From: Tom Phoenix Date: Fri, 28 Feb 1997 21:18:25 +0000 (-0800) Subject: Clarify C documentation X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=f827b0a6c8a4efd53c50a16b098f59ac04fd670d;p=p5sagit%2Fp5-mst-13.2.git Clarify C documentation From a suggestion I got in private e-mail. Most readers of this list will quickly recognize this as the part of perlfunc which explains crypt. This should apply over my previous patch, but if you've already fixed my typo, you'll have to fix something by hand. p5p-msgid: Pine.GSO.3.95q.970228131112.12357D-100000@kelly.teleport.com --- diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod index cf5dd8f..b55e1b1 100644 --- a/pod/perlfunc.pod +++ b/pod/perlfunc.pod @@ -606,8 +606,10 @@ 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 there is no corresponding decrypt, so this fucntion isn't -all that useful for cryptography. (For that, see your nearby CPAN mirror.) +Note that crypt is intended to be a one-way function, much like breaking +eggs to make an omelette. Thus, there is no (known) corresponding +decrypt function, so this function isn't all that useful for +cryptography. (For that, see your nearby CPAN mirror.) Here's an example that makes sure that whoever runs this program knows their own password: diff --git a/pod/perlfunc.pod.orig b/pod/perlfunc.pod.orig new file mode 100644 index 0000000..cf5dd8f --- /dev/null +++ b/pod/perlfunc.pod.orig @@ -0,0 +1,3819 @@ +=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+3; # Prints 6. + print(1+2) + 3; # Prints 3. + print (1+2)+3; # Also prints 3! + print +(1+2)+3; # Prints 6. + print ((1+2)+3); # Prints 6. + +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, +non-abortive 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 rule: + +=over 8 + +=item I + +=back + +Each operator and function decides which sort of value it would be most +appropriate to return in a scalar context. Some operators return the +length of the list that would have been returned in a list context. Some +operators return the first value in the list. Some operators return the +last value in the list. Some operators return a count of successful +operations. In general, they do what you want, unless you want +consistency. + +=head2 Perl Functions by Category + +Here are Perl's functions (including things that look like +functions, like some of the keywords and named operators) +arranged by category. Some functions appear in more +than one place. + +=over + +=item Functions for SCALARs or strings + +chomp, chop, chr, crypt, hex, index, lc, lcfirst, length, +oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex, +sprintf, substr, tr///, uc, ucfirst, y/// + +=item Regular expressions and pattern matching + +m//, pos, quotemeta, s///, split, study + +=item Numeric functions + +abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, +srand + +=item Functions for real @ARRAYs + +pop, push, shift, splice, unshift + +=item Functions for list data + +grep, join, map, qw/STRING/, reverse, sort, unpack + +=item Functions for real %HASHes + +delete, each, exists, keys, values + +=item Input and output functions + +binmode, close, closedir, dbmclose, dbmopen, die, eof, +fileno, flock, format, getc, print, printf, read, readdir, +rewinddir, seek, seekdir, select, syscall, sysread, +syswrite, tell, telldir, truncate, warn, write + +=item Functions for fixed length data or records + +pack, read, syscall, sysread, syswrite, unpack, vec + +=item Functions for filehandles, files, or directories + +I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, +lstat, mkdir, open, opendir, readlink, rename, rmdir, +stat, symlink, umask, unlink, utime + +=item Keywords related to the control flow of your perl program + +caller, continue, die, do, dump, eval, exit, goto, last, +next, redo, return, sub, wantarray + +=item Keywords related to scoping + +caller, import, local, my, package, use + +=item Miscellaneous functions + +defined, dump, eval, formline, local, my, reset, scalar, +undef, wantarray + +=item Functions for processes and process groups + +alarm, exec, fork, getpgrp, getppid, getpriority, kill, +pipe, qx/STRING/, setpgrp, setpriority, sleep, system, +times, wait, waitpid + +=item Keywords related to perl modules + +do, import, no, package, require, use + +=item Keywords related to classes and object-orientedness + +bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use + +=item Low-level socket functions + +accept, bind, connect, getpeername, getsockname, +getsockopt, listen, recv, send, setsockopt, shutdown, +socket, socketpair + +=item System V interprocess communication functions + +msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, +shmctl, shmget, shmread, shmwrite + +=item Fetching user and group info + +endgrent, endhostent, endnetent, endpwent, getgrent, +getgrgid, getgrnam, getlogin, getpwent, getpwnam, +getpwuid, setgrent, setpwent + +=item Fetching network info + +endprotoent, endservent, gethostbyaddr, gethostbyname, +gethostent, getnetbyaddr, getnetbyname, getnetent, +getprotobyname, getprotobynumber, getprotoent, +getservbyname, getservbyport, getservent, sethostent, +setnetent, setprotoent, setservent + +=item Time-related functions + +gmtime, localtime, time, times + +=item Functions new in perl5 + +abs, bless, chomp, chr, exists, formline, glob, import, lc, +lcfirst, map, my, no, prototype, qx, qw, readline, readpipe, +ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use + +* - C was a keyword in perl4, but in perl5 it is an +operator which can be used in expressions. + +=item Functions obsoleted in perl5 + +dbmclose, dbmopen + +=back + +=head2 Alphabetical Listing of Perl Functions + +=over 8 + +=item -X FILEHANDLE + +=item -X EXPR + +=item -X + +A file test, where X is one of the letters listed below. This unary +operator takes one argument, either a filename or a filehandle, and +tests the associated file to see if something is true about it. If the +argument is omitted, tests $_, except for 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: + + -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 non-zero size (returns size). + + -f File is a plain file. + -d File is a directory. + -l File is a symbolic link. + -p File is a named pipe (FIFO). + -S File is a socket. + -b File is a block special file. + -c File is a character special file. + -t Filehandle is opened to a tty. + + -u File has setuid bit set. + -g File has setgid bit set. + -k File has sticky bit set. + + -T File is a text file. + -B File is a binary file (opposite of -T). + + -M Age of file in days when script started. + -A Same for access time. + -C Same for inode change time. + +The interpretation of the file permission operators 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, +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. + +Example: + + while (<>) { + chop; + next unless -f $_; # ignore specials + ... + } + +Note that C<-s/a/b/> does not do a negated substitution. Saying +C<-exp($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 odd 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 