-
=head1 NAME
perlfunc - Perl builtin functions
returning the undefined value, and in a list context by returning the
null list.
-Remember the following rule:
-
-=over 8
-
-=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
-
-=back
-
+Remember the following important rule: There is B<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 a scalar context. Some operators return the
-length of the list that would have been returned in a list context. Some
+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 C<(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 C<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 C<$!>
+variable on failure. Other functions do not, except accidentally.
+
=head2 Perl Functions by Category
Here are Perl's functions (including things that look like
-functions, like some of the keywords and named operators)
+functions, like some keywords and named operators)
arranged by category. Some functions appear in more
than one place.
C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
* - C<sub> was a keyword in perl4, but in perl5 it is an
-operator which can be used in expressions.
+operator, which can be used in expressions.
=item Functions obsoleted in perl5
The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
uids and gids of the user. There may be other reasons you can't actually
-read, write or execute the file. Also note that, for the superuser,
+read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
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
while (<>) {
chop;
next unless -f $_; # ignore specials
- ...
+ #...
}
Note that C<-s/a/b/> does not do a negated substitution. Saying
The C<-T> and C<-B> switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
-characters with the high bit set. If too many odd characters (E<gt>30%)
+characters with the high bit set. If too many strange characters (E<gt>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
containing null in the first block is considered a binary file. If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
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 $! set to EINTR because Perl sets up signal handlers to
-restart system calls on some systems. Using eval/die always works.
+fail with C<$!> 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 L<perlipc/"Signals">.
eval {
- local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
+ local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
- die if $@ && $@ ne "alarm\n"; # propagate errors
if ($@) {
+ die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
and similarly archaic systems, it may be imperative--otherwise your
MS-DOS-damaged C library may mangle your file. The key distinction between
systems that need binmode and those that don't is their text file
-formats. Systems like Unix and Plan9 that delimit lines with a single
+formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
character, and that encode that character in C as '\n', do not need
C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
is taken as the name of the filehandle.
is specified, which is often the case. It returns the reference for
convenience, because a bless() is often the last thing in a constructor.
Always use the two-argument version if the function doing the blessing
-might be inherited by a derived class. See L<perlobj> for more about the
-blessing (and blessings) of objects.
+might be inherited by a derived class. See L<perltoot> and L<perlobj>
+for more about the blessing (and blessings) of objects.
=item caller EXPR
=item caller
-Returns the context of the current subroutine call. In a scalar context,
+Returns the context of the current subroutine call. In scalar context,
returns the caller's package name if there is a caller, that is, if
we're in a subroutine or eval() or require(), and the undefined value
-otherwise. In a list context, returns
+otherwise. In list context, returns
($package, $filename, $line) = caller;
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
- ...
+ # ...
}
You can actually chomp anything that's an lvalue, including an assignment:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
- ...
+ #...
}
You can actually chop anything that's an lvalue, including an assignment:
print "User: ";
chop($user = <STDIN>);
- print "Files: "
+ print "Files: ";
chop($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
- @ary = <${pattern}>; # expand filenames
+ @ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the
=item chroot
-This function works as the system call by the same name: it makes the
+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 of its children. (It doesn't
+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 chroot to $_.
+omitted, does a chroot to $_.
=item close FILEHANDLE
If the file handle came from a piped open C<close> will additionally
return FALSE if one of the other system calls involved fails or if the
program exits with non-zero status. (If the only problem was that the
-program exited non-zero $! will be set to 0.) Also, closing a pipe will
-wait for the process executing on the pipe to complete, in case you
+program exited non-zero $! will be set to 0.) Also, closing a pipe
+waits for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards. Closing a pipe
explicitly also puts the exit status value of the command into C<$?>.
+
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
- ... # print stuff to output
+ #... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
-FILEHANDLE may be an expression whose value gives the real filehandle name.
+FILEHANDLE may be an expression whose value can be used as an indirect
+filehandle, usually the real filehandle name.
=item closedir DIRHANDLE
-Closes a directory opened by opendir().
+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.
=item connect SOCKET,NAME
continued via the C<next> statement (which is similar to the C C<continue>
statement).
+C<last>, C<next>, or C<redo> may appear within a C<continue>
+block. C<last> and C<redo> will behave as if they had been executed within
+the main block. So will C<next>, but since it will execute a C<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 C<continue> section is semantically equivalent to using an
+empty one, logically enough. In that case, C<next> goes directly back
+to check the condition at the top of the loop.
+
=item cos EXPR
-Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
+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()
[This function has been superseded by the tie() function.]
-This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
+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 I<NOT> a filehandle, even though it looks like one). DBNAME
is the name of the database (without the F<.dir> or F<.pag> extension if
either use file tests or try setting a dummy hash entry inside an eval(),
which will trap the error.
-Note that functions such as keys() and values() may return huge array
-values when used on large DBM files. You may prefer to use the each()
+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
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.
-You may also use defined() to check whether a subroutine exists. On
-the other hand, use of defined() upon aggregates (hashes and arrays)
-is not guaranteed to produce intuitive results, and should probably be
-avoided.
+You may also use defined() to check whether a subroutine exists, by
+saying C<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 L</exists> for the latter
"ab" =~ /a(.*)b/;
-the pattern match succeeds, and $1 is defined, despite the fact that it
+The pattern match succeeds, and $1 is defined, despite the fact that it
matched "nothing". But it didn't really match nothing--rather, it
matched something that happened to be 0 characters long. This is all
very above-board and honest. When a function returns an undefined value,
if (%a_hash) { print "has hash members\n" }
Using undef() on these, however, does clear their memory and then report
-them as not defined anymore, but you shoudln't do that unless you don't
+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.
+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 behaviour of defined() on aggregates may be
+This counterintuitive behavior of defined() on aggregates may be
changed, fixed, or broken in a future release of Perl.
See also L</undef>, L</exists>, L</ref>.
delete @HASH{keys %HASH}
-(But both of these are slower than the undef() command.) Note that the
-EXPR can be arbitrarily complicated as long as the final operation is a
-hash element lookup or hash slice:
+(But both of these are slower than just assigning the empty list, or
+using undef().) Note that the EXPR can be arbitrarily complicated as
+long as the final operation is a hash element lookup or hash slice:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
=item die LIST
-Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
+Outside an eval(), prints the value of LIST to C<STDERR> and exits with
the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
is 0, exits with 255. Inside an eval(), the error message is stuffed into
-C<$@>, and the eval() is terminated with the undefined value; this makes
+C<$@> and the eval() is terminated with the undefined value. This makes
die() the way to raise an exception.
