perlfunc - Perl builtin functions
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 or 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 such list argument.) For
instance, splice() has three scalar arguments followed by a
list, whereas gethostbyname() has four scalar arguments.
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.
A few functions take no arguments at all, and therefore work as neither
unary nor list operators. These include such functions as time
and endpwent. For example, time+86_400 always means
time() + 86_400.
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.
Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in 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.
An named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context. You can't get a list
like (1,2,3) into being in scalar context, because the compiler knows the context at
compile time. It would generate the scalar comma operator there, not the
list construction version of the comma. That means it was never a list to
start with.
In general, functions in Perl that serve as wrappers for system calls of
the same name (like chown(2), fork(2),
closedir(2), etc.) all return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. This is
different from the C interfaces, which return -1 on failure. Exceptions to this rule are wait(),
waitpid(), and syscall(). System calls also set the special $!
variable on failure. Other functions do not, except accidentally.
Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.
chomp, chop, chr, crypt, hex, index, lc, lcfirst,
length, oct, ord, pack, q/STRING/, qq/STRING/, reverse,
rindex, sprintf, substr, tr///, uc, ucfirst, y///
abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand
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
-<EM>X</EM>, chdir, chmod, chown, chroot, fcntl, glob,
ioctl, link, lstat, mkdir, open, opendir,
readlink, rename, rmdir, stat, symlink, umask,
unlink, utime
caller, continue, die, do, dump, eval, exit, goto, last, next, redo, return, sub, wantarray
defined, dump, eval, formline, local, my, reset, scalar, undef, wantarray
alarm, exec, fork, getpgrp, getppid, getpriority, kill,
pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
times, wait, waitpid
bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, send, setsockopt, shutdown, socket, socketpair
msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite
endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent
endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent
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.
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available, or details of the available functionality may differ slightly. The Perl functions affected by this are:
-X, binmode, chmod, chown, chroot, crypt,
dbmclose, dbmopen, dump, endgrent, endhostent,
endnetent, endprotoent, endpwent, endservent, exec,
fcntl, flock, fork, getgrent, getgrgid, gethostent,
getlogin, getnetbyaddr, getnetbyname, getnetent,
getppid, getprgp, getpriority, getprotobynumber,
getprotoent, getpwent, getpwnam, getpwuid,
getservbyport, getservent, getsockopt, glob, ioctl,
kill, link, lstat, msgctl, msgget, msgrcv,
msgsnd, open, pipe, readlink, rename, select, semctl,
semget, semop, setgrent, sethostent, setnetent,
setpgrp, setpriority, setprotoent, setpwent,
setservent, setsockopt, shmctl, shmget, shmread,
shmwrite, socket, socketpair, stat, symlink, syscall,
sysopen, system, times, truncate, umask, unlink,
utime, wait, waitpid
For more information about the portability of these functions, see the perlport manpage and other available platform-specific documentation.
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:
X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
-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), or Filehandle is a pipe.
-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).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators -r, -R,
-w, -W, -x, and -X is by default based solely on the mode of the file and the uids and gids of
the user. There may be other reasons you can't actually read, write, or
execute the file. Such reasons may be for example network filesystem access
controls, ACLs (access control lists), read-only filesystems, and
unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the -r,
-R, -w, and -W tests always return 1, and -x and -X return 1 if any execute bit is set in the mode. Scripts run by the
superuser may thus need to do a stat() to determine the actual
mode of the file, or temporarily set their effective uid to something else.
Note that -s/a/b/ does not do a negated substitution. Saying
-exp($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 strange 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 $a || -w _ || -x _;
stat($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 _;
Returns the absolute value of its argument. If VALUE is omitted, uses $_.
Accepts an incoming socket connect, just as the accept(2)
system call does. Returns the packed address if it succeeded, FALSE
otherwise. See the example in Sockets: Client/Server Communication.
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
four-arugment version of select() leaving the first three
arguments undefined, or you might be able to use the syscall() interface to access setitimer(2) if your system supports it.
The Time::HiRes module from CPAN may also prove useful.
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, modulo the caveats given in Signals.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the POSIX::tan()
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
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 Sockets: Client/Server Communication.
