=head1 NAME perlfunc - Perl builtin functions =head1 DESCRIPTION The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in L.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides a scalar context to its argument, while a list operator may provide either scalar and list contexts for its arguments. If it does both, the scalar arguments will be first, and the list argument will follow. (Note that there can ever be only one list argument.) For instance, splice() has three scalar arguments followed by a list. In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas. Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It I like a function, therefore it I 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 B<-w> switch it can warn you about this. For example, the third line above produces: print (...) interpreted as function at - line 1. Useless use of integer addition in void context at - line 1. For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list. Remember the following important rule: There is B 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 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 otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return C<-1> on failure. Exceptions to this rule are C, C, and C. System calls also set the special C<$!> variable on failure. Other functions do not, except accidentally. =head2 Perl Functions by Category Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place. =over =item Functions for SCALARs or strings C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C =item Regular expressions and pattern matching C, C, C, C, C, C, C =item Numeric functions C, C, C, C, C, C, C, C, C, C, C, C =item Functions for real @ARRAYs C, C, C, C, C =item Functions for list data C, C, C, C, C, C, C =item Functions for real %HASHes C, C, C, C, C =item Input and output functions C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C For more information about the portability of these functions, see L and other available platform-specific documentation. =head2 Alphabetical Listing of Perl Functions =over 8 =item I<-X> FILEHANDLE =item I<-X> EXPR =item I<-X> A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename or a filehandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or the undefined value if the file doesn't exist. Despite the funny names, precedence is the same as any other named unary operator, and the argument may be parenthesized like any other unary operator. The operator may be any of: X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> -r File is readable by effective uid/gid. -w File is writable by effective uid/gid. -x File is executable by effective uid/gid. -o File is owned by effective uid. -R File is readable by real uid/gid. -W File is writable by real uid/gid. -X File is executable by real uid/gid. -O File is owned by real uid. -e File exists. -z File has zero size. -s File has nonzero size (returns size). -f File is a plain file. -d File is a directory. -l File is a symbolic link. -p File is a named pipe (FIFO), or Filehandle is a pipe. -S File is a socket. -b File is a block special file. -c File is a character special file. -t Filehandle is opened to a tty. -u File has setuid bit set. -g File has setgid bit set. -k File has sticky bit set. -T File is a text file. -B File is a binary file (opposite of -T). -M Age of file in days when script started. -A Same for access time. -C Same for inode change time. Example: while (<>) { chop; next unless -f $_; # ignore specials #... } The interpretation of the file permission operators C<-r>, C<-R>, C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file. Such reasons may be for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Also note that, for the superuser on the local filesystems, C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set the uid to something else. If you are using ACLs, there is a pragma called C that may produce more accurate results than the bare stat() mode bits. When under the C the above-mentioned filetests will test whether the permission can (not) be granted using the access() family of system calls. Also note that the C<-x> and C<-X> may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Read the documentation for the C pragma for more information. Note that C<-s/a/b/> does not do a negated substitution. Saying C<-exp($foo)> still works as expected, however--only single letters following a minus are interpreted as file tests. 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 strange characters (E30%) 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 rather than the first block. Both C<-T> and C<-B> return TRUE on a null file, or a file at EOF when testing a filehandle. Because you have to read a file to do the C<-T> test, on most occasions you want to use a C<-f> against the file first, as in C. If any of the file tests (or either the C or C operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with C<-t>, and you need to remember that lstat() and C<-l> will leave values in the stat structure for the 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 _; =item abs VALUE =item abs Returns the absolute value of its argument. If VALUE is omitted, uses C<$_>. =item accept NEWSOCKET,GENERICSOCKET Accepts an incoming socket connect, just as the accept(2) system call does. Returns the packed address if it succeeded, FALSE otherwise. See example in L. =item alarm SECONDS =item alarm Arranges to have a SIGALRM delivered to this process after the specified number of seconds have elapsed. If SECONDS is not specified, the value stored in C<$_> 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 C<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 C interface to access setitimer(2) if your system supports it, or else see L. It is usually a mistake to intermix C and C calls. If you want to use C to time out a system call you need to use an C/C pair. You can't rely on the alarm causing the system call to fail with C<$!> set to C because Perl sets up signal handlers to restart system calls on some systems. Using C/C always works, modulo the caveats given in L. eval { local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required alarm $timeout; $nread = sysread SOCKET, $buffer, $size; alarm 0; }; if ($@) { die unless $@ eq "alarm\n"; # propagate unexpected errors # timed out } else { # didn't } =item atan2 Y,X Returns the arctangent of Y/X in the range -PI to PI. For the tangent operation, you may use the C function, or use the familiar relation: sub tan { sin($_[0]) / cos($_[0]) } =item bind SOCKET,NAME Binds a network address to a socket, just as the bind system call does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in L. =item binmode FILEHANDLE Arranges for the file to be read or written in "binary" mode in operating systems that distinguish between binary and text files. Files that are not in binary mode have CR LF sequences translated to LF on input and LF translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS and similarly archaic systems, it may be imperative--otherwise your MS-DOS-damaged C library may mangle your file. The key distinction between systems that need C and those that don't is their text file formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single character, and that encode that character in C as C<"\n">, do not need C. The rest need it. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. =item bless REF,CLASSNAME =item bless REF This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package--or the current package if no CLASSNAME is specified, which is often the case. It returns the reference for convenience, because a C 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 and L 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, it is best to avoid such package names as well. See L. =item caller EXPR =item caller 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 C or C, and the undefined value otherwise. In list context, returns ($package, $filename, $line) = caller; With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one. ($package, $filename, $line, $subroutine, $hasargs, $wantarray, $evaltext, $is_require) = caller($i); Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine call, but an C. In such a case additional elements C<$evaltext> and C<$is_require> are set: C<$is_require> is true if the frame is created by a C or C statement, C<$evaltext> contains the text of the C statement. In particular, for a C statement, C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that each C statement creates a C frame inside an C) frame. Furthermore, when called from within the DB package, caller returns more detailed information: it sets the list variable C<@DB::args> to be the arguments with which the subroutine was invoked. Be aware that the optimizer might have optimized call frames away before C had a chance to get the information. That means that C might not return information about the call frame you expect it do, for C 1>. In particular, C<@DB::args> might have information from the previous time C was called. =item chdir EXPR Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to home directory. Returns TRUE upon success, FALSE otherwise. See example under C. =item chmod LIST Changes the permissions of a list of files. The first element of the list must be the numerical mode, which should probably be an octal number, and which definitely should I a string of octal digits: C<0644> is okay, C<'0644'> is not. Returns the number of files successfully changed. See also L, if all you have is a string. $cnt = chmod 0755, 'foo', 'bar'; chmod 0755, @executables; $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to # --w----r-T $mode = '0644'; chmod oct($mode), 'foo'; # this is better $mode = 0644; chmod $mode, 'foo'; # this is best =item chomp VARIABLE =item chomp LIST =item chomp This is a slightly safer version of L. It removes any line ending that corresponds to the current value of C<$/> (also known as $INPUT_RECORD_SEPARATOR in the C module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode (C<$/ = "">), it removes all trailing newlines from the string. If VARIABLE is omitted, it chomps C<$_>. Example: while (<>) { chomp; # avoid \n on last field @array = split(/:/); # ... } You can actually chomp anything that's an lvalue, including an assignment: chomp($cwd = `pwd`); chomp($answer = ); If you chomp a list, each element is chomped, and the total number of characters removed is returned. =item chop VARIABLE =item chop LIST =item chop Chops off the last character of a string and returns the character chopped. It's used primarily to remove the newline from the end of an input record, but is much more efficient than C because it neither scans nor copies the string. If VARIABLE is omitted, chops C<$_>. Example: while (<>) { chop; # avoid \n on last field @array = split(/:/); #... } You can actually chop anything that's an lvalue, including an assignment: chop($cwd = `pwd`); chop($answer = ); If you chop a list, each element is chopped. Only the value of the last C is returned. Note that C returns the last character. To return all but the last character, use C. =item chown LIST Changes the owner (and group) of a list of files. The first two elements of the list must be the I uid and gid, in that order. Returns the number of files successfully changed. $cnt = chown $uid, $gid, 'foo', 'bar'; chown $uid, $gid, @filenames; Here's an example that looks up nonnumeric uids in the passwd file: print "User: "; chop($user = ); print "Files: "; chop($pattern = ); ($login,$pass,$uid,$gid) = getpwnam($user) or die "$user not in passwd file"; @ary = glob($pattern); # expand filenames chown $uid, $gid, @ary; On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. =item chr NUMBER =item chr Returns the character represented by that NUMBER in the character set. For example, C is C<"A"> in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face (but only within the scope of a C). For the reverse, use L. If NUMBER is omitted, uses C<$_>. =item chroot FILENAME =item chroot 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 C<"/"> 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 C to C<$_>. =item close FILEHANDLE =item close Closes the file or pipe associated with the file handle, returning TRUE only if stdio successfully flushes buffers and closes the system file descriptor. Closes the currently selected filehandle if the argument is omitted. You don't have to close FILEHANDLE if you are immediately going to do another C on it, because C will close it for you. (See C.) However, an explicit C on an input file resets the line counter (C<$.>), while the implicit close done by C does not. If the file handle came from a piped open C 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 C<$!> will be set to C<0>.) Also, closing a pipe waits for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards. Closing a pipe explicitly also puts the exit status value of the command into C<$?>. Example: open(OUTPUT, '|sort >foo') # pipe to sort or die "Can't start sort: $!"; #... # print stuff to output 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: $!"; FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name. =item closedir DIRHANDLE Closes a directory opened by C and returns the success of that system call. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name. =item connect SOCKET,NAME Attempts to connect to a remote socket, just as the connect system call does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in L. =item continue BLOCK Actually a flow control statement rather than a function. If there is a C BLOCK attached to a BLOCK (typically in a L or L), it is always executed just before the conditional is about to be evaluated again, just like the third part of a L loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the C statement (which is similar to the C C statement). L, L, or L may appear within a C block. C and C will behave as if they had been executed within the main block. So will C, but since it will execute a C block, it may be more entertaining. while (EXPR) { ### redo always comes here do_something; } continue { ### next always comes here do_something_else; # then back the top to re-check EXPR } ### last always comes here Omitting the C section is semantically equivalent to using an empty one, logically enough. In that case, C goes directly back to check the condition at the top of the loop. See also L. =item cos EXPR Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of C<$_>. For the inverse cosine operation, you may use the C function, or use this relation: sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } =item crypt PLAINTEXT,SALT Encrypts a string exactly like the crypt(3) function in the C library (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 C 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 C). This allows your code to work with the standard C and with more exotic implementations. When choosing a new salt create a random two character string whose characters come from the set C<[./0-9A-Za-z]> (like C). 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($word = ); print "\n"; system "stty echo"; if (crypt($word, $pwd) ne $pwd) { die "Sorry...