Equivalent examples:
If $@ is empty then the string "Died" is used.
-You can arrange for a callback to be called just before the die() does
+You can arrange for a callback to be run just before the die() does
its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
will be called with the error text and can change the error message, if
it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
scalar eval `cat stat.pl`;
-except that it's more efficient, more concise, keeps track of the
+except that it's more efficient and concise, keeps track of the
current filename for error messages, and searches all the B<-I>
libraries if the file isn't in the current directory (see also the @INC
array in L<perlvar/Predefined Names>). It is also different in how
do this inside a loop.
Note that inclusion of library modules is better done with the
-use() and require() operators, which also do error checking
+use() and require() operators, which also do automatic error checking
and raise an exception if there's a problem.
+You might like to use C<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 $return;
+ }
+ }
+
=item dump LABEL
This causes an immediate core dump. Primarily this is so that you can
program. When the new binary is executed it will begin by executing a
C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
it as a goto with an intervening core dump and reincarnation. If LABEL
-is omitted, restarts the program from the top. WARNING: any files
+is omitted, restarts the program from the top. WARNING: Any files
opened at the time of the dump will NOT be open any more when the
program is reincarnated, with possible resulting confusion on the part
of Perl. See also B<-u> option in L<perlrun>.
QUICKSTART:
Getopt('f');
+This operator is largely obsolete, partly because it's very hard to
+convert a core file into an executable, and because the real perl-to-C
+compiler has superseded it.
+
=item each HASH
-When called in a list context, returns a 2-element array consisting of the
+When called in list context, returns a 2-element list consisting of the
key and value for the next element of a hash, so that you can iterate over
-it. When called in a scalar context, returns the key for only the next
+it. When called in scalar context, returns the key for only the "next"
element in the hash. (Note: Keys may be "0" or "", which are logically
false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
for this reason.)
Entries are returned in an apparently random order. When the hash is
entirely read, a null array is returned in list context (which when
-assigned produces a FALSE (0) value), and C<undef> is returned in a
+assigned produces a FALSE (0) value), and C<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
Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open. FILEHANDLE may be an expression whose value
-gives the real filehandle name. (Note that this function actually
-reads a character and then ungetc()s it, so it is not very useful in an
+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
C<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 C<eof> without an argument uses the last file read as argument.
-Empty parentheses () may be used to indicate the pseudo file formed of
+Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
the files listed on the command line, i.e., C<eof()> is reasonable to
use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
last file. Use C<eof(ARGV)> or eof without the parentheses to test
# reset line numbering on each input file
while (<>) {
+ next if /^\s*#/; # skip comments
print "$.\t$_";
- close(ARGV) if (eof); # Not eof().
+ } continue {
+ close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
- if (eof()) {
+ if (eof()) { # check for end of current file
print "--------------\n";
close(ARGV); # close or break; is needed if we
# are reading from the terminal
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators return undef when they run out of data.
+input operators return C<undef> when they run out of data.
=item eval EXPR
In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program. The value of the expression (which is itself
-determined within a scalar context) is first parsed, and if there are no
+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
the BLOCK.
In both forms, the value returned is the value of the last expression
-evaluated inside the mini-program, or a return statement may be used, just
+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 array context, depending on the context of the eval itself.
+in void, scalar, or list context, depending on the context of the eval itself.
See L</wantarray> for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a die() statement is
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
- eval { $answer = };
+ eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
eval '$x'; # CASE 3
eval { $x }; # CASE 4
- eval "\$$x++" # CASE 5
+ eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in
=item exec LIST
+=item exec PROGRAM LIST
+
The exec() function executes a system command I<AND NEVER RETURNS> -
use system() instead of exec() if you want it to return. It fails and
returns FALSE only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).
-If there is more than one argument in LIST, or if LIST is an array with
-more than one value, calls execvp(3) with the arguments in LIST. If
-there is only one scalar argument, the argument is checked for shell
-metacharacters, and if there are any, the entire argument is passed to
-the system's command shell for parsing (this is C</bin/sh -c> on Unix
-platforms, but varies on other platforms). If there are no shell
-metacharacters in the argument, it is split into words and passed
-directly to execvp(), which is more efficient. Note: exec() and
-system() do not flush your output buffer, so you may need to set C<$|>
-to avoid lost output. Examples:
+Since it's a common mistake to use system() instead of exec(), Perl
+warns you if there is a following statement which isn't die(), warn(),
+or exit() (if C<-w> is set - but you always do that). If you
+I<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 C</bin/sh -c> on Unix platforms, but varies on other platforms).
+If there are no shell metacharacters in the argument, it is split into
+words and passed directly to execvp(), which is more efficient. Note:
+exec() and system() do not flush your output buffer, so you may need to
+set C<$|> to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
be subject to its quirks and capabilities. See L<perlop/"`STRING`">
for details.
+Using an indirect object with C<exec> or C<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" );
+
+ system @args; # subject to shell escapes if @args == 1
+ system { $args[0] } @args; # safe even with one-arg list
+
+The first version, the one without the indirect object, ran the I<echo>
+program, passing it C<"surprise"> an argument. The second version
+didn't--it tried to run a program literally called I<"echo surprise">,
+didn't find it, and set C<$?> to a non-zero value indicating failure.
+
=item exists EXPR
Returns TRUE if the specified hash key exists in its hash array, even
Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:
- if (exists $ref->[$x][$y]{$key}) { ... }
+ if (exists $ref->{"A"}{"B"}{$key}) { ... }
+
+Although the last element will not spring into existence just because its
+existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
+C<$ref-E<gt>{"B"}> will spring into existence due to the existence
+test for a $key element. This autovivification may be fixed in a later
+release.
=item exit EXPR
someone might want to trap whatever error happened. Use die() instead,
which can be trapped by an eval().
+All C<END{}> blocks are run at exit time. See L<perlsub> for details.
+
=item exp EXPR
=item exp
use Fcntl;
first to get the correct function definitions. Argument processing and
-value return works just like ioctl() below. Note that fcntl() will produce
-a fatal error if used on a machine that doesn't implement fcntl(2).
+value return works just like ioctl() below.
For example:
use Fcntl;
- fcntl($filehandle, F_GETLK, $packed_return_buffer);
+ fcntl($filehandle, F_GETFL, $packed_return_buffer)
+ or die "can't fcntl F_GETFL: $!";
+
+You don't have to check for C<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 B<-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).
=item fileno FILEHANDLE
Returns the file descriptor for a filehandle. This is useful for
-constructing bitmaps for select(). If FILEHANDLE is an expression, the
-value is taken as the name of the filehandle.