Arranges for the file to be read or written in ``binary'' mode in operating
systems that distinguish between binary and text files. Files that are not
in binary mode have CR LF sequences translated to LF on input and LF
translated to CR LF on output. Binmode has no effect under many sytems, but
in MS-DOS and similarly archaic systems, it may be imperative--otherwise
your MS-DOS-damaged C library may mangle your file. The key distinction
between systems that need binmode() and those that don't is their text file formats. Systems like Unix, MacOS,
and Plan9 that delimit lines with a single character, and that encode that
character in C as "\n", do not need binmode(). The rest may need it. If FILEHANDLE is an expression, the value is taken
as the name of the filehandle.
If the system does care about it, using it when you shouldn't is just as
perilous as failing to use it when you should. Fortunately for most of us,
you can't go wrong using binmode() on systems that don't care
about it, though.
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If CLASSNAME is omitted, the current package is used. Because a bless() is often the last thing in a constructor. it returns the reference for convenience. Always use the two-argument version if the function doing the blessing might be inherited by a derived class. See the perltoot manpage and the perlobj manpage for more about the blessing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.
See Perl Modules.
Returns the context of the current subroutine call. In scalar context, returns the caller's package name if there is a caller, that is, if we're in a subroutine or eval() or require(), and the undefined value otherwise. In 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.
Be aware that the optimizer might have optimized call frames away before
caller() had a chance to get the information. That means that caller(N)
might not return information about the call frame you expect it do, for
N > 1. In particular, @DB::args might have information from the previous time caller() was called.
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the user's home directory. Returns TRUE upon success, FALSE otherwise. See the example under die().
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 oct, if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
This safer version of chop removes any trailing string 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.
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 $_. Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
#...
}
You can actually chop anything that's an lvalue, including an assignment:
chop($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).
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 "$user not in passwd file";
@ary = glob($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.
Returns the character represented by that NUMBER in the character set. For
example, chr(65) is "A" in ASCII. For the reverse, use ord.
If NUMBER is omitted, uses $_.
This function works like the system call by the same name: it makes the
named directory the new root directory for all further pathnames that begin
with a "/" by your process and all its children. (It doesn't change your current
working directory, which is unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omitted, does a chroot() to $_.
Closes the file or pipe associated with the file handle, returning TRUE only if stdio successfully flushes buffers and closes the system file descriptor. Closes the currently selected filehandle if the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to do
another open() on it, because open() will close it for you. (See
open().) However, an explicit close() on an input file resets the line counter ($.), 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.) Closing a pipe also waits for the process executing on the pipe to
complete, in case you want to look at the output of the pipe afterwards,
and implicitly puts the exit status value of that 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 can be used as an indirect filehandle, usually the real filehandle name.
Closes a directory opened by opendir() and returns the success of that system call.
DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle 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 Sockets: Client/Server Communication.
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).
last, next, or redo may appear within a continue block. last and redo will behave as if they had been executed within the main block. So will next, but since it will execute a continue block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the continue section is semantically equivalent to using an empty one, logically enough. In that case, next goes directly back to check the condition at the top of the loop.
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]), $_[0] ) }
Encrypts a string exactly like the crypt(3) function in the C
library (assuming that you actually have a version there that has not been
extirpated as a potential munition). This can prove useful for checking the
password file for lousy passwords, amongst other things. Only the guys
wearing white hats should do this.
Note that crypt() is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function. As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted
text as the salt (like crypt($plain, $crypted) eq $crypted). This allows your code to work with the standard crypt() and with more exotic implementations. When choosing a new salt create a
random two character string whose characters come from the set [./0-9A-Za-z]
(like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]).
Here's an example that makes sure that whoever runs this program knows their own password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
[This function has been largely superseded by the untie() function.]
Breaks the binding between a DBM file and a hash.
[This function has been largely superseded by the tie() function.]
This binds a dbm(3), ndbm(3),
sdbm(3), gdbm(3), or Berkeley DB file to a hash.
HASH is the name of the hash. (Unlike normal open(), the first argument is NOT a filehandle, even though it looks like one). DBNAME is the name of the
database (without the .dir or .pag extension if any). If the database does not exist, it is created with
protection specified by MODE (as modified by the umask()). If your system supports only the older DBM functions, you may perform
only one dbmopen() in your program. In older versions of Perl, if your system had neither DBM
nor ndbm, calling dbmopen() produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an eval(), which will trap the error.
