eval:
$symlink_exists = (eval {symlink("","")};, $@ eq '');
syscall LIST
Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call.
If unimplemented, produces a fatal error. The arguments are
interpreted as follows: if a given argument is numeric, the
argument is passed as an int. If not, the pointer to the string
value is passed. You are responsible to make sure a string is
pre-extended long enough to receive any result that might be
written into a string. If your integer arguments are not
literals and have never been interpreted in a numeric context,
you may need to add 0 to them to force them to look like numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
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 $!
(errno). Note that some system calls can legitimately return -1.
The proper way to handle such calls is to assign $!=0; before the
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PERLFUNC(1) PERLFUNC(1)
call and check the value of <$!> if syscall returns -1.
There's a problem with 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 pipe instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and
associates it with FILEHANDLE. If FILEHANDLE is an expression,
its value is used as the name of the real filehandle wanted.
This function calls the underlying operating system's open
function with the parameters FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module
Fcntl. However, for historical reasons, some values are
universal: zero means read-only, one means write-only, and two
means read/write.
If the file named by FILENAME does not exist and the open call
creates it (typically because MODE includes the O_CREAT flag),
then the value of PERMS specifies the permissions of the newly
created file. If PERMS is omitted, the default value is 0666,
which allows read and write for all. This default is reasonable:
see umask.
The IO::File module provides a more object-oriented approach, if
you're into that kind of thing.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
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,
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 placement at that many bytes counting backwards from
the end of the string. A positive OFFSET greater than the length
of SCALAR results in the string being padded to the required size
with "\0" bytes before the result of the read is appended.
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PERLFUNC(1) PERLFUNC(1)
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.
system LIST
Does exactly the same thing as "exec LIST" except that a fork is
done first, and the parent process waits for the child process to
complete. Note that argument processing varies depending on the
number of arguments. 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 the exec entry elsewhere in this
document . This is 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 the section on `STRING` in the perlop
manpage.
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.
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PERLFUNC(1) PERLFUNC(1)
$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 the section
on `STRING` in the perlop manpage for details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
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 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 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.
tell FILEHANDLE
tell Returns the current position for FILEHANDLE. FILEHANDLE may be
an expression whose value gives the name of the actual
filehandle. If FILEHANDLE is omitted, assumes the file last
read.
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PERLFUNC(1) PERLFUNC(1)
telldir DIRHANDLE
Returns the current position of the readdir() routines on
DIRHANDLE. Value may be given to seekdir() to access a
particular location in a directory. Has the same caveats about
possible directory compaction as the corresponding system library
routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that will
provide the implementation for the variable. VARIABLE is the
name of the variable to be enchanted. CLASSNAME is the name of a
class implementing objects of correct type. Any additional
arguments are passed to the "new" method of the class (meaning
TIESCALAR, TIEARRAY, or TIEHASH). Typically these are arguments
such as might be passed to the dbm_open() function of C. The
object returned by the "new" method is also returned by the tie()
function, which would be useful if you want to access other
methods in CLASSNAME.
Note that functions such as keys() and values() may return huge
array values when used on large objects, like DBM files. You may
prefer to use the each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
DELETE this, key
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
A class implementing an ordinary array should have the following
methods:
TIEARRAY classname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
[others TBD]
A class implementing a scalar should have the following methods:
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PERLFUNC(1) PERLFUNC(1)
TIESCALAR classname, LIST
DESTROY 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 the
DB_File manpage or the Config module for interesting tie()
implementations.
tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same
value that was originally returned by the tie() call which bound
the variable to a package.) Returns the undefined value if
VARIABLE isn't tied to a package.
time Returns the number of non-leap seconds since whatever time the
system considers to be the epoch (that's 00:00:00, January 1,
1904 for MacOS, and 00:00:00 UTC, January 1, 1970 for most other
systems). Suitable for feeding to gmtime() and localtime().
times Returns a four-element array giving the user and system times, in
seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
tr/// The translation operator. Same as y///. See the perlop manpage.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Produces a fatal error if truncate isn't
implemented on your system.
uc EXPR
uc Returns an uppercased version of EXPR. This is the internal
function implementing the \U escape in double-quoted strings.
Respects current LC_CTYPE locale if use locale in force. See the
perllocale manpage.
If EXPR is omitted, uses $_.
ucfirst EXPR
ucfirst Returns the value of EXPR with the first character uppercased.
This is the internal function implementing the \u escape in
double-quoted strings. Respects current LC_CTYPE locale if use
locale in force. See the perllocale manpage.
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PERLFUNC(1) PERLFUNC(1)
If EXPR is omitted, uses $_.
umask EXPR
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
not a string of octal digits. See also the oct entry elsewhere
in this document if all you have is a string.
undef EXPR
undef 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'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
unlink LIST
unlink Deletes a list of files. Returns the number of files
successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you are superuser
and the -U flag is supplied to Perl. Even if these conditions
are met, be warned that unlinking a directory can inflict damage
on your filesystem. Use rmdir instead.