stat() or lstat() operators) are given +the special filehandle consisting of a solitary underline, then the stat +structure of the previous file test (or stat operator) is used, saving +a system call. (This doesn't work with 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 $_. + +=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 $_ is used. (On some machines, +unfortunately, the elapsed time may be up to one second less than you +specified because of how seconds are counted.) Only one timer may be +counting at once. Each call disables the previous timer, and an +argument of 0 may be supplied to cancel the previous timer without +starting a new one. The returned value is the amount of time remaining +on the previous timer. + +For delays of finer granularity than one second, you may use Perl's +syscall() interface to access setitimer(2) if your system supports it, +or else see L below. It is not advised to intermix alarm() +and sleep() calls. + +If you want to use alarm() to time out a system call you need to use an +eval/die pair. You can't rely on the alarm causing the system call to +fail with $! set to EINTR because Perl sets up signal handlers to +restart system calls on some systems. Using eval/die always works. + + eval { + local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required + alarm $timeout; + $nread = sysread SOCKET, $buffer, $size; + alarm 0; + }; + die if $@ && $@ ne "alarm\n"; # propagate errors + if ($@) { + # timed out + } + else { + # didn't + } + +=item atan2 Y,X + +Returns the arctangent of Y/X in the range -PI to PI. + +For the tangent operation, you may use the POSIX::tan() +function, or use the familiar relation: + + sub tan { sin($_[0]) / cos($_[0]) } + +=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 DOS +and similarly archaic systems, it may be imperative--otherwise your +DOS-damaged C library may mangle your file. The key distinction between +systems that need binmode and those that don't is their text file +formats. Systems like Unix and Plan9 that delimit lines with a single +character, and that encode that character in C as '\n', do not need +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 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 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 caller's package name if there is a caller, that is, if +we're in a subroutine or eval() or require(), and the undefined value +otherwise. In a list context, returns + + ($package, $filename, $line) = caller; + +With EXPR, it returns some extra information that the debugger uses to +print a stack trace. The value of EXPR indicates how many call frames +to go back before the current one. + + ($package, $filename, $line, $subroutine, + $hasargs, $wantarray, $evaltext, $is_require) = caller($i); + +Here $subroutine may be C<"(eval)"> if the frame is not a subroutine +call, but C>. In such a case additional elements $evaltext and +$is_require are set: $is_require is true if the frame is created by +C> or C> statement, $evaltext contains the text of +C> statement. In particular, for C> +statement $filename is C<"(eval)">, but $evaltext is undefined. (Note +also that 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 @DB::args to be the +arguments with which that subroutine was invoked. + +=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 die(). + +=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 $mode, 'foo'; # this is best + +=item chomp VARIABLE + +=item chomp LIST + +=item chomp + +This is a slightly safer version of chop (see below). It removes any +line ending that corresponds to the current value of C<$/> (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 $_. Example: + + while (<>) { + chomp; # avoid \n on last field + @array = split(/:/); + ... + } + +You can actually chomp anything that's an lvalue, including an assignment: + + chomp($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 $_. +Example: + + while (<>) { + chop; # avoid \n on last field + @array = split(/:/); + ... + } + +You can actually chop anything that's an lvalue, including an assignment: + + chop($cwd = `pwd`); + chop($answer = ); + +If you chop a list, each element is chopped. Only the value of the +last chop is returned. + +Note that chop 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, $gid, @filenames; + +Here's an example that looks up non-numeric uids in the passwd file: + + print "User: "; + chop($user = ); + print "Files: " + chop($pattern = ); + + ($login,$pass,$uid,$gid) = getpwnam($user) + or die "$user not in passwd file"; + + @ary = <${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 "A" in ASCII. For the reverse, use L. + +If NUMBER is omitted, uses $_. + +=item chroot FILENAME + +=item chroot + +This function works as the system call by the same name: it makes the +named directory the new root directory for all further pathnames that +begin with a "/" by your process and all of its children. (It doesn't +change your current working directory, which is unaffected.) For security +reasons, this call is restricted to the superuser. If FILENAME is +omitted, does chroot to $_. + +=item close FILEHANDLE + +Closes the file or pipe associated with the file handle, returning TRUE +only if stdio successfully flushes buffers and closes the system file +descriptor. You don't have to close FILEHANDLE if you are immediately +going to do another open() on it, because open() will close it for you. (See +open().) However, an explicit close on an input file resets the line +counter ($.), while the implicit close done by open() does not. Also, +closing a pipe will wait for the process executing on the pipe to +complete, in case you want to look at the output of the pipe +afterwards. Closing a pipe explicitly also puts the status value of +the command into C<$?>. Example: + + open(OUTPUT, '|sort >foo'); # pipe to sort + ... # print stuff to output + close OUTPUT; # wait for sort to finish + open(INPUT, 'foo'); # get sort's results + +FILEHANDLE may be an expression whose value gives the real filehandle name. + +=item closedir DIRHANDLE + +Closes a directory opened by opendir(). + +=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 C or +C), it is always executed just before the conditional is about to +be evaluated again, just like the third part of a C 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). + +=item cos EXPR + +Returns the cosine of EXPR (expressed in radians). If EXPR is omitted +takes cosine of $_. + +For the inverse cosine operation, you may use the POSIX::acos() +function, or use this relation: + + sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } + +=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 there is no corresponding decrypt, so this fucntion isn't +all that useful for cryptography. (For that, see your nearby CPAN mirror.) + +Here's an example that makes sure that whoever runs this program knows +their own password: + + $pwd = (getpwuid($<))[1]; + $salt = substr($pwd, 0, 2); + + system "stty -echo"; + print "Password: "; + chop($word = ); + print "\n"; + system "stty echo"; + + if (crypt($word, $salt) ne $pwd) { + die "Sorry...