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perlfunc - Perl builtin functions |
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///
m//, pos, quotemeta, s///, split, study, qr//
abs, atan2, cos, exp, hex, int, log, oct, rand,
sin, sqrt, srand
pop, push, shift, splice, unshift
grep, join, map, qw/STRING/, reverse, sort, unpack
delete, each, exists, keys, values
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
pack, read, syscall, sysread, syswrite, unpack, vec
-X, 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
caller, import, local, my, our, package, use
defined, dump, eval, formline, local, my, our, reset,
scalar, undef, wantarray
alarm, exec, fork, getpgrp, getppid, getpriority, kill,
pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
times, wait, waitpid
do, import, no, package, require, use
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
gmtime, localtime, time, times
abs, bless, chomp, chr, exists, formline, glob,
import, lc, lcfirst, map, my, no, our, 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.
dbmclose, dbmopen
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.
$_, 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 (is empty).
-s File has nonzero size (returns size in bytes).
-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 an ASCII 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 (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators -r, -R,
-w, -W, -x, and -X is by default based solely on the mode
of the file and the uids and gids of the user. There may be other
reasons you can't actually read, write, or execute the file. Such
reasons may be for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and unrecognized
executable formats.
Also note that, for the superuser on the local filesystems, the -r,
-R, -w, and -W tests always return 1, and -x and -X return 1
if any execute bit is set in the mode. Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file,
or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called filetest that may
produce more accurate results than the bare stat() mode bits.
When under the use filetest 'access' the above-mentioned filetests
will test whether the permission can (not) be granted using the
access() family of system calls. Also note that the -x and -X may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs). This strangeness is
due to the underlying system calls' definitions. Read the
documentation for the filetest pragma for more information.
Note that -s/a/b/ does not do a negated substitution. Saying
-exp($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 _;
$_.
accept(2) system call
does. Returns the packed address if it succeeded, false otherwise.
See the example in Sockets: Client/Server Communication in the perlipc manpage.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in the perlvar manpage.
$_ 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-argument 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.
(sleep may be internally implemented in your system with alarm)
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 in the perlipc manpage.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For the tangent operation, you may use the Math::Trig::tan
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
":raw" for
binary mode or ":crlf" for ``text'' mode. If the DISCIPLINE is
omitted, it defaults to ":raw".
binmode() should be called after open() but before any I/O is done on
the filehandle.
On many systems binmode() currently has no effect, but in future, it
will be extended to support user-defined input and output disciplines.
On some systems binmode() is necessary when you're not working with a
text file. For the sake of portability it is a good idea to always use
it when appropriate, and to never use it when it isn't appropriate.
In other words: Regardless of platform, use binmode() on binary
files, and do not use binmode() on text files.
The open pragma can be used to establish default disciplines.
See the open manpage.
The operating system, device drivers, C libraries, and Perl run-time
system all work together to let the programmer treat a single
character (\n) as the line terminator, irrespective of the external
representation. On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of \n is made up of more than
one character.
Mac OS and all variants of Unix use a single character to end each line
in the external representation of text (even though that single
character is not necessarily the same across these platforms).
Consequently binmode() has no effect on these operating systems. In
other systems like VMS, MS-DOS and the various flavors of MS-Windows
your program sees a \n as a simple \cJ, but what's stored in text
files are the two characters \cM\cJ. That means that, if you don't
use binmode() on these systems, \cM\cJ sequences on disk will be
converted to \n on input, and any \n in your program will be
converted back to \cM\cJ on output. This is what you want for text
files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that if your binary
data contains \cZ, the I/O subsystem will regard it as the end of
the file, unless you use binmode().
binmode() is not only important for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see the perlport manpage for more details). See the $/ and $\ variables
in the perlvar manpage for how to manually set your input and output
line-termination sequences.
bless is often the last thing in a constructor,
it returns the reference for convenience. Always use the two-argument
version if the function doing the blessing might be inherited by a
derived class. See the perltoot manpage and the perlobj manpage for more about the blessing
(and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.
eval or require, and the undefined value
otherwise. In list context, returns
($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, $hints, $bitmask) = caller($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 an 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. $hasargs is true if a new instance of @_ was set up for the
frame. $hints and $bitmask contain pragmatic hints that the caller
was compiled with. The $hints and $bitmask values are subject to
change between versions of Perl, and are not meant for external use.
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.
$ENV{HOME}, if set; if not,
changes to the directory specified by $ENV{LOGDIR}. If neither is
set, chdir does nothing. It returns true upon success, false
otherwise. See the example under die.
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
You can also import the symbolic S_I* constants from the Fcntl
module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
$/ (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.
When in slurp mode ($/ = undef) or fixed-length record mode ($/ is
a reference to an integer or the like, see the perlvar manpage) chomp() won't
remove anything.