\n"; } else { print "ok\n"; } Of course, typing in your own password to whoever asks you for it is unwise. =item dbmclose HASH [This function has been superseded by the C function.] Breaks the binding between a DBM file and a hash. =item dbmopen HASH,DBNAME,MODE [This function has been superseded by the C function.] This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal C, the first argument is I a filehandle, even though it looks like one). DBNAME is the name of the database (without the F<.dir> or F<.pag> extension if any). If the database does not exist, it is created with protection specified by MODE (as modified by the C). If your system supports only the older DBM functions, you may perform only one C in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling C produced a fatal error; it now falls back to sdbm(3). If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an C, which will trap the error. Note that functions such as C and C may return huge lists when used on large DBM files. You may prefer to use the C function to iterate over large DBM files. Example: # print out history file offsets dbmopen(%HIST,'/usr/lib/news/history',0666); while (($key,$val) = each %HIST) { print $key, ' = ', unpack('L',$val), "\n"; } dbmclose(%HIST); See also L for a more general description of the pros and cons of the various dbm approaches, as well as L for a particularly rich implementation. =item defined EXPR =item defined Returns a Boolean value telling whether EXPR has a value other than the undefined value C. If EXPR is not present, C<$_> will be checked. Many operations return C to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish C from other values. (A simple Boolean test will not distinguish among C, zero, the empty string, and C<"0">, which are all equally false.) Note that since C is a valid scalar, its presence doesn't I indicate an exceptional condition: C returns C when its argument is an empty array, I when the element to return happens to be C. You may also use C to check whether a subroutine exists, by saying C without parentheses. On the other hand, use of C 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 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 C, and then are surprised to discover that the number C<0> and C<""> (the zero-length string) are, in fact, defined values. For example, if you say "ab" =~ /a(.*)b/; The pattern match succeeds, and C<$1> is defined, despite the fact that it matched "nothing". But it didn't really match nothing--rather, it matched something that happened to be C<0> characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use C only when you're questioning the integrity of what you're trying to do. At other times, a simple comparison to C<0> or C<""> is what you want. Currently, using C on an entire array or hash reports whether memory for that aggregate has ever been allocated. So an array you set to the empty list appears undefined initially, and one that once was full and that you then set to the empty list still appears defined. You should instead use a simple test for size: if (@an_array) { print "has array elements\n" } if (%a_hash) { print "has hash members\n" } Using C on these, however, does clear their memory and then report them as not defined anymore, but you shouldn't do that unless you don't plan to use them again, because it saves time when you load them up again to have memory already ready to be filled. The normal way to free up space used by an aggregate is to assign the empty list. This counterintuitive behavior of C on aggregates may be changed, fixed, or broken in a future release of Perl. See also L, L, L. =item delete EXPR Deletes the specified key(s) and their associated values from a hash. For each key, returns the deleted value associated with that key, or the undefined value if there was no such key. Deleting from C<$ENV{}> modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. (But deleting from a Cd hash doesn't necessarily return anything.) The following deletes all the values of a hash: foreach $key (keys %HASH) { delete $HASH{$key}; } And so does this: delete @HASH{keys %HASH} (But both of these are slower than just assigning the empty list, or using C.) 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, $key2, @morekeys}; =item die LIST Outside an C, prints the value of LIST to C and exits with the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of C<($? EE 8)> (backtick `command` status). If C<($? EE 8)> is C<0>, exits with C<255>. Inside an C the error message is stuffed into C<$@> and the C is terminated with the undefined value. This makes C the way to raise an exception. Equivalent examples: die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" If the value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Hint: sometimes appending C<", stopped"> to your message will cause it to make better sense when the string C<"at foo line 123"> is appended. Suppose you are running script "canasta". die "/etc/games is no good"; die "/etc/games is no good, stopped"; produce, respectively /etc/games is no good at canasta line 123. /etc/games is no good, stopped at canasta line 123. See also C and C. If LIST is empty and C<$@> already contains a value (typically from a previous eval) that value is reused after appending C<"\t...propagated">. This is useful for propagating exceptions: eval { ... }; die unless $@ =~ /Expected exception/; If C<$@> is empty then the string C<"Died"> is used. You can arrange for a callback to be run just before the C does its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler will be called with the error text and can change the error message, if it sees fit, by calling C again. See L for details on setting C<%SIG> entries, and L<"eval BLOCK"> for some examples. Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed blocks/strings. If one wants the hook to do nothing in such situations, put die @_ if $^S; as the first line of the handler (see L). =item do BLOCK Not really a function. Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by a loop modifier such as L or L, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.) C does I count as a loop, so the loop control statements L, L or L cannot be used to leave or restart the block. =item do SUBROUTINE(LIST) A deprecated form of subroutine call. See L. =item do EXPR Uses the value of EXPR as a filename and executes the contents of the file as a Perl script. Its primary use is to include subroutines from a Perl subroutine library. do 'stat.pl'; is just like scalar eval `cat stat.pl`; except that it's more efficient and concise, keeps track of the current filename for error messages, and searches all the B<-I> libraries if the file isn't in the current directory (see also the @INC array in L). It is also different in how code evaluated with C doesn't see lexicals in the enclosing scope like C does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop. If C cannot read the file, it returns undef and sets C<$!