\n"; + } else { + print "ok\n"; + } + +Of course, typing in your own password to whomever asks you +for it is unwise. + +=item dbmclose HASH + +[This function has been superseded by the untie() function.] + +Breaks the binding between a DBM file and a hash. + +=item dbmopen HASH,DBNAME,MODE + +[This function has been superseded by the tie() function.] + +This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a +hash. HASH is the name of the hash. (Unlike normal open, the first +argument is I a filehandle, even though it looks like one). DBNAME +is the name of the database (without the F<.dir> or F<.pag> extension if +any). If the database does not exist, it is created with protection +specified by MODE (as modified by the umask()). If your system supports +only the older DBM functions, you may perform only one dbmopen() in your +program. In older versions of Perl, if your system had neither DBM nor +ndbm, calling dbmopen() produced a fatal error; it now falls back to +sdbm(3). + +If you don't have write access to the DBM file, you can only read hash +variables, not set them. If you want to test whether you can write, +either use file tests or try setting a dummy hash entry inside an eval(), +which will trap the error. + +Note that functions such as keys() and values() may return huge array +values when used on large DBM files. You may prefer to use the each() +function to iterate over large DBM files. Example: + + # print out history file offsets + dbmopen(%HIST,'/usr/lib/news/history',0666); + while (($key,$val) = each %HIST) { + print $key, ' = ', unpack('L',$val), "\n"; + } + dbmclose(%HIST); + +See also 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 "0", which are all equally +false.) Note that since C is a valid scalar, its presence +doesn't I indicate an exceptional condition: pop() +returns C when its argument is an empty array, I when the +element to return happens to be C. + +You may also use defined() to check whether a subroutine exists. On +the other hand, use of defined() upon aggregates (hashes and arrays) +is not guaranteed to produce intuitive results, and should probably be +avoided. + +When used on a hash element, it tells you whether the value is defined, +not whether the key exists in the hash. Use 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 $sym); + eval '@foo = ()' if defined(@foo); + die "No XYZ package defined" unless defined %_XYZ; + sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } + $debugging = 0 unless defined $debugging; + +Note: Many folks tend to overuse defined(), and then are surprised to +discover that the number 0 and "" (the zero-length string) are, in fact, +defined values. For example, if you say + + "ab" =~ /a(.*)b/; + +the pattern match succeeds, and $1 is defined, despite the fact that it +matched "nothing". But it didn't really match nothing--rather, it +matched something that happened to be 0 characters long. This is all +very above-board and honest. When a function returns an undefined value, +it's an admission that it couldn't give you an honest answer. So you +should use defined() only when you're questioning the integrity of what +you're trying to do. At other times, a simple comparison to 0 or "" is +what you want. + +Currently, using defined() on an entire array or hash reports whether +memory for that aggregate has ever been allocated. So an array you set +to the empty list appears undefined initially, and one that once was full +and that you then set to the empty list still appears defined. You +should instead use a simple test for size: + + if (@an_array) { print "has array elements\n" } + if (%a_hash) { print "has hash members\n" } + +Using undef() on these, however, does clear their memory and then report +them as not defined anymore, but you shoudln't do that unless you don't +plan to use them again, because it saves time when you load them up +again to have memory already ready to be filled. + +This counter-intuitive behaviour of defined() on aggregates may be +changed, fixed, or broken in a future release of Perl. + +See also L, L, L. + +=item delete EXPR + +Deletes the specified key(s) and their associated values from a hash. +For each key, returns the deleted value associated with that key, or +the undefined value if there was no such key. Deleting from C<$ENV{}> +modifies the environment. Deleting from a hash tied to a DBM file +deletes the entry from the DBM file. (But deleting from a tie()d hash +doesn't necessarily return anything.) + +The following deletes all the values of a hash: + + foreach $key (keys %HASH) { + delete $HASH{$key}; + } + +And so does this: + + delete @HASH{keys %HASH} + +(But both of these are slower than the undef() command.) Note that the +EXPR can be arbitrarily complicated as long as the final operation is a +hash element lookup or hash slice: + + delete $ref->[$x][$y]{$key}; + delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; + +=item die LIST + +Outside of an eval(), prints the value of LIST to C and exits with +the current value of C<$!> (errno). If C<$!> is 0, exits with the value of +C<($? EE 8)> (back-tick `command` status). If C<($? EE 8)> +is 0, exits with 255. Inside an eval(), the error message is stuffed into +C<$@>, and the eval() is terminated with the undefined value; this makes +die() the way to raise an exception. + +Equivalent examples: + + die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; + chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" + +If the value of EXPR does not end in a newline, the current script line +number and input line number (if any) are also printed, and a newline +is supplied. Hint: sometimes appending ", stopped" to your message +will cause it to make better sense when the string "at foo line 123" is +appended. Suppose you are running script "canasta". + + die "/etc/games is no good"; + die "/etc/games is no good, stopped"; + +produce, respectively + + /etc/games is no good at canasta line 123. + /etc/games is no good, stopped at canasta line 123. + +See also exit() and warn(). + +You can arrange for a callback to be called just before the die() does +its deed, by setting the C<$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 for details on +setting C<%SIG> entries, and eval() for some examples. + +=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, executes the BLOCK once before testing the loop condition. +(On other statements the loop modifiers test the conditional first.) + +=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 + + eval `cat stat.pl`; + +except that it's more efficient, more concise, keeps track of the +current filename for error messages, and searches all the B<-I> +libraries if the file isn't in the current directory (see also the @INC +array in L). It's the same, however, in that it does +re-parse the file every time you call it, so you probably don't want to +do this inside a loop. + +Note that inclusion of library modules is better done with the +use() and require() operators, which also do error checking +and raise an exception if there's a problem. + +=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 LABEL +is omitted, restarts the program from the top. WARNING: any files +opened at the time of the dump will NOT be open any more when the +program is reincarnated, with possible resulting confusion on the part +of Perl. See also B<-u> option in L. + +Example: + + #!