If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
s/.$//s because it neither
scans nor copies the string. If VARIABLE is omitted, chops $_.
If VARIABLE is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the
last chop is returned.
Note that chop returns the last character. To return all but the last
character, use substr($string, 0, -1).
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($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. On POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr(65) is "A" in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face (but only within the scope of
a use utf8). For the reverse, use ord.
See the utf8 manpage for more about Unicode.
If NUMBER is omitted, uses $_.
/ 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 $_.
You don't have to close FILEHANDLE if you are immediately going to do
another open on it, because open will close it for you. (See
open.) However, an explicit close on an input file resets the line
counter ($.), 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 $?.
Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end can't handle that, be sure to read all the data before closing the pipe.
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.
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.
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.
$_.
For the inverse cosine operation, you may use the Math::Trig::acos()
function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt(3) function in the C library
(assuming that you actually have a version there that has not been
extirpated as a potential munition). This can prove useful for checking
the password file for lousy passwords, amongst other things. Only the
guys wearing white hats should do this.
Note that crypt is intended to be a one-way function, much like breaking
eggs to make an omelette. There is no (known) corresponding decrypt
function. As a result, this function isn't all that useful for
cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted
text as the salt (like crypt($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 $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.
untie function.]
Breaks the binding between a DBM file and a hash.
tie function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash. HASH is the name of the hash. (Unlike normal open, the first
argument is not a filehandle, even though it looks like one). DBNAME
is the name of the database (without the .dir or .pag extension if
any). If the database does not exist, it is created with protection
specified by MASK (as modified by the umask). If your system supports
only the older DBM functions, you may perform only one dbmopen in your
program. In older versions of Perl, if your system had neither DBM nor
ndbm, calling dbmopen produced a fatal error; it now falls back to
sdbm(3).
If you don't have write access to the DBM file, you can only read hash
variables, not set them. If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an eval,
which will trap the error.
Note that functions such as keys and values may return huge lists
when used on large DBM files. You may prefer to use the each
function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($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: $!";
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(&func) to check whether subroutine &func
has ever been defined. The return value is unaffected by any forward
declarations of &foo. Note that a subroutine which is not defined
may still be callable: its package may have an AUTOLOAD method that
makes it spring into existence the first time that it is called -- see
the perlsub manpage.
Use of defined on aggregates (hashes and arrays) is deprecated. It
used to report whether memory for that aggregate has ever been
allocated. This behavior may disappear in future versions of Perl.
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" }
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.
element(s) from the hash or array.
In the case of an array, if the array elements happen to be at the end,
the size of the array will shrink to the highest element that tests
true for exists() (or 0 if no such element exists).
Returns each element so deleted or the undefined value if there was no such
element. Deleting from $ENV{} modifies the environment. Deleting from
a hash tied to a DBM file deletes the entry from the DBM file. Deleting
from a tied hash or array may not necessarily return anything.
Deleting an array element effectively returns that position of the array
to its initial, uninitialized state. Subsequently testing for the same
element with exists() will return false. Note that deleting array
elements in the middle of an array will not shift the index of the ones
after them down--use splice() for that. See exists.
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element, array element, hash slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
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 $/ in the perlvar manpage and $. in the perlvar manpage.
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 arbitrary 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
}
}
Because 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} in the perlvar manpage for details on setting %SIG entries, and
eval BLOCK for some examples. Although this feature was meant
to be run only right before your program was to exit, this is not
currently the case--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 in the perlvar manpage). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release.
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.
SUBROUTINE(LIST)
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 in the perlvar manpage 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 $return;
}
}
goto LABEL (with all the restrictions that goto suffers).
Think of it as a goto with an intervening core dump and reincarnation.
If LABEL is omitted, restarts the program from the top.
WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very hard to convert a core file into an executable, and because the real compiler backends for generating portable bytecode and compilable C code have superseded it.
If you're looking to use the dump manpage to speed up your program, consider
generating bytecode or native C code as described in perlcc. If
you're just trying to accelerate a CGI script, consider using the
mod_perl extension to Apache, or the CPAN module, Fast::CGI.
You might also consider autoloading or selfloading, which at least
make your program appear to run faster.
Entries are returned in an apparently random order. The actual random
order is subject to change in future versions of perl, but it is guaranteed
to be in the same order as either the keys or values function
would produce on the same (unmodified) hash.
When the hash is entirely read, a null array is returned in list context
(which when assigned produces a false (0) value), and undef in
scalar context. The next call to each after that will start iterating
again. There is a single iterator for each hash, shared by all each,
keys, and values function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating keys HASH or
values HASH. If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so
don't. Exception: It is always safe to delete the item most recently
returned by each(), which means that the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
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";
}
ungetcs 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. File types such
as terminals may lose the end-of-file condition if you do.