> to the error. If C can read the file but cannot compile it, it returns undef and sets an error message in C<$@>. If the file is successfully compiled, C returns the value of the last expression evaluated. Note that inclusion of library modules is better done with the C and C operators, which also do automatic error checking and raise an exception if there's a problem. You might like to use C 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 use the B program to turn your core dump into an executable binary after having initialized all your variables at the beginning of the program. When the new binary is executed it will begin by executing a C (with all the restrictions that C suffers). Think of it as a goto with an intervening core dump and reincarnation. If C function, which C is the opposite of. =item oct EXPR =item oct Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to start off with C<0x>, interprets it as a hex string instead.) The following will handle decimal, octal, and hex in the standard Perl or C notation: $val = oct($val) if $val =~ /^0/; If EXPR is omitted, uses C<$_>. This function is commonly used when a string such as C<644> needs to be converted into a file mode, for example. (Although perl will automatically convert strings into numbers as needed, this automatic conversion assumes base 10.) =item open FILEHANDLE,EXPR =item open FILEHANDLE Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. (Note that lexical variables--those declared with C--will not work for this purpose; so if you're using C, specify EXPR in your call to open.) If the filename begins with C<'E'> or nothing, the file is opened for input. If the filename begins with C<'E'>, the file is truncated and opened for output, being created if necessary. If the filename begins with C<'EE'>, the file is opened for appending, again being created if necessary. You can put a C<'+'> in front of the C<'E'> or C<'E'> to indicate that you want both read and write access to the file; thus C<'+E'> is almost always preferred for read/write updates--the C<'+E'> mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable length records. See the B<-i> switch in L for a better approach. The file is created with permissions of C<0666> modified by the process' C value. The prefix and the filename may be separated with spaces. These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. If the filename begins with C<'|'>, the filename is interpreted as a command to which output is to be piped, and if the filename ends with a C<'|'>, the filename is interpreted See L for more examples of this. (You are not allowed to C to a command that pipes both in I out, but see L, L, and L for alternatives.) Opening C<'-'> opens STDIN and opening C<'E-'> opens STDOUT. Open returns nonzero upon success, the undefined value otherwise. If the C involved a pipe, the return value happens to be the pid of the subprocess. If you're unfortunate enough to be running Perl on a system that distinguishes between text files and binary files (modern operating systems don't care), then you should check out L for tips for dealing with this. The key distinction between systems that need C 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 C<"\n">, do not need C. The rest need it. When opening a file, it's usually a bad idea to continue normal execution if the request failed, so C is frequently used in connection with C. Even if C won't do what you want (say, in a CGI script, where you want to make a nicely formatted error message (but there are modules that can help with that problem)) you should always check the return value from opening a file. The infrequent exception is when working with an unopened filehandle is actually what you want to do. Examples: $ARTICLE = 100; open ARTICLE or die "Can't find article $ARTICLE: $!\n"; while (
) {... open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) # if the open fails, output is discarded open(DBASE, '+/tmp/Tmp$$") # $$ is our process id or die "Can't start sort: $!"; # process argument list of files along with any includes foreach $file (@ARGV) { process($file, 'fh00'); } sub process { my($filename, $input) = @_; $input++; # this is a string increment unless (open($input, $filename)) { print STDERR "Can't open $filename: $!\n"; return; } local $_; while (<$input>) { # note use of indirection if (/^#include "(.*)"/) { process($1, $input); next; } #... # whatever } } You may also, in the Bourne shell tradition, specify an EXPR beginning with C<'E&'>, in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped and opened. You may use C<&> after C>, CE>, C>, C<+E>, C<+EE>, and C<+E>. The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of stdio buffers.) Here is a script that saves, redirects, and restores STDOUT and STDERR: #!/usr/bin/perl open(OLDOUT, ">&STDOUT"); open(OLDERR, ">&STDERR"); open(STDOUT, ">foo.out") || die "Can't redirect stdout"; open(STDERR, ">&STDOUT") || die "Can't dup stdout"; select(STDERR); $| = 1; # make unbuffered select(STDOUT); $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too close(STDOUT); close(STDERR); open(STDOUT, ">&OLDOUT"); open(STDERR, ">&OLDERR"); print STDOUT "stdout 2\n"; print STDERR "stderr 2\n"; If you specify C<'E&=N'>, where C is a number, then Perl will do an equivalent of C's C of that file descriptor; this is more parsimonious of file descriptors. For example: open(FILEHANDLE, "<&=$fd") If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and C<0> within the child process. (Use C to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following pairs are more or less equivalent: open(FOO, "|tr '[a-z]' '[A-Z]'"); open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; open(FOO, "cat -n '$file'|"); open(FOO, "-|") || exec 'cat', '-n', $file; See L for more examples of this. NOTE: On any operation that may do a fork, any unflushed buffers remain unflushed in both processes, which means you may need to set C<$|> to avoid duplicate output. 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 $^F. See L. Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in C<$?>. The filename passed to open will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of F<"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 C (see L on your system), then you should use the C 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: ", ; Using the constructor from the C package (or one of its subclasses, such as C or C), 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 $handle = new IO::File; open($handle, "myfile") or die "myfile: $!"; $first = <$handle> or return (); # Automatically closed here. mung $first or die "mung failed"; # Or here. return $first, <$handle> if $ALL; # Or here. $first; # Or here. } See L for some details about mixing reading and writing. =item opendir DIRHANDLE,EXPR Opens a directory named EXPR for processing by C, C, C, C, and C. Returns TRUE if successful. DIRHANDLEs have their own namespace separate from FILEHANDLEs. =item ord EXPR =item ord Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If EXPR is omitted, uses C<$_>. For the reverse, see L. =item pack TEMPLATE,LIST Takes an array or list of values and packs it into a binary structure, returning the string containing the structure. The TEMPLATE is a sequence of characters that give the order and type of values, as follows: A An ascii string, will be space padded. a An ascii string, will be null padded. b A bit string (ascending bit order, like vec()). B A bit string (descending bit order). h A hex string (low nybble first). H A hex string (high nybble first). c A signed char value. C An unsigned char value. Only does bytes. See U for Unicode. s A signed short value. S An unsigned short value. (This 'short' is _exactly_ 16 bits, which may differ from what a local C compiler calls 'short'.) i A signed integer value. I An unsigned integer value. (This 'integer' is _at_least_ 32 bits wide. Its exact size depends on what a local C compiler calls 'int', and may even be larger than the 'long' described in the next item.) l A signed long value. L An unsigned long value. (This 'long' is _exactly_ 32 bits, which may differ from what a local C compiler calls 'long'.) n A short in "network" (big-endian) order. N A long in "network" (big-endian) order. v A short in "VAX" (little-endian) order. V A long in "VAX" (little-endian) order. (These 'shorts' and 'longs' are _exactly_ 16 bits and _exactly_ 32 bits, respectively.) q A signed quad (64-bit) value. Q An unsigned quad value. (Available only if your system supports 64-bit integer values _and_ if Perl has been compiled to support those. Causes a fatal error otherwise.) f A single-precision float in the native format. d A double-precision float in the native format. p A pointer to a