If LIST is omitted, uses $_.
unpack TEMPLATE,EXPR
Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the
array value. (In a scalar context, it returns merely the first
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PERLFUNC(1) PERLFUNC(1)
value produced.) The TEMPLATE has the same format as in the pack
function. Here's a subroutine that does substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition, you may prefix a field with a %<number> to indicate
that you want a <number>-bit checksum of the items instead of the
items themselves. Default is a 16-bit checksum. For example,
the following computes the same number as the System V sum
program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
The following efficiently counts the number of set bits in a bit
vector:
$setbits = unpack("%32b*", $selectmask);
untie VARIABLE
Breaks the binding between a variable and a package. (See
tie().)
unshift ARRAY,LIST
Does the opposite of a shift. Or the opposite of a push,
depending on how you look at it. Prepends list to the front of
the array, and returns the new number of elements in the array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so
the prepended elements stay in the same order. Use reverse to do
the reverse.
use Module LIST
use Module
use Module VERSION LIST
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PERLFUNC(1) PERLFUNC(1)
use VERSION
Imports some semantics into the current package from the named
module, generally by aliasing certain subroutine or variable
names into your package. It is exactly equivalent to
BEGIN { require Module; import Module LIST; }
except that Module must be a bareword.
If the first argument to use is a number, it is treated as a
version number instead of a module name. If the version of the
Perl interpreter is less than VERSION, then an error message is
printed and Perl exits immediately. This is often useful if you
need to check the current Perl version before useing library
modules which have changed in incompatible ways from older
versions of Perl. (We try not to do this more than we have to.)
The BEGIN forces the require and import to happen at compile
time. The require makes sure the module is loaded into memory if
it hasn't been yet. The import is not a builtin--it's just an
ordinary static method call into the "Module" package to tell the
module to import the list of 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 the Exporter manpage. 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:
use Module ();
That is exactly equivalent to
BEGIN { require Module; }
If the VERSION argument is present between Module and LIST, then
the use will call the VERSION method in class Module with the
given version as an argument. The default VERSION method,
inherited from the Universal class, croaks if the given version
is larger than the value of the variable $Module::VERSION. (Note
that there is not a comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler
directives) are also implemented this way. Currently implemented
pragmas are:
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PERLFUNC(1) PERLFUNC(1)
use integer;
use diagnostics;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
These pseudo-modules import semantics into the current block
scope, unlike ordinary modules, which import symbols into the
current package (which are effective through the end of the
file).
There's a corresponding "no" command that unimports meanings
imported by use, i.e., it calls unimport Module LIST instead of
import.
no integer;
no strict 'refs';
If no unimport method can be found the call fails with a fatal
error.
See the perlmod manpage for a list of standard modules and
pragmas.
utime LIST
Changes the access and modification times on each file of a list
of files. The first two elements of the list must be the
NUMERICAL access and modification times, in that order. Returns
the number of files successfully changed. The inode modification
time of each file is set to the current time. Example of a
"touch" command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values HASH
Returns a normal array consisting of all the values of the named
hash. (In a scalar context, returns the number of values.) The
values are returned in an apparently random order, but it is the
same order as either the keys() or each() function would produce
on the same hash. As a side effect, it resets HASH's iterator.
See also keys(), each(), and sort().
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a vector of unsigned integers, and
returns the value of the bit field specified by OFFSET. BITS
specifies the number of bits that are reserved for each entry in
the bit vector. This must be a power of two from 1 to 32. vec()
may also be assigned to, in which case parentheses are needed to
give the expression the correct precedence as in
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vec($image, $max_x * $x + $y, 8) = 3;
Vectors created with vec() can also be manipulated with the
logical operators |, &, and ^, which will assume a bit vector
operation is desired when both operands are strings.
To transform a bit vector into a string or array of 0's and 1's,
use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of
the *.
wait Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes. The
status is returned in $?.
waitpid PID,FLAGS
Waits for a particular child process to terminate and returns the
pid of the deceased process, or -1 if there is no such child
process. The status is returned in $?. If you say
use POSIX ":sys_wait_h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-
blocking wait is available on machines supporting either the
waitpid(2) or wait4(2) system calls. However, waiting for a
particular pid with FLAGS of 0 is implemented everywhere. (Perl
emulates the system call by remembering the status values of
processes that have exited but have not been harvested by the
Perl script yet.)
wantarray
Returns TRUE if the context of the currently executing subroutine
is looking for a list value. Returns FALSE if the context is
looking for a scalar. Returns the undefined value if the context
is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
warn LIST
Produces a message on STDERR just like die(), but doesn't exit or
throw an exception.
No message is printed if there is a $SIG{__WARN__} handler
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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 __WARN__
hooks are not called from inside one.
You will find this behavior is slightly different from that of
$SIG{__DIE__} handlers (which don't suppress the error text, but
can instead call die() again to change it).
Using a __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 the perlvar manpage for details on setting %SIG entries, and
for more examples.
write FILEHANDLE
write EXPR
write 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 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 $~ variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted record,
the page is advanced by writing a form feed, a special top-of-
page format is used to format the new page header, and then the
record is written. By default the top-of-page format is the name
of the filehandle with "_TOP" appended, but it may be dynamically
set to the format of your choice by assigning the name to the $^
variable while the filehandle is selected. The number of lines
remaining on the current page is in variable $-, which can be set
to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default
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output channel, which starts out as STDOUT but may be changed by
the select operator. If the FILEHANDLE is an EXPR, then the
expression is evaluated and the resulting string is used to look
up the name of the FILEHANDLE at run time. For more on formats,
see the perlform manpage.
Note that write is NOT the opposite of read. Unfortunately.
y/// The translation operator. Same as tr///. See the perlop
manpage.
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