An eof without an argument uses the last file read. Using eof()
with empty parentheses is very different. It refers to the pseudo file
formed from the files listed on the command line and accessed via the
<> operator. Since <> isn't explicitly opened,
as a normal filehandle is, an eof() before <> has been
used will cause @ARGV to be examined to determine if input is
available.
In a while (<>) loop, eof or eof(ARGV) can be used to
detect the end of each file, eof() will only detect the end of the
last file. 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 typically return undef when they run out of data, or if
there was an error.
$_. 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 counterintuitive 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.
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.
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` in the perlop manpage for details.
Using an indirect object with exec or system is also more
secure. This usage (which also works fine with system()) 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.
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before the exec, but this may not be supported on some platforms
(see the perlport manpage). To be safe, you may need to set $| ($AUTOFLUSH
in English) or call the autoflush() method of IO::Handle on any
open handles in order to avoid lost output.
Note that exec will not call your END blocks, nor will it call
any DESTROY methods in your objects.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine,
returns true if the specified subroutine has ever been declared, even
if it is undefined. Mentioning a subroutine name for exists or defined
does not count as declaring it. Note that a subroutine which does not
exist may still be callable: its package may have an AUTOLOAD
method that makes it spring into existence the first time that it is
called -- see the perlsub manpage.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into existence
just because its existence was tested, any intervening ones will.
Thus $ref->{"A"} and $ref->{"A"}->{"B"} will spring
into existence due to the existence test for the $key element above.
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.
See Pseudo-hashes: Using an array as a hash in the perlref manpage for specifics
on how exists() acts when used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an argument
to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
$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 perlmod manpage for details.
exp($_).
fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and
value return works just like ioctl below.
For example:
use Fcntl;
fcntl($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.
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";
}
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 you 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 bitwise-or'ed with
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.
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.
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before forking the child process, but this may not be supported
on some platforms (see the perlport manpage). To be safe, you may need to set
$| ($AUTOFLUSH in English) or call the autoflush() method of
IO::Handle on any open handles in order 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 launched from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue.
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.
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 $^A back to "". Note that a format typically
does one formline per line of form, but the formline function itself
doesn't care how many newlines are embedded in the PICTURE. This means
that the ~ and ~~ tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.
Be careful if you put double quotes around the picture, because an @
character may be taken to mean the beginning of an array name.
formline always returns true. See the perlform manpage for other examples.
if ($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 in the perlmodlib manpage.
getpwuid.
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider getlogin for authentication: it is not as
secure as getpwuid.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
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.
($name,$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.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see the perlsec manpage). The $passwd and $shell, user's encrypted password and login shell, are also tainted, because of the same reason.
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 or if there exists
the shadow(3) functions as found in System V ( this includes Solaris
and Linux.) Those systems which implement a proprietary shadow password
facility are unlikely to be 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 provided
in standard modules: File::stat, Net::hostent, Net::netent,
Net::protoent, Net::servent, Time::gmtime, Time::localtime,
and User::grent. These override the normal built-ins, supplying
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.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
<*.c> operator, but you can use it directly.
If EXPR is omitted, $_ is used. The <*.c> operator is
discussed in more detail in I/O Operators in the perlop manpage.
Beginning with v5.6.0, this operator is implemented using the standard
File::Glob extension. See the File::Glob manpage for details.
# 0 1 2 3 4 5 6 7
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
gmtime(time);
All list elements are numeric, and come straight out of the C `struct
tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
specified time. $mday is the day of the month, and $mon is the month
itself, in the range 0..11 with 0 indicating January and 11
indicating December. $year is the number of years since 1900. That
is, $year is 123 in year 2023. $wday is the day of the week, with
0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
the year, in the range 0..364 (or 0..365 in leap years.)
Note that the $year element is 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?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, gmtime() uses the current time (gmtime(time)).
In scalar context, gmtime() 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);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b escapes, which represent the short forms
of the day of the week and the month of the year, may not necessarily
be three characters wide in all locales.
goto-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop. It
also can't be used to go into a construct that is optimized away,
or to get out of a block or subroutine given to sort.
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as last or die. The author of Perl has never felt the
need to use this form of goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be resolved
dynamically. This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The goto-&NAME form is quite different from the other forms of goto.
In fact, it isn't a goto in the normal sense at all, and doesn't have
the stigma associated with other gotos. Instead, it
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.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference, or a block which evaluates to a code reference.
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 $_ is an alias to the list value, so it can be used to
modify the elements of the LIST. While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
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 a list composed of the results of the BLOCK or EXPR.
$_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a warning.
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.
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.
$_.
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().
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 may be needed to manipulate the values of structures used by
ioctl.
The return value of