+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";
+ }
=item flock FILEHANDLE,OPERATION
is Perl's portable file locking interface, although it locks only entire
files, not records.
+On many platforms (including most versions or clones of Unix), locks
+established by flock() are B<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(). Windows NT and OS/2 are among the platforms which
+enforce mandatory locking. See your local documentation for details.
+
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,
=item fork
-Does a fork(2) system call. Returns the child pid to the parent process
-and 0 to the child process, or C<undef> if the fork is unsuccessful.
+Does a fork(2) system call. Returns the child pid to the parent process,
+0 to the child process, or C<undef> if the fork is unsuccessful.
+
Note: unflushed buffers remain unflushed in both processes, which means
you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
method of IO::Handle to avoid duplicate output.
Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
-if you exit, the remote server (such as, say, httpd or rsh) won't think
+if you exit, then the remote server (such as, say, httpd or rsh) won't think
you're done. You should reopen those to /dev/null if it's any issue.
=item format
-Declare a picture format with use by the write() function. For
+Declare a picture format for use by the write() function. For
example:
format Something =
See L<perlform> for many details and examples.
-
=item formline PICTURE,LIST
-This is an internal function used by C<format>s, though you may call it
+This is an internal function used by C<format>s, though you may call it,
too. It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or $ACCUMULATOR in English).
is left as an exercise to the reader.
The POSIX::getattr() function can do this more portably on systems
-alleging POSIX compliance.
+purporting POSIX compliance.
See also the C<Term::ReadKey> module from your nearest CPAN site;
details on CPAN can be found on L<perlmod/CPAN>.
=item getlogin
-Returns the current login from F</etc/utmp>, if any. If null, use
-getpwuid().
+Implements the C library function of the same name, which on most
+systems returns the current login from F</etc/utmp>, if any. If null,
+use getpwuid().
$login = getlogin || getpwuid($<) || "Kilroy";
=item endservent
These routines perform the same functions as their counterparts in the
-system library. Within a list context, the return values from 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) = getpw*
+ $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
(If the entry doesn't exist you get a null list.)
-Within a scalar context, you get the name, unless the function was a
+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 = getpwuid
- $name = getpwent
- $gid = getgrnam
- $name = getgrgid
- $name = getgrent
- etc.
+ $uid = getpwnam($name);
+ $name = getpwuid($num);
+ $name = getpwent();
+ $gid = getgrnam($name);
+ $name = getgrgid($num;
+ $name = getgrent();
+ #etc.
+
+In I<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 which meaning
+your $quota and $comment fields have 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.
The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.
($a,$b,$c,$d) = unpack('C4',$addr[0]);
+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.
+
=item getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection.
=item getsockopt SOCKET,LEVEL,OPTNAME
-Returns the socket option requested, or undefined if there is an error.
+Returns the socket option requested, or undef if there is an error.
=item glob EXPR
=item glob
-Returns the value of EXPR with filename expansions such as a shell would
+Returns the value of EXPR with filename expansions such as the standard Unix shell /bin/sh would
do. This is the internal function implementing the C<E<lt>*.cE<gt>>
operator, but you can use it directly. If EXPR is omitted, $_ is used.
The C<E<lt>*.cE<gt>> operator is discussed in more detail in
If EXPR is omitted, does C<gmtime(time())>.
-In a scalar context, returns the ctime(3) value:
+In scalar context, returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
to modify the elements of the array. While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array. Similarly, grep returns aliases into the original list,
-much like the way that L<Foreach Loops>'s index variable aliases the list
+much like the way that a for loops's index variable aliases the list
elements. That is, modifying an element of a list returned by grep
(for example, in a C<foreach>, C<map> or another C<grep>)
actually modifies the element in the original list.
=item import
-There is no builtin import() function. It is merely an ordinary
+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 L</use()>, L<perlmod>, and L<Exporter>.
=item int
Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
+You should not use this for rounding, because it truncates
+towards 0, and because machine representations of floating point
+numbers can sometimes produce counterintuitive results. Usually sprintf() or printf(),
+or the POSIX::floor or POSIX::ceil functions, would serve you better.
=item ioctl FILEHANDLE,FUNCTION,SCALAR
first to get the correct function definitions. If F<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 F<E<lt>sys/ioctl.hE<gt>>.
-(There is a Perl script called B<h2ph> that comes with the Perl kit which
+(There is a Perl script called B<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
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
+The special string "0 but true" is excempt from B<-w> complaints
+about improper numeric conversions.
+
=item join EXPR,LIST
Joins the separate strings of LIST into a single string with
=item keys HASH
-Returns a normal array consisting of all the keys of the named hash. (In
-a scalar context, returns the number of keys.) The keys are returned in
+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, but it is the same order as either the
values() or each() function produces (given that the hash has not been
modified). As a side effect, it resets HASH's iterator.
To sort an array by value, you'll need to use a C<sort> function.
Here's a descending numeric sort of a hash by its values:
- foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
+ foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
keys %hash = 200;
-then C<%hash> will have at least 200 buckets allocated for it. These
+then C<%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 C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
- ...
+ #...
}
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
=item lc EXPR
=item lc
=item length
-Returns the length in characters of the value of EXPR. If EXPR is
+Returns the length in bytes of the value of EXPR. If EXPR is
omitted, returns length of $_.
=item link OLDFILE,NEWFILE
-Creates a new filename linked to the old filename. Returns 1 for
-success, 0 otherwise.
+Creates a new filename linked to the old filename. Returns TRUE for
+success, FALSE otherwise.
=item listen SOCKET,QUEUESIZE
=item local EXPR
-A local modifies the listed variables to be local to the enclosing block,
-subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
-list must be placed in parentheses. See L<perlsub/"Temporary Values via
-local()"> for details, including issues with tied arrays and hashes.
+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 L<perlsub/"Temporary Values via local()">
+for details, including issues with tied arrays and hashes.
-But you really probably want to be using my() instead, because local() isn't
-what most people think of as "local"
). See L<perlsub/"Private Variables
+You really probably want to be using my() instead, because local() isn't
+what most people think of as "local". See L<perlsub/"Private Variables
via my()"> for details.
=item localtime EXPR
If EXPR is omitted, uses the current time (C<localtime(time)>).
-In a scalar context, returns the ctime(3) value:
+In scalar context, returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
-This scalar value is B<not> locale dependent, see L<perllocale>,
-but instead a Perl builtin.
-Also see the Time::Local module, and the strftime(3) and mktime(3)
-function available via the POSIX module.