null-terminated string. P A pointer to a structure (fixed-length string). u A uuencoded string. U A Unicode character number. Encodes to UTF-8 internally. Works even if C is not in effect. w A BER compressed integer. Its bytes represent an unsigned integer in base 128, most significant digit first, with as few digits as possible. Bit eight (the high bit) is set on each byte except the last. x A null byte. X Back up a byte. @ Null fill to absolute position. Each letter may optionally be followed by a number giving a repeat count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the pack function will gobble up that many values from the LIST. A C<*> for the repeat count means to use however many items are left. The C<"a"> and C<"A"> types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a string that many nybbles long. The C<"p"> type packs a pointer to a null- terminated string. You are responsible for ensuring the string is not a temporary value (which can potentially get deallocated before you get around to using the packed result). The C<"P"> packs a pointer to a structure of the size indicated by the length. A NULL pointer is created if the corresponding value for C<"p"> or C<"P"> is C. Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of floating formats around, and the lack of a standard "network" representation, no facility for interchange has been made. This means that packed floating point data written on one machine may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). Note that Perl uses doubles internally for all numeric calculation, and converting from double into float and thence back to double again will lose precision (i.e., C) will not in general equal C<$foo>). Examples: $foo = pack("CCCC",65,66,67,68); # foo eq "ABCD" $foo = pack("C4",65,66,67,68); # same thing $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); # same thing with Unicode circled letters $foo = pack("ccxxcc",65,66,67,68); # foo eq "AB\0\0CD" $foo = pack("s2",1,2); # "\1\0\2\0" on little-endian # "\0\1\0\2" on big-endian $foo = pack("a4","abcd","x","y","z"); # "abcd" $foo = pack("aaaa","abcd","x","y","z"); # "axyz" $foo = pack("a14","abcdefg"); # "abcdefg\0\0\0\0\0\0\0" $foo = pack("i9pl", gmtime); # a real struct tm (on my system anyway) sub bintodec { unpack("N", pack("B32", substr("0" x 32 . shift, -32))); } The same template may generally also be used in the unpack function. =item package =item package NAMESPACE Declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block (the same scope as the C operator). All further unqualified dynamic identifiers will be in this namespace. A package statement affects only dynamic variables--including those you've used C on--but I lexical variables created with C. Typically it would be the first declaration in a file to be included by the C or C operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: C<$Package::Variable>. If the package name is null, the C
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, since it also extends to function names. See L for more information about packages, modules, and classes. See L for other scoping issues. =item pipe READHANDLE,WRITEHANDLE Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, deadlock can occur unless you are very careful. In addition, note that Perl's pipes use stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE after each command, depending on the application. See L, L, and L for examples of such things. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors as determined by the value of $^F. See L. =item pop ARRAY =item pop Pops and returns the last value of the array, shortening the array by 1. Has a similar effect to $tmp = $ARRAY[$#ARRAY--]; If there are no elements in the array, returns the undefined value. If ARRAY is omitted, pops the C<@ARGV> array in the main program, and the C<@_> array in subroutines, just like C. =item pos SCALAR =item pos Returns the offset of where the last C search left off for the variable is in question (C<$_> is used when the variable is not specified). May be modified to change that offset. Such modification will also influence the C<\G> zero-width assertion in regular expressions. See L and L. =item print FILEHANDLE LIST =item print LIST =item print Prints a string or a comma-separated list of strings. Returns TRUE if successful. FILEHANDLE may be a scalar variable name, in which case the variable contains the name of or a reference to the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is omitted, prints by default to standard output (or to the last selected output channel--see L). If LIST is also omitted, prints C<$_> to 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 list context, and any subroutine that you call will have one or more of its expressions 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 C<+> or put parentheses around all the arguments. Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a block returning its value instead: print { $files[$i] } "stuff\n"; print { $OK ? STDOUT : STDERR } "stuff\n"; =item printf FILEHANDLE FORMAT, LIST =item printf FORMAT, LIST Equivalent to C, except that C<$\> (the output record separator) is not appended. The first argument of the list will be interpreted as the C format. If C is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See L. Don't fall into the trap of using a C when a simple C would do. The C is more efficient and less error prone. =item prototype FUNCTION Returns the prototype of a function as a string (or C if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is a string starting with C, the rest is taken as a name for Perl builtin. If builtin is not I (such as C) or its arguments cannot be expressed by a prototype (such as C) - in other words, the builtin does not behave like a Perl function - returns C. Otherwise, the string describing the equivalent prototype is returned. =item push ARRAY,LIST Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as for $value (LIST) { $ARRAY[++$#ARRAY] = $value; } but is more efficient. Returns the new number of elements in the array. =item q/STRING/ =item qq/STRING/ =item qr/STRING/ =item qx/STRING/ =item qw/STRING/ Generalized quotes. See L. =item quotemeta EXPR =item quotemeta Returns the value of EXPR with all non-alphanumeric characters backslashed. (That is, all characters not matching C will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the C<\Q> escape in double-quoted strings. If EXPR is omitted, uses C<$_>. =item rand EXPR =item rand Returns a random fractional number greater than or equal to C<0> and less than the value of EXPR. (EXPR should be positive.) If EXPR is omitted, the value C<1> is used. Automatically calls C unless C has already been called. See also C. (Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.) =item read FILEHANDLE,SCALAR,LENGTH,OFFSET =item read FILEHANDLE,SCALAR,LENGTH Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE. Returns the number of bytes actually read, C<0> at end of file, or undef if there was an error. SCALAR will be grown or shrunk to the length actually read. An OFFSET may be specified to place the read data at some other place than the beginning of the string. This call is actually implemented in terms of stdio's fread(3) call. To get a true read(2) system call, see C. =item readdir DIRHANDLE Returns the next directory entry for a directory opened by C. 