Note that functions such as keys() and values() may return huge lists when used on large DBM files. You may prefer to use the each() function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($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.
You can control which DBM library you use by loading that library before
you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
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
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, by saying defined &func without parentheses. 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 exists 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 zero
characters long. This is all very above-board and honest. When a function
returns an undefined value, it's an admission that it couldn't give you an
honest answer. So you should use 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 shouldn't do that unless you don't plan to use them again, because it saves time when you load them up again to have memory already ready to be filled. The normal way to free up space used by an aggregate is to assign the empty list.
This counterintuitive behavior of defined() on aggregates may be changed, fixed, or broken in a future release of Perl.
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 just assigning the empty list or undefining it:
%hash = (); # completely empty %hash
undef %hash; # forget %hash every existed
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};
Outside 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. Note that the ``input line number'' (also known as ``chunk'') is
subject to whatever notion of ``line'' happens to be currently in effect,
and is also available as the special variable $.. See $/ and $..
Hint: sometimes appending ", 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(), warn(), and the Carp module.
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.
die() can also be called with a reference argument. If this
happens to be trapped within an eval(), $@ contains the
reference. This behavior permits a more elaborate exception handling
implementation using objects that maintain arbitary state about the nature
of the exception. Such a scheme is sometimes preferable to matching
particular string values of $@ using regular expressions. Here's an
example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Since perl will stringify uncaught exception messages before displaying them, you may want to overload stringification operations on such custom exception objects. See the overload manpage for details about that.
You can arrange for a callback to be run 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 $SIG{expr} for details on setting %SIG entries, and eval BLOCK for some examples.
Note that the $SIG{__DIE__} hook is currently called even inside eval()ed blocks/strings!
If one wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see $^S). Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
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 BLOCK does not count as a loop, so the loop control statements
next, last, or redo cannot be used to leave or restart the block. See the perlsyn manpage for alternative strategies.
A deprecated form of subroutine call. See the perlsub manpage.
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 and concise, keeps track of the current
filename for error messages, searches the @INC libraries, and
updates
%INC if the file is found. See Predefined Names for these variables. It also differs in that code evaluated with do FILENAME
cannot see lexicals in the enclosing scope; eval STRING does. 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.
If do cannot read the file, it returns undef and sets $! to the error. If do can read the file but cannot compile it, it returns undef and sets an error
message in $@. If the file is successfully compiled, do returns the value of the last expression evaluated.
Note that inclusion of library modules is better done with the use() and require() operators, which also do automatic error checking and raise an exception if there's a problem.
You might like to use do to read in a program configuration file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
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 $ARGV[0] eq '-d';
QUICKSTART:
Getopt('f');
This operator is largely obsolete, partly because it's very hard to convert a core file into an executable, and because the real perl-to-C compiler has superseded it.
When called in list context, returns a 2-element list consisting of the key
and value for the next element of a hash, so that you can iterate over it.
When called in scalar context, returns the key for only the ``next''
element in the hash. (Note: Keys may be "0" or "", which are logically false; you may wish to avoid constructs like while ($k = each %foo) {}
for this reason.)
Entries are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be in the same order as either the keys() or values() function would produce on the same (unmodified) hash.
When the hash is entirely read, a null array is returned in list context
(which when assigned produces a FALSE (0) value), and undef in 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";
}
Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives
the real filehandle. (Note that this function actually reads a character
and then ungetc()s it, so isn't 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. Using eof() with empty parentheses is very different. It indicates 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:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of current file
print "--------------\n";
close(ARGV); # close or last; 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 false values when they run out of data, or if there was an error.
In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program. The value of the expression (which is itself
determined within scalar context) is first parsed, and if there weren't any
errors, executed in the context of the current Perl program, so that any
variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the eval executes. If EXPR is
omitted, evaluates $_. This form is typically used to delay parsing and subsequent execution of
the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed only once--at the same time the code surrounding the eval itself was parsed--and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first (see below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may be also used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a die() statement is executed, an undefined value is returned by eval(), and $@ is set to the error message. If there was no error, $@ is guaranteed to be a null string. 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:
# 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 { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Due to the current arguably broken state of __DIE__ hooks, 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 lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuive behavior may be fixed in a future release.