+This scalar value is B<not> locale dependent, see L<perllocale>, but
+instead a Perl builtin. Also see the Time::Local module, and the
+strftime(3) and mktime(3) function available via the POSIX module. To
+get somewhat similar but locale dependent date strings, set up your
+locale environment variables appropriately (please see L<perllocale>)
+and try for example:
+
+ use POSIX qw(strftime);
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
+
+Note that the C<%a> and C<%b>, the short forms of the day of the week
+and the month of the year, may not necessarily be three characters wide.
=item log EXPR
=item log
-Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
+Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
of $_.
=item lstat FILEHANDLE
=item lstat
-Does the same thing as the stat() function, but stats a symbolic link
-instead of the file the symbolic link points to. If symbolic links are
-unimplemented on your system, a normal stat() is done.
+Does the same thing as the stat() function (including setting the
+special C<_> 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 $_.
=item mkdir FILENAME,MODE
Creates the directory specified by FILENAME, with permissions specified
-by MODE (as modified by umask). If it succeeds it returns 1, otherwise
-it returns 0 and sets C<$!> (errno).
+by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
+it returns FALSE and sets C<$!> (errno).
=item msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
-must be a variable which will hold the returned msqid_ds structure.
+must be a variable that 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.
=item my EXPR
A "my" declares the listed variables to be local (lexically) to the
-enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
+enclosing block, file, or C<eval>. If
more than one value is listed, the list must be placed in parentheses. See
L<perlsub/"Private Variables via my()"> for details.
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
- ...
+ #...
}
Note that if there were a C<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.
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
=item no Module LIST
See the "use" function, which "no" is the opposite of.
If the filename begins with '<' or nothing, the file is opened for input.
If the filename begins with '>', the file is truncated and opened for
-output. If the filename begins with '>>', the file is opened for
-appending. You can put a '+' in front of the '>' or '<' to indicate that
+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. The prefix and the filename may be separated with spaces.
+file first. You can't usually use either read-write mode for updating
+textfiles, since they have variable length records. See the B<-i>
+switch in L<perlrun> for a better approach.
+
+The prefix and the filename may be separated with spaces.
These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
'w+', 'a', and 'a+'.
-If the filename begins with "|", the filename is interpreted as a command
-to which output is to be piped, and if the filename ends with a "|", the
-filename is interpreted See L<perlipc/"Using open() for IPC"> for more
-examples of this. as command which pipes input to us. (You may not have
-a raw open() to a command that pipes both in I<and> out, but see
-L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
-for alternatives.)
+If the filename begins with "|", the filename is interpreted as a
+command to which output is to be piped, and if the filename ends with a
+"|", the filename is interpreted See L<perlipc/"Using open() for IPC">
+for more examples of this. (You are not allowed to open() to a command
+that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
+and L<perlipc/"Bidirectional Communication"> for alternatives.)
Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
nonzero upon success, the undefined value otherwise. If the open
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</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 and
-Plan9 that delimit lines with a single character, and that encode that
+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 C<binmode>. The rest need it.
When opening a file, it's usually a bad idea to continue normal execution
if the request failed, so C<open> is frequently used in connection with
C<die>. Even if C<die> won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
-modules which can help with that problem)) you should always check
+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.
}
sub process {
- local($filename, $input) = @_;
+ 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
+ #... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR beginning
with "E<gt>&", in which case the rest of the string is interpreted as the
-name of a filehandle (or file descriptor, if numeric) which is to be
+name of a filehandle (or file descriptor, if numeric) to be
duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
+E<gt>E<gt>, and +E<lt>. The
mode you specify should match the mode of the original filehandle.
STDERR:
#!/usr/bin/perl
- open(SAVEOUT, ">&STDOUT");
- open(SAVEERR, ">&STDERR");
+ open(OLDOUT, ">&STDOUT");
+ open(OLDERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
close(STDOUT);
close(STDERR);
- open(STDOUT, ">&SAVEOUT");
- open(STDERR, ">&SAVEERR");
+ open(STDOUT, ">&OLDOUT");
+ open(STDERR, ">&OLDERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
See L<perlipc/"Safe Pipe Opens"> for more examples of this.
-NOTE: On any operation which may do a fork, unflushed buffers remain
+NOTE: On any operation that may do a fork, any unflushed buffers remain
unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.
Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?>.
+The filename passed to open will have leading and trailing
+whitespace deleted, and the normal redirection chararacters
+honored. This property, known as "magic open",
+can often be used to good effect. A user could specify a filename of
+"rsh cat file |", or you could change certain filenames as needed:
+
+ $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
+ open(FH, $filename) or die "Can't open $filename: $!";
+
+However, to open a file with arbitrary weird characters in it, it's
+necessary to protect any leading and trailing whitespace:
+
+ $file =~ s#^(\s)#./$1#;
+ open(FOO, "< $file\0");
+
+If you want a "real" C open() (see L<open(2)> on your system), then you
+should use the sysopen() function, which involves no such magic. This is
+another way to protect your filenames from interpretation. For example:
+
+ use IO::Handle;
+ sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
+ or die "sysopen $path: $!";
+ $oldfh = select(HANDLE); $| = 1; select($oldfh);
+ print HANDLE "stuff $$\n");
+ seek(HANDLE, 0, 0);
+ print "File contains: ", <HANDLE>;
+
Using the constructor from the IO::Handle package (or one of its
-subclasses, such as IO::File or IO::Socket),
-you can generate anonymous filehandles which have the scope of whatever
-variables hold references to them, and automatically close whenever
-and however you leave that scope:
+subclasses, such as IO::File or IO::Socket), you can generate anonymous
+filehandles that have the scope of whatever variables hold references to
+them, and automatically close whenever and however you leave that scope:
use IO::File;
- ...
+ #...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
$first; # Or here.
}
-The filename that is passed to open will have leading and trailing
-whitespace deleted. To open a file with arbitrary weird
-characters in it, it's necessary to protect any leading and trailing
-whitespace thusly:
-
- $file =~ s#^(\s)#./$1#;
- open(FOO, "< $file\0");
-
-If you want a "real" C open() (see L<open(2)> on your system), then
-you should use the sysopen() function. This is another way to
-protect your filenames from interpretation. For example:
-
- use IO::Handle;
- sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
- or die "sysopen $path: $!";
- HANDLE->autoflush(1);
- HANDLE->print("stuff $$\n");
- seek(HANDLE, 0, 0);
- print "File contains: ", <HANDLE>;
-
See L</seek()> for some details about mixing reading and writing.
=item opendir DIRHANDLE,EXPR
X Back up a byte.