/usr/bin/perl + require 'getopt.pl'; + require 'stat.pl'; + %days = ( + 'Sun' => 1, + 'Mon' => 2, + 'Tue' => 3, + 'Wed' => 4, + 'Thu' => 5, + 'Fri' => 6, + 'Sat' => 7, + ); + + dump QUICKSTART if $ARGV[0] eq '-d'; + + QUICKSTART: + Getopt('f'); + +=item each HASH + +When called in a list context, returns a 2-element array consisting of the +key and value for the next element of a hash, so that you can iterate over +it. When called in a scalar context, returns the key for only the next +element in the hash. (Note: Keys may be "0" or "", which are logically +false; you may wish to avoid constructs like C +for this reason.) + +Entries are returned in an apparently random order. When the hash is +entirely read, a null array is returned in list context (which when +assigned produces a FALSE (0) value), and C is returned in a +scalar context. The next call to each() after that will start iterating +again. There is a single iterator for each hash, shared by all each(), +keys(), and values() function calls in the program; it can be reset by +reading all the elements from the hash, or by evaluating C or +C. If you add or delete elements of a hash while you're +iterating over it, you may get entries skipped or duplicated, so don't. + +The following prints out your environment like the printenv(1) program, +only in a different order: + + while (($key,$value) = each %ENV) { + print "$key=$value\n"; + } + +See also keys() and values(). + +=item eof FILEHANDLE + +=item eof () + +=item eof + +Returns 1 if the next read on FILEHANDLE will return end of file, or if +FILEHANDLE is not open. FILEHANDLE may be an expression whose value +gives the real filehandle name. (Note that this function actually +reads a character and then ungetc()s it, so it is not very useful in an +interactive context.) Do not read from a terminal file (or call +C on it) after end-of-file is reached. Filetypes such +as terminals may lose the end-of-file condition if you do. + +An C without an argument uses the last file read as argument. +Empty parentheses () may be used to indicate the pseudo file formed of +the files listed on the command line, i.e., C is reasonable to +use inside a CE)> loop to detect the end of only the +last file. Use C or eof without the parentheses to test +I file in a while (EE) loop. Examples: + + # reset line numbering on each input file + while (<>) { + print "$.\t$_"; + close(ARGV) if (eof); # Not eof(). + } + + # insert dashes just before last line of last file + while (<>) { + if (eof()) { + print "--------------\n"; + close(ARGV); # close or break; is needed if we + # are reading from the terminal + } + print; + } + +Practical hint: you almost never need to use C in Perl, because the +input operators return undef when they run out of data. + +=item eval EXPR + +=item eval BLOCK + +EXPR is parsed and executed as if it were a little Perl program. It +is executed in the context of the current Perl program, so that any +variable settings or subroutine and format definitions remain afterwards. +The value returned is the value of the last expression evaluated, or a +return statement may be used, just as with subroutines. The last +expression is evaluated in scalar or array context, depending on the +context of the eval. + +If there is a syntax error or runtime error, or a die() statement is +executed, an undefined value is returned by eval(), and C<$@> is set to the +error message. If there was no error, C<$@> is guaranteed to be a null +string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if +any, may be omitted from the expression. Beware that using eval() +neither silences perl from printing warnings to STDERR, nor does it +stuff the text of warning messages into C<$@>. To do either of those, +you have to use the C<$SIG{__WARN__}> facility. See warn() and L. + +Note that, because eval() traps otherwise-fatal errors, it is useful for +determining whether a particular feature (such as socket() or symlink()) +is implemented. It is also Perl's exception trapping mechanism, where +the die operator is used to raise exceptions. + +If the code to be executed doesn't vary, you may use the eval-BLOCK +form to trap run-time errors without incurring the penalty of +recompiling each time. The error, if any, is still returned in C<$@>. +Examples: + + # make divide-by-zero non-fatal + eval { $answer = $a / $b; }; warn $@ if $@; + + # same thing, but less efficient + eval '$answer = $a / $b'; warn $@ if $@; + + # a compile-time error + eval { $answer = }; + + # a run-time error + eval '$answer ='; # sets $@ + +When using the eval{} form as an exception trap in libraries, you may +wish not to trigger any C<__DIE__> hooks that user code may have +installed. You can use the C construct for this +purpose, as shown in this example: + + # a very private exception trap for divide-by-zero + eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@; + +This is especially significant, given that C<__DIE__> hooks can call +die() again, which has the effect of changing their error messages: + + # __DIE__ hooks may modify error messages + { + local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; + eval { die "foo foofs here" }; + print $@ if $@; # prints "bar barfs here" + } + +With an eval(), you should be especially careful to remember what's +being looked at when: + + eval $x; # CASE 1 + eval "$x"; # CASE 2 + + eval '$x'; # CASE 3 + eval { $x }; # CASE 4 + + eval "\$$x++" # CASE 5 + $$x++; # CASE 6 + +Cases 1 and 2 above behave identically: they run the code contained in +the variable $x. (Although case 2 has misleading double quotes making +the reader wonder what else might be happening (nothing is).) Cases 3 +and 4 likewise behave in the same way: they run the code '$x', which +does nothing but return the value of C<$x>. (Case 4 is preferred for +purely visual reasons, but it also has the advantage of compiling at +compile-time instead of at run-time.) Case 5 is a place where +normally you I like to use double quotes, except that in that +particular situation, you can just use symbolic references instead, as +in case 6. + +=item exec LIST + +The exec() function executes a system command I, +unless the command does not exist and is executed directly instead of +via C (see below). Use system() instead of exec() if you +want it to return. + +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. If there are any, the entire argument is passed to +C for parsing. If there are none, the argument 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"; + +If you don't really want to execute the first argument, but want to lie +to the program you are executing about its own name, you can specify +the program you actually want to run