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 situation, you
can just use symbolic references instead, as in case 6.
eval BLOCK does not count as a loop, so the loop control statements
next, last, or redo cannot be used to leave or restart the block.
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).
Since it's a common mistake to use exec() instead of system(), Perl warns you if there is a following statement which isn't die(), warn(), or exit() (if -w is set - but you always do that). If you
really want to follow an exec() with some other statement, you can use one of these styles to avoid the
warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
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 or an array with one element in
it, 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 `STRING` for details.
Using an indirect object with exec() or system() is also more secure. This usage forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo
program, passing it "surprise" an argument. The second version didn't--it tried to run a program literally
called "echo surprise", didn't find it, and set $? to a non-zero value indicating failure.
Note that exec() will not call your END blocks, nor will it call any DESTROY methods in your objects.
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 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->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
Although the last element will not spring into existence just because its
existence was tested, intervening ones will. Thus $ref->{"A"}
and $ref->{"A"}->{"B"} will spring into existence due to the existence test for a
$key element. This happens anywhere the arrow operator is
used, including even
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a future release.
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also die(). If EXPR is omitted, exits with 0 status. The only universally recognized values for EXPR are 0 for success and 1
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running. For example, exiting 69
(EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered,
but that's not true everywhere.
Don'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().
The exit() function does not always exit immediately. It calls
any defined END routines first, but these END routines may not themselves abort the exit. Likewise any object destructors
that need to be called are called before the real exit. If this is a
problem, you can call POSIX:_exit($status) to avoid END and destructor processing. See the perlsub manpage for details.
Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives exp($_).
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value return works just like ioctl() below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for defined() on the return from fnctl(). Like ioctl(), it maps a 0 return from the system call into ``0
but true'' in Perl. This string is true in boolean context and 0
in numeric context. It is also exempt from the normal -w warnings on improper numeric conversions.
Note that fcntl() will produce a fatal error if used on a machine that doesn't implement
fcntl(2). See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. This is mainly useful for constructing bitmaps for select() and low-level POSIX tty-handling operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
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.
Two potentially non-obvious but traditional flock semantics are that it waits indefinitely until the lock is granted, and that its locks merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with flock() may be modified by programs that do not also use flock(). See the perlport manpage, your port's specific documentation, or your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called ``features''). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.)
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 miscoordination, Perl now flushes FILEHANDLE before locking or unlocking 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 if not
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: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are inherited
across fork() calls, whereas those that must resort to the
more capricious fcntl() function lose the locks, making it
harder to write servers.
See also the DB_File manpage for other flock() examples.
Does a fork(2) system call to create a new process running the
same program at the same point. It returns the child pid to the parent
process, 0 to the child process, or undef if the fork is unsuccessful. File descriptors (and sometimes locks on those
descriptors) are shared, while everything else is copied. On most systems
supporting fork(), great care has gone into making it
extremely efficient (for example, using copy-on-write technology on data
pages), making it the dominant paradigm for multitasking over the last few
decades.
Note: unflushed buffers remain unflushed in both processes, which means you
may need to set $| ($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. On some
systems, you can avoid this by setting
$SIG{CHLD} to "IGNORE". 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, then the remote server (such as, say, a CGI script or a backgrounded job launced from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue.
Declare a picture format for use by the write() function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See the perlform manpage for many details and examples.
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.
Returns the next character from the input file attached to FILEHANDLE, or the undefined value at end of file, or if there was an error. If FILEHANDLE is omitted, reads from STDIN. This is not particularly efficient. However, it cannot be used by itself to fetch single characters without waiting for the user to hit enter. 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";
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 purporting POSIX compliance.
See also the Term::ReadKey
module from your nearest CPAN site; details on CPAN can be found on
CPAN.
Implements the C library function of the same name, which on most systems 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().
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);
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.
Returns the process id of the parent process.
Returns the current priority for a process, a process group, or a user.
(See getpriority(2).) Will raise a fatal exception if used on a machine that doesn't implement
getpriority(2).