@ Null fill to absolute position.
-Each letter may optionally be followed by a number which gives a repeat
+Each letter may optionally be followed by a number giving a repeat
count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
pack function will gobble up that many values from the LIST. A * for the
repeat count means to use however many items are left. The "a" and "A"
The same template may generally also be used in the unpack function.
+=item package
+
=item package NAMESPACE
Declares the compilation unit as being in the given namespace. The scope
local() on--but I<not> lexical variables created with my(). Typically it
would be the first declaration in a file to be included by the C<require>
or C<use> operator. You can switch into a package in more than one place;
-it influences merely which symbol table is used by the compiler for the
+it merely influences which symbol table is used by the compiler for the
rest of that block. You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a double
colon: C<$Package::Variable>. If the package name is null, the C<main>
package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
+If NAMESPACE is omitted, then there is no current package, and all
+identifiers must be fully qualified or lexicals. This is stricter
+than C<use strict>, since it also extends to function names.
+
See L<perlmod/"Packages"> for more information about packages, modules,
and classes. See L<perlsub> for other scoping issues.
interpose a + or put parentheses around the arguments.) If FILEHANDLE is
omitted, prints by default to standard output (or to the last selected
output channel--see L</select>). If LIST is also omitted, prints $_ to
-STDOUT. To set the default output channel to something other than
+the currently selected output channel. To set the default output channel to something other than
STDOUT use the select operation. Note that, because print takes a
-LIST, anything in the LIST is evaluated in a list context, and any
+LIST, anything in the LIST is evaluated in list context, and any
subroutine that you call will have one or more of its expressions
-evaluated in a list context. Also be careful not to follow the print
+evaluated in list context. Also be careful not to follow the print
keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--interpose a + or
put parentheses around all the arguments.
=item printf FORMAT, LIST
-Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
+Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
+(the output record separator) is not appended. The first argument
of the list will be interpreted as the printf format. If C<use locale> is
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale. See L<perllocale>.
Don't fall into the trap of using a printf() when a simple
-print() would do. The print() is more efficient, and less
+print() would do. The print() is more efficient and less
error prone.
=item prototype FUNCTION
undef if there was an error. SCALAR will be grown or shrunk to the
length actually read. An OFFSET may be specified to place the read
data at some other place than the beginning of the string. This call
-is actually implemented in terms of stdio's fread call. To get a true
-read system call, see sysread().
+is actually implemented in terms of stdio's fread(3) call. To get a true
+read(2) system call, see sysread().
=item readdir DIRHANDLE
Returns the next directory entry for a directory opened by opendir().
-If used in a list context, returns all the rest of the entries in the
+If used in list context, returns all the rest of the entries in the
directory. If there are no more entries, returns an undefined value in
-a scalar context or a null list in a list context.
+scalar context or a null list in list context.
If you're planning to filetest the return values out of a readdir(), you'd
better prepend the directory in question. Otherwise, because we didn't
=item readline EXPR
-Reads from the file handle EXPR. In scalar context, a single line
+Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
is read and returned. In list context, reads until end-of-file is
reached and returns a list of lines (however you've defined lines
with $/ or $INPUT_RECORD_SEPARATOR).
operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
operator is discussed in more detail in L<perlop/"I/O Operators">.
+ $line = <STDIN>;
+ $line = readline(*STDIN); # same thing
+
=item readlink EXPR
=item readlink
=item readpipe EXPR
-EXPR is interpolated and then executed as a system command.
+EXPR is executed as a system command.
The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially
multi-line) string. In list context, returns a list of lines
Receives a message on a socket. Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.
-Actually does a C recvfrom(), so that it can returns the address of the
+Actually does a C recvfrom(), so that it can return the address of the
sender. Returns the undefined value if there's an error. SCALAR will
be grown or shrunk to the length actually read. Takes the same flags
as the system call of the same name.
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
- s|^|$front{|;
+ s|^|$front\{|;
redo LINE;
}
}
print;
}
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
=item ref EXPR
=item ref
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
- if (!ref ($r) {
+ if (!ref($r)) {
print "r is not a reference at all.\n";
}
subroutine:
sub require {
- local($filename) = @_;
+ my($filename) = @_;
return 1 if $INC{$filename};
- local($realfilename,$result);
+ my($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
- $result;
+ return $result;
}
Note that the file will not be included twice under the same specified
to make it easy to load standard modules. This form of loading of
modules does not risk altering your namespace.
-For a yet-more-powerful import facility, see L</use> and
-L<perlmod>.
+In other words, if you try this:
+
+ require Foo::Bar; # a splendid bareword
+
+The require function will actually look for the "Foo/Bar.pm" file in the
+directories specified in the @INC array.
+
+But if you try this:
+
+ $class = 'Foo::Bar';
+ require $class; # $class is not a bareword
+ #or
+ require "Foo::Bar"; # not a bareword because of the ""
+
+The require function will look for the "Foo::Bar" file in the @INC array and
+will complain about not finding "Foo::Bar" there. In this case you can do:
+
+ eval "require $class";
+
+For a yet-more-powerful import facility, see L</use> and L<perlmod>.
=item reset EXPR
=item return
-Returns from a subroutine, eval(), or do FILE with the value of the
-given EXPR. Evaluation of EXPR may be in a list, scalar, or void
+Returns from a subroutine, eval(), or C<do FILE> with the value
+given in EXPR. Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see wantarray()). If no EXPR
-is given, returns an empty list in a list context, an undefined value in
-a scalar context, or nothing in a void context.
+is given, returns an empty list in list context, an undefined value in
+scalar context, or nothing in a void context.
(Note that in the absence of a return, a subroutine, eval, or do FILE
will automatically return the value of the last expression evaluated.)
=item reverse LIST
-In a list context, returns a list value consisting of the elements
-of LIST in the opposite order. In a scalar context, concatenates the
+In list context, returns a list value consisting of the elements
+of LIST in the opposite order. In scalar context, concatenates the
elements of LIST, and returns a string value consisting of those bytes,
but in the opposite order.
=item rmdir
-Deletes the directory specified by FILENAME if it is empty. If it
-succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
+Deletes the directory specified by FILENAME if that directory is empty. If it
+succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
FILENAME is omitted, uses $_.
=item s///
=item scalar EXPR
-Forces EXPR to be interpreted in a scalar context and returns the value
+Forces EXPR to be interpreted in scalar context and returns the value
of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to
-be interpolated in a list context because it's in practice never
+be interpolated in list context because it's in practice never
needed. If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.
subroutine:
sub fhbits {
- local(@fhlist) = split(' ',$_[0]);
- local($bits);
+ my(@fhlist) = split(' ',$_[0]);
+ my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in $timeleft, so
-calling select() in a scalar context just returns $nfound.