as an "indirect object" (without a +comma) in front of the LIST. (This always forces interpretation of the +LIST as a multi-valued list, even if there is only a single scalar in +the list.) Example: + + $shell = '/bin/csh'; + exec $shell '-sh'; # pretend it's a login shell + +or, more directly, + + exec {'/bin/csh'} '-sh'; # pretend it's a login shell + +=item exists EXPR + +Returns TRUE if the specified hash key exists in its hash array, even +if the corresponding value is undefined. + + print "Exists\n" if exists $array{$key}; + print "Defined\n" if defined $array{$key}; + print "True\n" if $array{$key}; + +A hash element can be TRUE only if it's defined, and defined if +it exists, but the reverse doesn't necessarily hold true. + +Note that the EXPR can be arbitrarily complicated as long as the final +operation is a hash key lookup: + + if (exists $ref->[$x][$y]{$key}) { ... } + +=item exit EXPR + +Evaluates EXPR and exits immediately with that value. (Actually, it +calls any defined C routines first, but the C routines may not +abort the exit. Likewise any object destructors that need to be called +are called before exit.) Example: + + $ans = ; + exit 0 if $ans =~ /^[Xx]/; + +See also die(). If EXPR is omitted, exits with 0 status. The only +univerally portable values for EXPR are 0 for success and 1 for error; +all other values are subject to unpredictable interpretation depending +on the environment in which the Perl program is running. + +You shouldn't use exit() to abort a subroutine if there's any chance that +someone might want to trap whatever error happened. Use die() instead, +which can be trapped by an eval(). + +=item exp EXPR + +=item exp + +Returns I (the natural logarithm base) to the power of EXPR. +If EXPR is omitted, gives C. + +=item fcntl FILEHANDLE,FUNCTION,SCALAR + +Implements the fcntl(2) function. You'll probably have to say + + use Fcntl; + +first to get the correct function definitions. Argument processing and +value return works just like ioctl() below. Note that fcntl() will produce +a fatal error if used on a machine that doesn't implement fcntl(2). +For example: + + use Fcntl; + fcntl($filehandle, F_GETLK, $packed_return_buffer); + +=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. + +=item flock FILEHANDLE,OPERATION + +Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for +success, FALSE on failure. Will produce a fatal error if used on a +machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). +flock() is Perl's portable file locking interface, although it will lock +only entire files, not records. + +OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with +LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but +you can use the symbolic names if you pull them in with an explicit +request to the Fcntl module. The names can be requested as a group with +the :flock tag (or they can be requested individually, of course). +LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and +LOCK_UN releases a previously requested lock. If LOCK_NB is added to +LOCK_SH or LOCK_EX then flock() will return immediately rather than +blocking waiting for the lock (check the return status to see if you got +it). + +Note that the emulation built with lockf(3) doesn't provide shared +locks, and it requires that FILEHANDLE be open with write intent. These +are the semantics that lockf(3) implements. Most (all?) systems +implement lockf(3) in terms of fcntl(2) locking, though, so the +differing semantics shouldn't bite too many people. + +Note also that some versions of flock() cannot lock things over the +network; you would need to use the more system-specific fcntl() for +that. If you like you can force Perl to ignore your system's flock(2) +function, and so provide its own fcntl(2)-based emulation, by passing +the switch C<-Ud_flock> to the F program when you configure +perl. + +Here's a mailbox appender for BSD systems. + + use Fcntl ':flock'; # import LOCK_* constants + + sub lock { + flock(MBOX,LOCK_EX); + # and, in case someone appended + # while we were waiting... + seek(MBOX, 0, 2); + } + + sub unlock { + flock(MBOX,LOCK_UN); + } + + open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") + or die "Can't open mailbox: $!"; + + lock(); + print MBOX $msg,"\n\n"; + unlock(); + +See also L for other flock() examples. + +=item fork + +Does a fork(2) system call. Returns the child pid to the parent process +and 0 to the child process, or C 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. + +If you fork() without ever waiting on your children, you will accumulate +zombies: + + $SIG{CHLD} = sub { wait }; + +There's also the double-fork trick (error checking on +fork() returns omitted); + + unless ($pid = fork) { + unless (fork) { + exec "what you really wanna do"; + die "no exec"; + # ... or ... + ## (some_perl_code_here) + exit 0; + } + exit 0; + } + waitpid($pid,0); + +See also L for more examples of forking and reaping +moribund children. + +Note that if your forked child inherits system file descriptors like +STDIN and STDOUT that are actually connected by a pipe or socket, even +if you exit, the remote server (such as, say, httpd or rsh) won't think +you're done. You should reopen those to /dev/null if it's any issue. + +=item format + +Declare a picture format with use by the write() function. For +example: + + format Something = + Test: @<<<<<<<< @||||| @>>>>> + $str, $%, '$' . int($num) + . + + $str = "widget"; + $num = $cost/$quantity; + $~ = 'Something'; + write; + +See L for many details and examples. + + +=item formline PICTURE, LIST + +This is an internal function used by Cs, though you may call it +too. It formats (see L) a list of values according to the +contents of PICTURE, placing the output into the format output +accumulator, C<$^A> (or $ACCUMULATOR in English). +Eventually, when a write() is done, the contents of +C<$^A> are written to some filehandle, but you could also read C<$^A> +yourself and then set C<$^A> back to "". Note that a format typically +does one formline() per line of form, but the formline() function itself +doesn't care how many newlines are embedded in the PICTURE. This means +that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. +You may therefore need to use multiple formlines to implement a single +record format, just like the format compiler. + +Be careful if you put double quotes around the picture, because an "C<@>" +character may be taken to mean the beginning of an array name. +formline() always returns TRUE. See L for other examples. + +=item getc FILEHANDLE + +=item getc + +Returns the next character from the input file attached to FILEHANDLE, +or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN. +This is not particularly efficient. It cannot be used to get unbuffered +single-characters, however. For that, try something more like: + + if ($BSD_STYLE) { + system "stty cbreak /dev/tty 2>&1"; + } + else { + system "stty", '-icanon', 'eol', "\001"; + } + + $key = getc(STDIN); + + if ($BSD_STYLE) { + system "stty -cbreak /dev/tty 2>&1"; + } + else { + system "stty", 'icanon', 