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 scalar context or a null list in list context. If you're planning to filetest the return values out of a C, you'd better prepend the directory in question. Otherwise, because we didn't C there, it would have been testing the wrong file. opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); closedir DIR; =item readline EXPR Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line is read and returned. In list context, reads until end-of-file is reached and returns a list of lines (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). This is the internal function implementing the CEXPRE> operator, but you can use it directly. The CEXPRE> operator is discussed in more detail in L. $line = ; $line = readline(*STDIN); # same thing =item readlink EXPR =item readlink Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal error. If there is some system error, returns the undefined value and sets C<$!> (errno). If EXPR is omitted, uses C<$_>. =item readpipe EXPR EXPR is executed as a system command. The collected standard output of the command is returned. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). This is the internal function implementing the C operator, but you can use it directly. The C operator is discussed in more detail in L. =item recv SOCKET,SCALAR,LEN,FLAGS Receives a message on a socket. Attempts to receive LENGTH bytes of data into variable SCALAR from the specified SOCKET filehandle. Actually does a C C, 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. See L for examples. =item redo LABEL =item redo The C command restarts the loop block without evaluating the conditional again. The L block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. This command is normally used by programs that want to lie to themselves about what was just input: # a simpleminded Pascal comment stripper # (warning: assumes no { or } in strings) LINE: while () { while (s|({.*}.*){.*}|$1 |) {} s|{.*}| |; if (s|{.*| |) { $front = $_; while () { if (/}/) { # end of comment? s|^|$front\{|; redo LINE; } } } print; } C cannot be used to retry a block which returns a value such as C, C or C. See also L for an illustration of how L, L, and C work. See also L. =item ref EXPR =item ref Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR is not specified, C<$_> will be used. The value returned depends on the type of thing the reference is a reference to. Builtin types include: REF SCALAR ARRAY HASH CODE GLOB If the referenced object has been blessed into a package, then that package name is returned instead. You can think of C as a C operator. if (ref($r) eq "HASH") { print "r is a reference to a hash.\n"; } if (!ref($r)) { print "r is not a reference at all.\n"; } See also L. =item rename OLDNAME,NEWNAME Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will not work across file system boundaries. =item require EXPR =item require Demands some semantics specified by EXPR, or by C<$_> if EXPR is not supplied. If EXPR is numeric, demands that the current version of Perl (C<$]> or $PERL_VERSION) be equal or greater than EXPR. Otherwise, demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of C. Has semantics similar to the following subroutine: sub require { my($filename) = @_; return 1 if $INC{$filename}; my($realfilename,$result); ITER: { foreach $prefix (@INC) { $realfilename = "$prefix/$filename"; if (-f $realfilename) { $result = do $realfilename; last ITER; } } die "Can't find $filename in \@INC"; } die $@ if $@; die "$filename did not return true value" unless $result; $INC{$filename} = $realfilename; return $result; } Note that the file will not be included twice under the same specified name. The file must return TRUE as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with "C<1;>" unless you're sure it'll return TRUE otherwise. But it's better just to put the "C<1;>", in case you add more statements. If EXPR is a bareword, the require assumes a "F<.pm>" extension and replaces "F<::>" with "F" in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace. In other words, if you try this: require Foo::Bar; # a splendid bareword The require function will actually look for the "F" file in the directories specified in the C<@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 "F" file in the @INC array and will complain about not finding "F" there. In this case you can do: eval "require $class"; For a yet-more-powerful import facility, see L and L. =item reset EXPR =item reset Generally used in a C block at the end of a loop to clear variables and reset C searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (C) are reset to match again. Resets only variables or searches in the current package. Always returns 1. Examples: reset 'X'; # reset all X variables reset 'a-z'; # reset lower case variables reset; # just reset ?? searches Resetting C<"A-Z"> is not recommended because you'll wipe out your C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See L. =item return EXPR =item return Returns from a subroutine, C, or C 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 C). If no EXPR 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 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 with all the characters in the opposite order. print reverse <>; # line tac, last line first undef $/; # for efficiency of <> print scalar reverse <>; # character tac, last line tsrif This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash. %by_name = reverse %by_address; # Invert the hash =item rewinddir DIRHANDLE Sets the current position to the beginning of the directory for the C routine on DIRHANDLE. =item rindex STR,SUBSTR,POSITION =item rindex STR,SUBSTR Works just like index except that it returns the position of the LAST occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence at or before that position. =item rmdir FILENAME =item rmdir Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If FILENAME is omitted, uses C<$_>. =item s/// The substitution operator. See L. =item scalar EXPR Forces EXPR to be interpreted in scalar context and returns the value of EXPR. @counts = ( scalar @a, scalar @b, scalar @c ); There is no equivalent operator to force an expression to be interpolated in list context because it's in practice never needed. If you really wanted to do so, however, you could use the construction C<@{[ (some expression) ]}>, but usually a simple C<(some expression)> suffices. =item seek FILEHANDLE,POSITION,WHENCE Sets FILEHANDLE's position, just like the C call of C. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are C<0> to set the new position to POSITION, C<1> to set it to the current position plus POSITION, and C<2> to set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants C, C, and C from either the C or the POSIX module. Returns C<1> upon success, C<0> otherwise. If you want to position file for C or C, don't use C -- buffering makes its effect on the file's system position unpredictable and non-portable. Use C instead. On some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of C<1> (C) is useful for not moving the file position: seek(TEST,0,1); This is also useful for applications emulating C. Once you hit EOF on your read, and then sleep for a while, you might have to stick in a seek() to reset things. The C doesn't change the current position, but it I clear the end-of-file condition on the handle, so that the next CFILEE> makes Perl try again to read something. We hope. If that doesn't work (some stdios are particularly cantankerous), then you may need something more like this: for (;;) { for ($curpos = tell(FILE); $_ = ; $curpos = tell(FILE)) { # search for some stuff and put it into files } sleep($for_a_while); seek(FILE, $curpos, 0); } =item seekdir DIRHANDLE,POS Sets the current position for the C routine on DIRHANDLE. POS must be a value returned by C. Has the same caveats about possible directory compaction as the corresponding system library routine. =item select FILEHANDLE =item select Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a C or a C without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have