+calling select() in scalar context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is
in seconds, which may be fractional. Note: not all implementations are
select(undef, undef, undef, 0.25);
-B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
-with select(). You have to use sysread() instead.
+B<WARNING>: One should not attempt to mix buffered I/O (like read()
+or E<lt>FHE<gt>) with select(), except as permitted by POSIX, and even
+then only on POSIX systems. You have to use sysread() instead.
=item semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl. If CMD is &IPC_STAT or
-&GETALL, then ARG must be a variable which will hold the returned
+&GETALL, then ARG must be a variable that will hold the returned
semid_ds structure or semaphore value array. Returns like ioctl: the
undefined value for error, "0 but true" for zero, or the actual return
value otherwise.
=item shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
-must be a variable which will hold the returned shmid_ds structure.
+must be a variable that will hold the returned shmid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.
Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
-detaching from it. When reading, VAR must be a variable which will
+detaching from it. When reading, VAR must be a variable that will
hold the data read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the system call of the same name.
+ shutdown(SOCKET, 0); # I/we have stopped reading data
+ shutdown(SOCKET, 1); # I/we have stopped writing data
+ shutdown(SOCKET, 2); # I/we have stopped using this socket
+
+This is useful with sockets when you want to tell the other
+side you're done writing but not done reading, or vice versa.
+It's also a more insistent form of close because it also
+disables the filedescriptor in any forked copies in other
+processes.
+
=item sin EXPR
=item sin
=item sleep
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
-May be interrupted by sending the process a SIGALRM. Returns the
-number of seconds actually slept. You probably cannot mix alarm() and
-sleep() calls, because sleep() is often implemented using alarm().
+May be interrupted if the process receives a signal such as SIGALRM.
+Returns the number of seconds actually slept. You probably cannot
+mix alarm() and sleep() calls, because sleep() is often implemented
+using alarm().
On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds. Most modern systems
-always sleep the full amount.
+always sleep the full amount. They may appear to sleep longer than that,
+however, because your process might not be scheduled right away in a
+busy multitasking system.
For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if your system supports it,
-or else see L</select()> below.
+or else see L</select()> above.
See also the POSIX module's sigpause() function.
for the system call of the same name. If unimplemented, yields a fatal
error. Returns TRUE if successful.
+Some systems defined pipe() in terms of socketpair, in which a call
+to C<pipe(Rdr, Wtr)> is essentially:
+
+ use Socket;
+ socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
+ shutdown(Rdr, 1); # no more writing for reader
+ shutdown(Wtr, 0); # no more reading for writer
+
+See L<perlipc> for an example of socketpair use.
+
=item sort SUBNAME LIST
=item sort BLOCK LIST
less than, equal to, or greater than 0, depending on how the elements
of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
operators are extremely useful in such routines.) SUBNAME may be a
-scalar variable name, in which case the value provides the name of the
-subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
-an anonymous, in-line sort subroutine.
+scalar variable name (unsubscripted), in which case the value provides
+the name of (or a reference to) the actual subroutine to use. In place
+of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
+subroutine.
In the interests of efficiency the normal calling code for subroutines is
bypassed, with the following effects: the subroutine may not be a
=item splice ARRAY,OFFSET
Removes the elements designated by OFFSET and LENGTH from an array, and
-replaces them with the elements of LIST, if any. Returns the elements
-removed from the array. The array grows or shrinks as necessary. If
-LENGTH is omitted, removes everything from OFFSET onward. The
-following equivalences hold (assuming C<$[ == 0>):
+replaces them with the elements of LIST, if any. In list context,
+returns the elements removed from the array. In scalar context,
+returns the last element removed, or C<undef> if no elements are
+removed. The array grows or shrinks as necessary. If LENGTH is
+omitted, removes everything from OFFSET onward. The following
+equivalences hold (assuming C<$[ == 0>):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
- $a[$x] = $y splice(@a,$x,1,$y);
+ $a[$x] = $y splice(@a,$x,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
- local(@a) = splice(@_,0,shift);
- local(@b) = splice(@_,0,shift);
+ my(@a) = splice(@_,0,shift);
+ my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
=item split
-Splits a string into an array of strings, and returns it.
+Splits a string into an array of strings, and returns it. By default,
+empty leading fields are preserved, and empty trailing ones are deleted.
-If not in a list context, returns the number of fields found and splits into
-the @_ array. (In a list context, you can force the split into @_ by
-using C<??> as the pattern delimiters, but it still returns the array
-value.) The use of implicit split to @_ is deprecated, however.
+If not in list context, returns the number of fields found and splits into
+the @_ array. (In list context, you can force the split into @_ by
+using C<??> as the pattern delimiters, but it still returns the list
+value.) The use of implicit split to @_ is deprecated, however, because
+it clobbers your subroutine arguments.
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace). Anything
matching PATTERN is taken to be a delimiter separating the fields. (Note
that the delimiter may be longer than one character.)
-If LIMIT is specified and is not negative, splits into no more than
-that many fields (though it may split into fewer). If LIMIT is
-unspecified, trailing null fields are stripped (which potential users
+If LIMIT is specified and positive, splits into no more than that
+many fields (though it may split into fewer). If LIMIT is unspecified
+or zero, trailing null fields are stripped (which potential users
of pop() would do well to remember). If LIMIT is negative, it is
treated as if an arbitrarily large LIMIT had been specified.
Example:
- open(passwd, '/etc/passwd');
- while (<passwd>) {
- ($login, $passwd, $uid, $gid, $gcos,
- $home, $shell) = split(/:/);
- ...
+ open(PASSWD, '/etc/passwd');
+ while (<PASSWD>) {
+ ($login, $passwd, $uid, $gid, $gcos,$home, $shell) = split(/:/);
+ #...
}
(Note that $shell above will still have a newline on it. See L</chop>,
C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
on your system for an explanation of the general principles.