These routines perform the same functions as their counterparts in the system library. In list context, the return values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = 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.)
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num;
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special cases in the sense that in many systems they are unsupported.
If the $quota is unsupported, it is an empty scalar. If it is supported, it usually
encodes the disk quota. If the $comment
field is unsupported, it is an empty scalar. If it is supported it usually
encodes some administrative comment about the user. In some systems the
$quota field may be $change or $age, fields that have to do with password aging. In some systems the $comment field may be $class. The $expire field, if present, encodes the expiration period of the account or the
password. For the availability and the exact meaning of these fields in
your system, please consult your getpwnam(3) documentation and
your pwd.h file. You can also find out from within Perl what your $quota and $comment fields mean and whether you have the $expire field by using the Config module and the values d_pwquota, d_pwage, d_pwchange, d_pwcomment, and d_pwexpire. Shadow password files are only supported if your vendor has implemented
them in the intuitive fashion that calling the regular C library routines
gets the shadow versions if you're running under privilege. Those that
incorrectly implement a separate library call are not supported.
The $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]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr");
If you get tired of remembering which element of the return list contains
which return value, by-name interfaces are also provided in modules:
File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent,
Time::gmtime, Time::localtime, and User::grent. These override the normal built-in, replacing them with versions that
return objects with the appropriate names for each field. For example:
use File::stat; use User::pwent; $is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid), they aren't,
because a File::stat object is different from a User::pwent object.
Returns the packed sockaddr address of this end of the SOCKET connection.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
Returns the socket option requested, or undef if there is an error.
Returns the value of EXPR with filename expansions such as the standard
Unix shell /bin/csh 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 I/O Operators.
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, that is, $year is 123 in year 2023,
not simply the last two digits of the year. If you assume it is, then you
create non-Y2K-compliant programs--and you wouldn't want to do that, would
you?
If EXPR is omitted, does gmtime(time()).
In 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.
This scalar value is not locale dependent (see the perllocale manpage), but is instead a Perl builtin. Also see the Time::Local module, and the strftime(3) and mktime(3)
functions available via the POSIX module. To get somewhat similar but
locale dependent date strings, set up your locale environment variables
appropriately (please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b escapes, which represent the short forms of the day of the week and the
month of the year, may not necessarily be three characters wide in all
locales.
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).
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.
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 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 as a for loop's index variable aliases the
list elements. That is, modifying an element of a list returned by grep
(for example, in a foreach, map()
or another grep()) actually modifies the element in the original list. This is usually
something to be avoided when writing clear code.
See also map for an array composed of the results of the BLOCK or EXPR.
Interprets EXPR as a hex string and returns the corresponding value. (To
convert strings that might start with either 0, 0x, or 0b, see
oct.) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
There is no builtin import() function. It is just 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 use(), the perlmod manpage, and the Exporter manpage.
The index function searches for one string within another, but without the
wildcard-like behavior of a full regular-expression pattern match. It
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.
Returns the integer portion of EXPR. If EXPR is omitted, uses $_. You should not use this function for rounding: one because it truncates
towards 0, and two because machine representations of floating point numbers can
sometimes produce counterintuitive results. For example,
int(-6.725/0.025) produces -268 rather than the correct -269; that's because it's really more
like -268.99999999999994315658 instead. Usually, the sprintf(), printf(), or the POSIX::floor and POSIX::ceil
functions will serve you better than will int().
Implements the ioctl(2) function. You'll probably first have
to say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
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 that may help you in this, but it's
nontrivial.) SCALAR will be read and/or written depending on the
FUNCTION--a pointer to the string value of SCALAR will be passed as the
third argument of the actual 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.
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(...) || -1;
printf "System returned %d\n", $retval;
The special string ``0 but true'' is exempt from -w complaints about improper numeric conversions.
Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split.
Returns a list 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. The actual random order is subject to change in future versions of perl, but it is guaranteed to be the same order as either the 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";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
To sort a hash 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--256 of them, in fact,
since it rounds up to the next power of two. 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).
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 Signals for details.
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:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
last cannot be used to exit a block which returns a value such as
eval {}, sub {} or do {}, and should not be used to exit a grep() or
map() operation.
See also continue for an illustration of how last, next, and redo work.
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 $_.