'eol', '^@'; # ASCII null + } + print "\n"; + +Determination of whether $BSD_STYLE should be set +is left as an exercise to the reader. + +The POSIX::getattr() function can do this more portably on systems +alleging POSIX compliance. +See also the C module from your nearest CPAN site; +details on CPAN can be found on L. + +=item getlogin + +Returns the current login from F, if any. If null, use +getpwuid(). + + $login = getlogin || getpwuid($<) || "Kilroy"; + +Do not consider getlogin() for authentication: it is not as +secure as getpwuid(). + +=item getpeername SOCKET + +Returns the packed sockaddr address of other end of the SOCKET connection. + + use Socket; + $hersockaddr = getpeername(SOCK); + ($port, $iaddr) = unpack_sockaddr_in($hersockaddr); + $herhostname = gethostbyaddr($iaddr, AF_INET); + $herstraddr = inet_ntoa($iaddr); + +=item getpgrp PID + +Returns the current process group for the specified PID. Use +a PID of 0 to get the current process group for the +current process. Will raise an exception if used on a machine that +doesn't implement getpgrp(2). If PID is omitted, returns process +group of current process. Note that the POSIX version of getpgrp() +does not accept a PID argument, so only PID==0 is truly portable. + +=item getppid + +Returns the process id of the parent process. + +=item getpriority WHICH,WHO + +Returns the current priority for a process, a process group, or a user. +(See L.) Will raise a fatal exception if used on a +machine that doesn't implement getpriority(2). + +=item getpwnam NAME + +=item getgrnam NAME + +=item gethostbyname NAME + +=item getnetbyname NAME + +=item getprotobyname NAME + +=item getpwuid UID + +=item getgrgid GID + +=item getservbyname NAME,PROTO + +=item gethostbyaddr ADDR,ADDRTYPE + +=item getnetbyaddr ADDR,ADDRTYPE + +=item getprotobynumber NUMBER + +=item getservbyport PORT,PROTO + +=item getpwent + +=item getgrent + +=item gethostent + +=item getnetent + +=item getprotoent + +=item getservent + +=item setpwent + +=item setgrent + +=item sethostent STAYOPEN + +=item setnetent STAYOPEN + +=item setprotoent STAYOPEN + +=item setservent STAYOPEN + +=item endpwent + +=item endgrent + +=item endhostent + +=item endnetent + +=item endprotoent + +=item endservent + +These routines perform the same functions as their counterparts in the +system library. Within a list context, the return values from the +various get routines are as follows: + + ($name,$passwd,$uid,$gid, + $quota,$comment,$gcos,$dir,$shell) = getpw* + ($name,$passwd,$gid,$members) = getgr* + ($name,$aliases,$addrtype,$length,@addrs) = gethost* + ($name,$aliases,$addrtype,$net) = getnet* + ($name,$aliases,$proto) = getproto* + ($name,$aliases,$port,$proto) = getserv* + +(If the entry doesn't exist you get a null list.) + +Within a scalar context, you get the name, unless the function was a +lookup by name, in which case you get the other thing, whatever it is. +(If the entry doesn't exist you get the undefined value.) For example: + + $uid = getpwnam + $name = getpwuid + $name = getpwent + $gid = getgrnam + $name = getgrgid + $name = getgrent + etc. + +The $members value returned by I is a space separated list of +the login names of the members of the group. + +For the I functions, if the C variable is supported in +C, it will be returned to you via C<$?> if the function call fails. The +@addrs value returned by a successful call is a list of the raw +addresses returned by the corresponding system library call. In the +Internet domain, each address is four bytes long and you can unpack it +by saying something like: + + ($a,$b,$c,$d) = unpack('C4',$addr[0]); + +=item getsockname SOCKET + +Returns the packed sockaddr address of this end of the SOCKET connection. + + use Socket; + $mysockaddr = getsockname(SOCK); + ($port, $myaddr) = unpack_sockaddr_in($mysockaddr); + +=item getsockopt SOCKET,LEVEL,OPTNAME + +Returns the socket option requested, or undefined if there is an error. + +=item glob EXPR + +=item glob + +Returns the value of EXPR with filename expansions such as a shell +would do. This is the internal function implementing the <*.c> +operator, except it's easier to use. If EXPR is omitted, $_ is used. + +=item gmtime EXPR + +Converts a time as returned by the time function to a 9-element array +with the time localized for the standard Greenwich time zone. +Typically used as follows: + + + ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = + gmtime(time); + +All array elements are numeric, and come straight out of a struct tm. +In particular this means that $mon has the range 0..11 and $wday has +the range 0..6. Also, $year is the number of years since 1900, I +simply the last two digits of the year. + +If EXPR is omitted, does C. + +In a scalar context, prints out the ctime(3) value: + + $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" + +Also see the F library, and the strftime(3) function available +via the POSIX module. + +=item goto LABEL + +=item goto EXPR + +=item goto &NAME + +The goto-LABEL form finds the statement labeled with LABEL and resumes +execution there. It may not be used to go into any construct that +requires initialization, such as a subroutine or a foreach loop. It +also can't be used to go into a construct that is optimized away, +or to get out of a block or subroutine given to sort(). +It can be used to go almost anywhere else within the dynamic scope, +including out of subroutines, but it's usually better to use some other +construct such as last or die. The author of Perl has never felt the +need to use this form of goto (in Perl, that is--C is another matter). + +The goto-EXPR form expects a label name, whose scope will be resolved +dynamically. This allows for computed gotos per FORTRAN, but isn't +necessarily recommended if you're optimizing for maintainability: + + goto ("FOO", "BAR", "GLARCH")[$i]; + +The goto-&NAME form is highly magical, and substitutes a call to the +named subroutine for the currently running subroutine. This is used by +AUTOLOAD subroutines that wish to load another subroutine and then +pretend that the other subroutine had been called in the first place +(except that any modifications to @_ in the current subroutine are +propagated to the other subroutine.) After the goto, not even caller() +will be able to tell that this routine was called first. + +=item grep BLOCK LIST + +=item grep EXPR,LIST + +This is similar in spirit to, but not the same as, L +and its relatives. In particular, it is not limited to using +regular expressions. + +Evaluates the BLOCK or EXPR for each element of LIST (locally setting +$_ to each element) and returns the list value consisting of those +elements for which the expression evaluated to TRUE. In a scalar +context, returns the number of times the expression was TRUE. + + @foo = grep(!/^#/, @bar); # weed out comments + +or equivalently, + + @foo = grep {!