-Perl does all of its own sprintf() formatting -- it emulates the C
+Perl does its own sprintf() formatting -- it emulates the C
function sprintf(), but it doesn't use it (except for floating-point
numbers, and even then only the standard modifiers are allowed). As a
result, any non-standard extensions in your local sprintf() are not
+ prefix positive number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
- # prefix octal with "0", hex with "0x"
+ # prefix non-zero octal with "0", non-zero hex with "0x"
number minimum field width
.number "precision": digits after decimal point for floating-point,
max length for string, minimum length for integer
ID, among other things. In versions of Perl prior to 5.004 the default
seed was just the current time(). This isn't a particularly good seed,
so many old programs supply their own seed value (often C<time ^ $$> or
-C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
+C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
In fact, it's usually not necessary to call srand() at all, because if
it is not called explicitly, it is called implicitly at the first use of
=item stat
-Returns a 13-element array giving the status info for a file, either the
-file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
-stats $_. Returns a null list if the stat fails. Typically used as
-follows:
-
+Returns a 13-element list giving the status info for a file, either
+the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
+it stats $_. Returns a null list if the stat fails. Typically used
+as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
(This works on machines only for which the device number is negative under NFS.)
+In scalar context, C<stat> returns a boolean value indicating success
+or failure, and, if successful, sets the information associated with
+the special filehandle C<_>.
+
=item study SCALAR
=item study
constructed from some C programs and English text. Only those places
that contain this "rarest" character are examined.)
-For example, here is a loop which inserts index producing entries
+For example, here is a loop that inserts index producing entries
before any line containing a certain pattern:
while (<>) {
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
- ...
+ # ...
print;
}
-In searching for /\bfoo\b/, only those locations in $_ that contain "f"
+In searching for C</\bfoo\b/>, only those locations in $_ that contain "f"
will be looked at, because "f" is rarer than "o". In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
value: the CODE ref of the closure you just created. See L<perlsub> and
L<perlref> for details.
+=item substr EXPR,OFFSET,LEN,REPLACEMENT
+
=item substr EXPR,OFFSET,LEN
=item substr EXPR,OFFSET
everything to the end of the string. If LEN is negative, leaves that
many characters off the end of the string.
-If you specify a substring which is partly outside the string, the part
+If you specify a substring that is partly outside the string, the part
within the string is returned. If the substring is totally outside
the string a warning is produced.
keep the string the same length you may need to pad or chop your value
using sprintf().
+An alternative to using substr() as an lvalue is to specify the
+replacement string as the 4th argument. This allows you to replace
+parts of the EXPR and return what was there before in one operation.
+In this case LEN can be C<undef> if you want to affect everything to
+the end of the string.
+
=item symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename.
symbolic links, produces a fatal error at run time. To check for that,
use eval:
- $symlink_exists = (eval {symlink("","")};, $@ eq '');
+ $symlink_exists = eval { symlink("",""); 1 };
=item syscall LIST
as follows: if a given argument is numeric, the argument is passed as
an int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to
-receive any result that might be written into a string. If your
+receive any result that might be written into a string. You can't use a
+string literal (or other read-only string) as an argument to syscall()
+because Perl has to assume that any string pointer might be written
+through. If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add 0 to them to force them to look
-like numbers.
+like numbers. This emulates the syswrite() function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
- syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
+ $s = "hi there\n";
+ syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.
If the system call fails, syscall returns -1 and sets C<$!> (errno).
Note that some system calls can legitimately return -1. The proper
way to handle such calls is to assign C<$!=0;> before the call and
-check the value of <$!> if syscall returns -1.
+check the value of C<$!> if syscall returns -1.
There's a problem with C<syscall(&SYS_pipe)>: it returns the file
number of the read end of the pipe it creates. There is no way
However, for historical reasons, some values are universal: zero means
read-only, one means write-only, and two means read/write.
-If the file named by FILENAME does not exist and the C<open> call
-creates it (typically because MODE includes the O_CREAT flag), then
-the value of PERMS specifies the permissions of the newly created
-file. If PERMS is omitted, the default value is 0666, which allows
-read and write for all. This default is reasonable: see C<umask>.
+If the file named by FILENAME does not exist and the C<open> call creates
+it (typically because MODE includes the O_CREAT flag), then the value of
+PERMS specifies the permissions of the newly created file. If you omit
+the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
+These permission values need to be in octal, and are modified by your
+process's current C<umask>. The C<umask> value is a number representing
+disabled permissions bits--if your C<umask> were 027 (group can't write;
+others can't read, write, or execute), then passing C<sysopen> 0666 would
+create a file with mode 0640 (C<0666 &~ 027> is 0640).
+
+If you find this C<umask> talk confusing, here's some advice: supply a
+creation mode of 0666 for regular files and one of 0777 for directories
+(in C<mkdir>) and executable files. This gives users the freedom of
+choice: if they want protected files, they might choose process umasks
+of 022, 027, or even the particularly antisocial mask of 077. Programs
+should rarely if ever make policy decisions better left to the user.
+The exception to this is when writing files that should be kept private:
+mail files, web browser cookies, I<.rhosts> files, and so on. In short,
+seldom if ever use 0644 as argument to C<sysopen> because that takes
+away the user's option to have a more permissive umask. Better to omit it.
The IO::File module provides a more object-oriented approach, if you're
into that kind of thing.
=item system LIST
+=item system PROGRAM LIST
+
Does exactly the same thing as "exec LIST" except that a fork is done
first, and the parent process waits for the child process to complete.
Note that argument processing varies depending on the number of
-arguments. The return value is the exit status of the program as
+arguments. If there is more than one argument in LIST, or if LIST is
+an array with more than one value, starts the program given by the
+first element of the list with arguments given by the rest of the list.
+If there is only one scalar argument, the argument is
+checked for shell metacharacters, and if there are any, the entire
+argument is passed to the system's command shell for parsing (this is
+C</bin/sh -c> on Unix platforms, but varies on other platforms). If
+there are no shell metacharacters in the argument, it is split into
+words and passed directly to execvp(), which is more efficient.
+
+The return value is the exit status of the program as
returned by the wait() call. To get the actual exit value divide by
256. See also L</exec>. This is I<NOT> what you want to use to capture
the output from a command, for that you should use merely backticks or
qx//, as described in L<perlop/"`STRING`">.
+Like exec(), system() allows you to lie to a program about its name if
+you use the "system PROGRAM LIST" syntax. Again, see L</exec>.
+
Because system() and backticks block SIGINT and SIGQUIT, killing the
program they're running doesn't actually interrupt your program.
system(@args) == 0
or die "system @args failed: $?"
-Here's a more elaborate example of analysing the return value from
-system() on a Unix system to check for all possibilities, including for
-signals and core dumps.