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 $_.
Returns the length in characters of the value of EXPR. If EXPR is omitted,
returns length of $_. Note that this cannot be used on an entire array or hash to find out how
many elements these have. For that, use scalar @array and scalar keys %hash respectively.
Creates a new filename linked to the old filename. Returns TRUE for success, FALSE otherwise.
Does the same thing that the listen system call does. Returns TRUE if it succeeded, FALSE otherwise. See the example in Sockets: Client/Server Communication.
You really probably want to be using my() instead, because local() isn't what most people think of as ``local''. See Private Variables via my() for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See Temporary Values via local() for details, including issues with tied arrays and hashes.
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, and not simply the last two digits of the year. If you assume it is, then you
create non-Y2K-compliant programs--and you wouldn't want to do that, would
you?
If EXPR is omitted, uses the current time (localtime(time)).
In 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. To get somewhat similar but locale
dependent date strings, set up your locale environment variables
appropriately (please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
Note that the %a and %b, the short forms of the day of the week and the month of the year, may not
necessarily be three characters wide.
Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns log of $_. To get the log of another base, use basic algebra: The base-N log of a
number is is equal to the natural log of that number divided by the natural
log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also exp for the inverse operation.
Does the same thing as the stat() function (including setting the special _ filehandle) but stats a symbolic link instead of the file the symbolic link
points to. If symbolic links are unimplemented on your system, a normal stat() is done.
If EXPR is omitted, stats $_.
The match operator. See the perlop manpage.
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.
In scalar context, returns the total number of elements so generated.
@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. Using a regular foreach loop for this purpose would be clearer in most cases. See also grep for an array composed of those items of the original list for which the
BLOCK or EXPR evaluates to true.
Creates the directory specified by FILENAME, with permissions specified by
MODE (as modified by umask). If it succeeds it returns TRUE, otherwise it returns FALSE and sets $! (errno).
In general, it is better to create directories with permissive MODEs, and
let the user modify that with their umask, than it is to supply a restrictive MODE and give the user no way to be
more permissive. The exceptions to this rule are when the file or directory
should be kept private (mail files, for instance). The
perlfunc(1) entry on
umask discusses the choice of MODE in more detail.
Calls the System V IPC function msgctl(2). You'll probably
have to say
use IPC::SysV;
first to get the correct constant definitions. 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. See also
IPC::SysV and IPC::Semaphore::Msg documentation.
Calls the System V IPC function msgget(2). Returns the message
queue id, or the undefined value if there is an error. See also IPC::SysV
and IPC::SysV::Msg documentation.
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. See also IPC::SysV
and IPC::SysV::Msg documentation.
Calls the System V IPC function msgrcv to receive a message from message
queue ID into variable VAR with a maximum message size of SIZE. Note that
if a message is received, the message type will be the first thing in VAR,
and the maximum length of VAR is SIZE plus the size of the message type.
Returns TRUE if successful, or FALSE if there is an error. See also IPC::SysV and IPC::SysV::Msg documentation.
A my() declares the listed variables to be local (lexically) to the enclosing block, file, or eval(). If more than one value is listed, the list must be placed in parentheses. See Private Variables via my() for details.
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.
next cannot be used to exit a block which returns a value such as
eval {}, sub {} or do {}, and should not be used to exit a grep() or
map() operation.
See also continue for an illustration of how last, next, and redo work.
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. If EXPR starts off with 0b, it is interpreted as a binary string.) The following will handle decimal,
binary, 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.)
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.) See the perlopentut manpage for a kinder, gentler explanation of opening files.
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, being created if necessary.
If the filename begins with '>>', the file is opened for appending, again being created if necessary. 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. You can't usually use either read-write
mode for updating textfiles, since they have variable length records. See
the -i
switch in the perlrun manpage for a better approach. The file is created with permissions of 0666 modified by the process' umask value.
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 as a command which pipes output to us. See Using open() for IPC
for more examples of this. (You are not allowed to open() to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and Bidirectional Communication 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 binmode 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, MacOS,
and Plan9, which delimit lines with a single character, and which 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 that 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
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 {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$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) 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(OLDOUT, ">&STDOUT");
open(OLDERR, ">&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
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&OLDOUT");