to set the top of form format for more than one output channel, you might do the following: select(REPORT1); $^ = 'report1_top'; select(REPORT2); $^ = 'report2_top'; FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus: $oldfh = select(STDERR); $| = 1; select($oldfh); Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as: use IO::Handle; STDERR->autoflush(1); =item select RBITS,WBITS,EBITS,TIMEOUT This calls the select(2) system call with the bit masks specified, which can be constructed using C and C, along these lines: $rin = $win = $ein = ''; vec($rin,fileno(STDIN),1) = 1; vec($win,fileno(STDOUT),1) = 1; $ein = $rin | $win; If you want to select on many filehandles you might wish to write a subroutine: sub fhbits { my(@fhlist) = split(' ',$_[0]); my($bits); for (@fhlist) { vec($bits,fileno($_),1) = 1; } $bits; } $rin = fhbits('STDIN TTY SOCK'); The usual idiom is: ($nfound,$timeleft) = select($rout=$rin, $wout=$win, $eout=$ein, $timeout); or to block until something becomes ready just do this $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); Most systems do not bother to return anything useful in C<$timeleft>, so calling select() in scalar context just returns C<$nfound>. Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning theC<$timeleft>. If not, they always return C<$timeleft> equal to the supplied C<$timeout>. You can effect a sleep of 250 milliseconds this way: select(undef, undef, undef, 0.25); B: One should not attempt to mix buffered I/O (like C or EFHE) with C, except as permitted by POSIX, and even then only on POSIX systems. You have to use C instead. =item semctl ID,SEMNUM,CMD,ARG Calls the System V IPC function C. You'll probably have to say use IPC::SysV; first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable which will hold the returned semid_ds structure or semaphore value array. Returns like C: the undefined value for error, "C<0> but true" for zero, or the actual return value otherwise. See also C and C documentation. =item semget KEY,NSEMS,FLAGS Calls the System V IPC function semget. Returns the semaphore id, or the undefined value if there is an error. See also C and C documentation. =item semop KEY,OPSTRING Calls the System V IPC function semop to perform semaphore operations such as signaling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with C. The number of semaphore operations is implied by the length of OPSTRING. Returns TRUE if successful, or FALSE if there is an error. As an example, the following code waits on semaphore C<$semnum> of semaphore id C<$semid>: $semop = pack("sss", $semnum, -1, 0); die "Semaphore trouble: $!\n" unless semop($semid, $semop); To signal the semaphore, replace C<-1> with C<1>. See also C and C documentation. =item send SOCKET,MSG,FLAGS,TO =item send SOCKET,MSG,FLAGS Sends a message on a socket. Takes the same flags as the system call of the same name. On unconnected sockets you must specify a destination to send TO, in which case it does a C C. Returns the number of characters sent, or the undefined value if there is an error. See L for examples. =item setpgrp PID,PGRP Sets the current process group for the specified PID, C<0> for the current process. Will produce a fatal error if used on a machine that doesn't implement setpgrp(2). If the arguments are omitted, it defaults to C<0,0>. Note that the POSIX version of C does not accept any arguments, so only setpgrp C<0,0> is portable. =item setpriority WHICH,WHO,PRIORITY Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Will produce a fatal error if used on a machine that doesn't implement setpriority(2). =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL Sets the socket option requested. Returns undefined if there is an error. OPTVAL may be specified as C if you don't want to pass an argument. =item shift ARRAY =item shift Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the C<@_> array within the lexical scope of subroutines and formats, and the C<@ARGV> array at file scopes or within the lexical scopes established by the C, C, C, and C constructs. See also C, C, and C. C and C do the same thing to the left end of an array that C and C do to the right end. =item shmctl ID,CMD,ARG Calls the System V IPC function shmctl. You'll probably have to say use IPC::SysV; first to get the correct constant definitions. If CMD is C, then ARG must be a variable which will hold the returned C structure. Returns like ioctl: the undefined value for error, "C<0> but true" for zero, or the actual return value otherwise. See also C documentation. =item shmget KEY,SIZE,FLAGS Calls the System V IPC function shmget. Returns the shared memory segment id, or the undefined value if there is an error. See also C documentation. =item shmread ID,VAR,POS,SIZE =item shmwrite ID,STRING,POS,SIZE Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable 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. See also C documentation. =item shutdown SOCKET,HOW Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the system call of the same name. 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 Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of C<$_>. For the inverse sine operation, you may use the C function, or use this relation: sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } =item sleep EXPR =item sleep Causes the script to sleep for EXPR seconds, or forever if no EXPR. May be interrupted if the process receives a signal such as C. Returns the number of seconds actually slept. You probably cannot mix C and C calls, because C is often implemented using C. 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. 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 C interface to access setitimer(2) if your system supports it, or else see L above. See also the POSIX module's C function. =item socket SOCKET,DOMAIN,TYPE,PROTOCOL Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. You should "C" first to get the proper definitions imported. See the example in L. =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. If unimplemented, yields a fatal error. Returns TRUE if successful. Some systems defined C in terms of C, in which a call to C 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 for an example of socketpair use. =item sort SUBNAME LIST =item sort BLOCK LIST =item sort LIST Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK is omitted, Cs in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns an integer less than, equal to, or greater than C<0>, depending on how the elements of the array are to be ordered. (The C=E> and C operators are extremely useful in such routines.) SUBNAME may be a 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 recursive subroutine, and the two elements to be compared are passed into the subroutine not via C<@_> but as the package global variables C<$a> and C<$b> (see example below). They are passed by reference, so don't modify C<$a> and C<$b>. And don't try to declare them as lexicals either. You also cannot exit out of the sort block or subroutine using any of the loop control operators described in L or with C. When C is in effect, C sorts LIST according to the current collation locale. See L. Examples: # sort lexically @articles = sort @files; # same thing, but with explicit sort routine @articles = sort {$a cmp $b} @files; # now case-insensitively @articles = sort {uc($a) cmp uc($b)} @files; # same thing in reversed order @articles = sort {$b cmp $a} @files; # sort numerically ascending @articles = sort {$a <=> $b} @files; # sort numerically descending @articles = sort {$b <=> $a} @files; # sort using explicit subroutine name sub byage { $age{$a} <=> $age{$b}; # presuming numeric } @sortedclass = sort byage @class; # this sorts the %age hash by value instead of key # using an in-line function @eldest = sort { $age{$b} <=> $age{$a} } keys %age; sub backwards { $b cmp $a; } @harry = ('dog','cat','x','Cain','Abel'); @george = ('gone','chased','yz','Punished','Axed'); print sort @harry; # prints AbelCaincatdogx print sort backwards @harry; # prints xdogcatCainAbel print sort @george, 'to', @harry; # prints AbelAxedCainPunishedcatchaseddoggonetoxyz # inefficiently sort by descending numeric compare using # the first integer after the first = sign, or the # whole record case-insensitively otherwise @new = sort { ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] || uc($a) cmp uc($b) } @old; # same thing, but much more efficiently; # we'll build auxiliary indices instead # for speed @nums = @caps = (); for (@old) { push @nums, /=(\d+)/; push @caps, uc($_); } @new = @old[ sort { $nums[$b] <=> $nums[$a] || $caps[$a] cmp $caps[$b] } 0..