+You can check all the failure possibilities by inspecting
+C<$?> like this:
- $rc = 0xffff & system @args;
- printf "system(%s) returned %#04x: ", "@args", $rc;
- if ($rc == 0) {
- print "ran with normal exit\n";
- }
- elsif ($rc == 0xff00) {
- print "command failed: $!\n";
- }
- elsif ($rc > 0x80) {
- $rc >>= 8;
- print "ran with non-zero exit status $rc\n";
- }
- else {
- print "ran with ";
- if ($rc & 0x80) {
- $rc &= ~0x80;
- print "core dump from ";
- }
- print "signal $rc\n"
- }
- $ok = ($rc != 0);
+ $exit_value = $? >> 8;
+ $signal_num = $? & 127;
+ $dumped_core = $? & 128;
When the arguments get executed via the system shell, results will
be subject to its quirks and capabilities. See L<perlop/"`STRING`">
-for details.
+and L</exec> for details.
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
returned by the tie() function, which would be useful if you want to
access other methods in CLASSNAME.
-Note that functions such as keys() and values() may return huge array
-values when used on large objects, like DBM files. You may prefer to
-use the each() function to iterate over such. Example:
+Note that functions such as keys() and values() may return huge lists
+when used on large objects, like DBM files. You may prefer to use the
+each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
=item tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value
-that was originally returned by the tie() call which bound the variable
+that was originally returned by the tie() call that bound the variable
to a package.) Returns the undefined value if VARIABLE isn't tied to a
package.
=item times
-Returns a four-element array giving the user and system times, in
+Returns a four-element list giving the user and system times, in
seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Produces a fatal error if truncate isn't implemented
-on your system.
+on your system. Returns TRUE if successful, the undefined value
+otherwise.
=item uc EXPR
=item umask
Sets the umask for the process to EXPR and returns the previous value.
-If EXPR is omitted, merely returns the current umask. Remember that a
+If EXPR is omitted, merely returns the current umask. If umask(2) is
+not implemented on your system, returns C<undef>. Remember that a
umask is a number, usually given in octal; it is I<not> a string of octal
digits. See also L</oct>, if all you have is a string.
=item undef
Undefines the value of EXPR, which must be an lvalue. Use only on a
-scalar value, an entire array, an entire hash, or a subroutine name (using
-"&"). (Using undef() will probably not do what you expect on most
-predefined variables or DBM list values, so don't do that.) Always
-returns the undefined value. You can omit the EXPR, in which case
-nothing is undefined, but you still get an undefined value that you
-could, for instance, return from a subroutine, assign to a variable or
-pass as a parameter. Examples:
+scalar value, an array (using "@"), a hash (using "%"), a subroutine
+(using "&"), or a typeglob (using "*"). (Saying C<undef $hash{$key}>
+will probably not do what you expect on most predefined variables or
+DBM list values, so don't do that; see L<delete>.) Always returns the
+undefined value. You can omit the EXPR, in which case nothing is
+undefined, but you still get an undefined value that you could, for
+instance, return from a subroutine, assign to a variable or pass as a
+parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
+ undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
+Note that this is a unary operator, not a list operator.
+
=item unlink LIST
=item unlink
Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
-value. (In a scalar context, it returns merely the first value
+value. (In scalar context, it returns merely the first value
produced.) The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:
sub substr {
- local($what,$where,$howmuch) = @_;
+ my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
number instead of a module name. If the version of the Perl interpreter
is less than VERSION, then an error message is printed and Perl exits
immediately. This is often useful if you need to check the current
-Perl version before C<use>ing library modules which have changed in
+Perl version before C<use>ing library modules that have changed in
incompatible ways from older versions of Perl. (We try not to do
this more than we have to.)
That is exactly equivalent to
- BEGIN { require Module; }
+ BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the
C<use> will call the VERSION method in class Module with the given
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
-These pseudo-modules import semantics into the current block scope, unlike
-ordinary modules, which import symbols into the current package (which are
-effective through the end of the file).
+Some of these these pseudo-modules import semantics into the current
+block scope (like C<strict> or C<integer>, unlike ordinary modules,
+which import symbols into the current package (which are effective
+through the end of the file).
There's a corresponding "no" command that unimports meanings imported
by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
files. The first two elements of the list must be the NUMERICAL access
and modification times, in that order. Returns the number of files
successfully changed. The inode modification time of each file is set
-to the current time. Example of a "touch" command:
+to the current time. This code has the same effect as the "touch"
+command if the files already exist:
#!/usr/bin/perl
$now = time;
=item values HASH
-Returns a normal array consisting of all the values of the named hash.
-(In a scalar context, returns the number of values.) The values are
-returned in an apparently random order, but it is the same order as either
-the keys() or each() function would produce on the same hash. As a side
-effect, it resets HASH's iterator. See also keys(), each(), and sort().
+Returns a list consisting of all the values of the named hash. (In a
+scalar context, returns the number of values.) The values are
+returned in an apparently random order, but it is the same order as
+either the keys() or each() function would produce on the same hash.
+As a side effect, it resets HASH's iterator. See also keys(), each(),
+and sort().
=item vec EXPR,OFFSET,BITS
operators |, &, and ^, which will assume a bit vector operation is
desired when both operands are strings.
+The following code will build up an ASCII string saying 'PerlPerlPerl'.
+The comments show the string after each step. Note that this code works
+in the same way on big-endian or little-endian machines.
+
+ my $foo = '';
+ vec($foo, 0, 32) = 0x5065726C; # 'Perl'
+ vec($foo, 2, 16) = 0x5065; # 'PerlPe'
+ vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
+ vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
+ vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
+ vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
+ vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72"
+ vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
+ vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
+ vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c"
+
To transform a bit vector into a string or array of 0's and 1's, use these:
$bits = unpack("b*", $vector);
status is returned in C<$?>. If you say
use POSIX ":sys_wait_h";
- ...
+ #...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-blocking wait
by remembering the status values of processes that have exited but have
not been harvested by the Perl script yet.)
+See L<perlipc> for other examples.
+
=item wantarray
Returns TRUE if the context of the currently executing subroutine is
Produces a message on STDERR just like die(), but doesn't exit or throw
an exception.
+If LIST is empty and $@ already contains a value (typically from a
+previous eval) that value is used after appending "\t...caught"
+to $@. This is useful for staying almost, but not entirely similar to
+die().
+
+If $@ is empty then the string "Warning: Something's wrong" is used.
+
No message is printed if there is a C<$SIG{__WARN__}> handler
installed. It is the handler's responsibility to deal with the message
as it sees fit (like, for instance, converting it into a die()). Most
=item write
-Writes a formatted record (possibly multi-line) to the specified file,
+Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
using the format associated with that file. By default the format for
a file is the one having the same name as the filehandle, but the
format for the current output channel (see the select() function) may be set
https://perl5.git.perl.org / perl5.git / blobdiff