$#old ]; # same thing using a Schwartzian Transform (no temps) @new = map { $_->[0] } sort { $b->[1] <=> $a->[1] || $a->[2] cmp $b->[2] } map { [$_, /=(\d+)/, uc($_)] } @old; If you're using strict, you I declare C<$a> and C<$b> as lexicals. They are package globals. That means if you're in the C
package, it's @articles = sort {$main::b <=> $main::a} @files; or just @articles = sort {$::b <=> $::a} @files; but if you're in the C package, it's @articles = sort {$FooPack::b <=> $FooPack::a} @files; The comparison function is required to behave. If it returns inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and sometimes saying the opposite, for example) the results are not well-defined. =item splice ARRAY,OFFSET,LENGTH,LIST =item splice ARRAY,OFFSET,LENGTH =item splice ARRAY,OFFSET Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. In list context, returns the elements removed from the array. In scalar context, returns the last element removed, or C if no elements are removed. The array grows or shrinks as necessary. If OFFSET is negative then it start that far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is negative, leave that many elements off the end of the array. The following equivalences hold (assuming C<$[ == 0>): push(@a,$x,$y) splice(@a,@a,0,$x,$y) pop(@a) splice(@a,-1) shift(@a) splice(@a,0,1) unshift(@a,$x,$y) splice(@a,0,0,$x,$y) $a[$x] = $y splice(@a,$x,1,$y) Example, assuming array lengths are passed before arrays: sub aeq { # compare two list values my(@a) = splice(@_,0,shift); my(@b) = splice(@_,0,shift); return 0 unless @a == @b; # same len? while (@a) { return 0 if pop(@a) ne pop(@b); } return 1; } if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } =item split /PATTERN/,EXPR,LIMIT =item split /PATTERN/,EXPR =item split /PATTERN/ =item split Splits a string into an array of strings, and returns it. By default, empty leading fields are preserved, and empty trailing ones are deleted. If not in list context, returns the number of fields found and splits into the C<@_> array. (In list context, you can force the split into C<@_> by using C as the pattern delimiters, but it still returns the list value.) The use of implicit split to C<@_> is deprecated, however, because it clobbers your subroutine arguments. If EXPR is omitted, splits the C<$_> 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 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 C would do well to remember). If LIMIT is negative, it is treated as if an arbitrarily large LIMIT had been specified. A pattern matching the null string (not to be confused with a null pattern C, which is just one member of the set of patterns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example: print join(':', split(/ */, 'hi there')); produces the output 'h:i:t:h:e:r:e'. The LIMIT parameter can be used to split a line partially ($login, $passwd, $remainder) = split(/:/, $_, 3); When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need. If the PATTERN contains parentheses, additional array elements are created from each matching substring in the delimiter. split(/([,-])/, "1-10,20", 3); produces the list value (1, '-', 10, ',', 20) If you had the entire header of a normal Unix email message in C<$header>, you could split it up into fields and their values this way: $header =~ s/\n\s+/ /g; # fix continuation lines %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); The pattern C may be replaced with an expression to specify patterns that vary at runtime. (To do runtime compilation only once, use C.) As a special case, specifying a PATTERN of space (C<' '>) will split on white space just as C with no arguments does. Thus, C can be used to emulate B's default behavior, whereas C will give you as many null initial fields as there are leading spaces. A C on C is like a C except that any leading whitespace produces a null first field. A C with no arguments really does a C internally. Example: open(PASSWD, '/etc/passwd'); while () { ($login, $passwd, $uid, $gid, $gcos, $home, $shell) = split(/:/); #... } (Note that C<$shell> above will still have a newline on it. See L, L, and L.) =item sprintf FORMAT, LIST Returns a string formatted by the usual C conventions of the C library function C. See L or L on your system for an explanation of the general principles. Perl does its own C formatting -- it emulates the C function C, 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 C are not available from Perl. Perl's C permits the following universally-known conversions: %% a percent sign %c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %o an unsigned integer, in octal %x an unsigned integer, in hexadecimal %e a floating-point number, in scientific notation %f a floating-point number, in fixed decimal notation %g a floating-point number, in %e or %f notation In addition, Perl permits the following widely-supported conversions: %X like %x, but using upper-case letters %E like %e, but using an upper-case "E" %G like %g, but with an upper-case "E" (if applicable) %p a pointer (outputs the Perl value's address in hexadecimal) %n special: *stores* the number of characters output so far into the next variable in the parameter list Finally, for backward (and we do mean "backward") compatibility, Perl permits these unnecessary but widely-supported conversions: %i a synonym for %d %D a synonym for %ld %U a synonym for %lu %O a synonym for %lo %F a synonym for %f Perl permits the following universally-known flags between the C<%> and the conversion letter: space prefix positive number with a space + prefix positive number with a plus sign - left-justify within the field 0 use zeros, not spaces, to right-justify # prefix 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 l interpret integer as C type "long" or "unsigned long" h interpret integer as C type "short" or "unsigned short" There is also one Perl-specific flag: V interpret integer as Perl's standard integer type Where a number would appear in the flags, an asterisk ("C<*>") may be used instead, in which case Perl uses the next item in the parameter list as the given number (that is, as the field width or precision). If a field width obtained through "C<*>" is negative, it has the same effect as the "C<->" flag: left-justification. If C is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See L. =item sqrt EXPR =item sqrt Return the square root of EXPR. If EXPR is omitted, returns square root of C<$_>. =item srand EXPR =item srand Sets the random number seed for the C operator. If EXPR is omitted, uses a semi-random value supplied by the kernel (if it supports the F device) or based on the current time and process ID, among other things. In versions of Perl prior to 5.004 the default seed was just the current C. This isn't a particularly good seed, so many old programs supply their own seed value (often C