/^#/} @bar; # weed out comments + +Note that, because $_ is a reference into the list value, it can be used +to modify the elements of the array. While this is useful and +supported, it can cause bizarre results if the LIST is not a named +array. Similarly, grep returns aliases into the original list, +much like the way that L's index variable aliases the list +elements. That is, modifying an element of a list returned by grep +actually modifies the element in the original list. + +=item hex EXPR + +=item hex + +Interprets EXPR as a hex string and returns the corresponding +value. (To convert strings that might start with either 0 or 0x +see L.) If EXPR is omitted, uses $_. + + print hex '0xAf'; # prints '175' + print hex 'aF'; # same + +=item import + +There is no built-in import() function. It is merely an ordinary +method (subroutine) defined (or inherited) by modules that wish to export +names to another module. The use() function calls the import() method +for the package used. See also L, L, and L. + +=item index STR,SUBSTR,POSITION + +=item index STR,SUBSTR + +Returns the position of the first occurrence of SUBSTR in STR at or after +POSITION. If POSITION is omitted, starts searching from the beginning of +the string. The return value is based at 0 (or whatever you've set the C<$[> +variable to--but don't do that). If the substring is not found, returns +one less than the base, ordinarily -1. + +=item int EXPR + +=item int + +Returns the integer portion of EXPR. If EXPR is omitted, uses $_. + +=item ioctl FILEHANDLE,FUNCTION,SCALAR + +Implements the ioctl(2) function. You'll probably have to say + + require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph + +first to get the correct function definitions. If F 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 Fsys/ioctl.hE>. +(There is a Perl script called B that comes with the Perl kit which +may help you in this, but it's non-trivial.) SCALAR will be read and/or +written depending on the FUNCTION--a pointer to the string value of SCALAR +will be passed as the third argument of the actual ioctl call. (If SCALAR +has no string value but does have a numeric value, that value will be +passed rather than a pointer to the string value. To guarantee this to be +TRUE, add a 0 to the scalar before using it.) The pack() and unpack() +functions are useful for manipulating the values of structures used by +ioctl(). The following example sets the erase character to DEL. + + require 'ioctl.ph'; + $getp = &TIOCGETP; + die "NO TIOCGETP" if $@ || !$getp; + $sgttyb_t = "ccccs"; # 4 chars and a short + if (ioctl(STDIN,$getp,$sgttyb)) { + @ary = unpack($sgttyb_t,$sgttyb); + $ary[2] = 127; + $sgttyb = pack($sgttyb_t,@ary); + ioctl(STDIN,&TIOCSETP,$sgttyb) + || die "Can't ioctl: $!"; + } + +The return value of ioctl (and fcntl) is as follows: + + if OS returns: then Perl returns: + -1 undefined value + 0 string "0 but true" + anything else that number + +Thus Perl returns TRUE on success and FALSE on failure, yet you can +still easily determine the actual value returned by the operating +system: + + ($retval = ioctl(...)) || ($retval = -1); + printf "System returned %d\n", $retval; + +=item join EXPR,LIST + +Joins the separate strings of LIST or ARRAY into a single string with +fields separated by the value of EXPR, and returns the string. +Example: + + $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); + +See L. + +=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 +an apparently random order, but it is the same order as either the +values() or each() function produces (given that the hash has not been +modified). As a side effect, it resets HASH's iterator. + +Here is yet another way to print your environment: + + @keys = keys %ENV; + @values = values %ENV; + while ($#keys >= 0) { + print pop(@keys), '=', pop(@values), "\n"; + } + +or how about sorted by key: + + foreach $key (sort(keys %ENV)) { + print $key, '=', $ENV{$key}, "\n"; + } + +To sort an array by value, you'll need to use a C function. +Here's a descending numeric sort of a hash by its values: + + foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) { + printf "%4d %s\n", $hash{$key}, $key; + } + +As an lvalue C allows you to increase the number of hash buckets +allocated for the given hash. This can gain you a measure of efficiency if +you know the hash is going to get big. (This is similar to pre-extending +an array by assigning a larger number to $#array.) If you say + + keys %hash = 200; + +then C<%hash> will have at least 200 buckets allocated for it. These +buckets will be retained even if you do C<%hash = ()>, use C 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 +C in this way (but you needn't worry about doing this by accident, +as trying has no effect). + +=item kill LIST + +Sends a signal to a list of processes. The first element of +the list must be the signal to send. Returns the number of +processes successfully signaled. + + $cnt = kill 1, $child1, $child2; + kill 9, @goners; + +Unlike in the shell, in Perl if the I is negative, it kills +process groups instead of processes. (On System V, a negative I +number will also kill process groups, but that's not portable.) That +means you usually want to use positive not negative signals. You may also +use a signal name in quotes. See L for details. + +=item last LABEL + +=item last + +The C command is like the C statement in C (as used in +loops); it immediately exits the loop in question. If the LABEL is +omitted, the command refers to the innermost enclosing loop. The +C block, if any, is not executed: + + LINE: while () { + last LINE if /^$/; # exit when done with header + ... + } + +=item lc EXPR + +=item lc + +Returns an lowercased version of EXPR. This is the internal function +implementing the \L escape in double-quoted strings. +Respects current LC_CTYPE locale if C in force. See L. + +If EXPR is omitted, uses $_. + +=item lcfirst EXPR + +=item lcfirst + +Returns the value of EXPR with the first character lowercased. This is +the internal function implementing the \l escape in double-quoted strings. +Respects current LC_CTYPE locale if C in force. See L. + +If EXPR is omitted, uses $_. + +=item length EXPR + +=item length + +Returns the length in characters 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. + +=item listen SOCKET,QUEUESIZE + +Does the same thing that the listen system call does. Returns TRUE if +it succeeded, FALSE otherwise. See example in L. + +=item local EXPR + +A local modifies the listed variables to be local to the enclosing block, +subroutine, C, or C. If more than one value is listed, the +list must be placed in parentheses. See L for details. + +But you really probably want to be using my() instead, because local() isn't +what most people think of as "local"). See L for details. + +=item localtime EXPR + +Converts a time as returned by the time function to a 9-element array +with the time analyzed for the local time zone. Typically used as +follows: + + ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = + localtime(time); + +All array elements are numeric, and come straight out of a struct tm. +In particular this means that $mon has the range 0..11 and $wday has +the range 0..6 and $year is year-1900, that is, $year is 123 in year +2023. If EXPR is omitted, uses the current time ("localtime(time)"). + +In a scalar context, returns the ctime(3) value: + + $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" + +Also see the Time::Local module, and the strftime(3) function available +via the POSIX module. + +=item log EXPR + +=item log + +Returns logarithm (base I) of EXPR. If EXPR is omitted, returns log +of $_. + +=item lstat FILEHANDLE + +=item lstat EXPR + +=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. + +If EXPR is omitted, stats $_. + +=item m// + +The match operator. See L. + +=item map BLOCK LIST + +=item map EXPR,LIST + +Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each +element) and returns the list value composed of the results of each such +evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST +may produce zero, one, or more elements in the returned value. + + @chars = map(chr, @nums); + +translates a list of numbers to the corresponding characters. And + + %hash = map { getkey($_) => $_ } @array; + +is just a funny way to write + + %hash = (); + foreach $_ (@array) { + $hash{getkey($_)} = $_; + } + +=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). + +=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. +Returns like ioctl: the undefined value for error, "0 but true" for +zero, or the actual return value otherwise. + +=item msgget KEY,FLAGS + +Calls the System V IPC function msgget(2). Returns the message queue id, +or the undefined value if there is an error. + +=item msgsnd ID,MSG,FLAGS + +Calls the System V IPC function msgsnd to send the message MSG to the +message queue ID. MSG must begin with the long integer message type, +which may be created with C. Returns TRUE if +successful, or FALSE if there is an error. + +=item msgrcv ID,VAR,SIZE,TYPE,FLAGS + +Calls the System V IPC function msgrcv to receive a message from +message queue ID into variable VAR with a maximum message size of +SIZE. Note that if a message is received, the message type will be the +first thing in VAR, and the maximum length of VAR is SIZE plus the size +of the message type. Returns TRUE if successful, or FALSE if there is +an error. + +=item my EXPR + +A "my" declares the listed variables to be local (lexically) to the +enclosing block, subroutine, C, or C'd file. If +more than one value is listed, the list must be placed in parentheses. See +L for details. + +=item next LABEL + +=item next + +The C command is like the C statement in C; it starts +the next iteration of the loop: + + LINE: while () { + next LINE if /^#/; # discard comments + ... + } + +Note that if there were a C 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. + +=item no Module LIST + +See the "use" function, which "no" 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 0x, interprets it as +a hex string instead.) The following will handle decimal, octal, and +hex in the standard Perl or C notation: + + $val = oct($val) if $val =~ /^0/; + +If EXPR is omitted, uses $_. This function is commonly used when +a string such as "644" needs to be converted into a file mode, for +example. (Although perl will automatically convert strings into +numbers as needed, this automatic conversion assumes base 10.) + +=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 '<' 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 +you want both read and write access to the file; thus '+<' is almost +always preferred for read/write updates--the '+>' mode would clobber the +file first. The prefix and the filename may be separated with spaces. +These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w', +'w+', 'a', and 'a+'. + +If the filename begins with "|", the filename is interpreted as a command +to which output is to be piped, and if the filename ends with a "|", the +filename is interpreted See L 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 out, but see +L, L, and L +for alternatives.) + +Opening '-' opens STDIN and opening 'E-' opens STDOUT. Open returns +non-zero upon success, the undefined value otherwise. If the open +involved a pipe, the return value happens to be the pid of the +subprocess. + +If you're unfortunate enough to be running Perl on a system that +distinguishes between text files and binary files (modern operating +systems don't care), then you should check out L for tips for +dealing with this. The key distinction between systems that need binmode +and those that don't is their text file formats. Systems like Unix and +Plan9 that delimit lines with a single character, and that encode that +character in C as '\n', do not need C. The rest need it. + +Examples: + + $ARTICLE = 100; + open ARTICLE or die "Can't find article $ARTICLE: $!\n"; + while (
) {... + + open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) + + open(DBASE, '+/tmp/Tmp$$"); # $$ is our process id + + # process argument list of files along with any includes + + foreach $file (@ARGV) { + process($file, 'fh00'); + } + + sub process { + local($filename, $input) = @_; + $input++; # this is a string increment + unless (open($input, $filename)) { + print STDERR "Can't open $filename: $!\n"; + return; + } + + while (<$input>) { # note use of indirection + if (/^#include "(.*)"/) { + process($1, $input); + next; + } + ... # whatever + } + } + +You may also, in the Bourne shell tradition, specify an EXPR beginning +with "E&", in which case the rest of the string is interpreted as the +name of a filehandle (or file descriptor, if numeric) which is to be +duped and opened. You may use & after E, EE, E, +E, ++EE, and +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(SAVEOUT, ">&STDOUT"); + open(SAVEERR, ">&STDERR"); + + open(STDOUT, ">foo.out") || die "Can't redirect stdout"; + open(STDERR, ">&STDOUT") || die "Can't dup stdout"; + + select(STDERR); $| = 1; # make unbuffered + select(STDOUT); $| = 1; # make unbuffered + + print STDOUT "stdout 1\n"; # this works for + print STDERR "stderr 1\n"; # subprocesses too + + close(STDOUT); + close(STDERR); + + open(STDOUT, ">&SAVEOUT"); + open(STDERR, ">&SAVEERR"); + + print STDOUT "stdout 2\n"; + print STDERR "stderr 2\n"; + + +If you specify "E&=N", where N is a number, then Perl will do an +equivalent of C's fdopen() of that file descriptor; this is more +parsimonious of file descriptors. For example: + + open(FILEHANDLE, "<&=$fd") + +If you open a pipe on the command "-", i.e., either "|-" or "-|", then +there is an implicit fork done, and the return value of open is the pid +of the child within the parent process, and 0 within the child +process. (Use 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. + +Explicitly closing any piped filehandle causes the parent process to +wait for the child to finish, and returns the status value in C<$?>. +Note: on any operation which may do a fork, unflushed buffers remain +unflushed in both processes, which means you may need to set C<$|> to +avoid duplicate output. + +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: + + use IO::File; + ... + sub read_myfile_munged { + my $ALL = shift; + my $handle = new IO::File; + open($handle, "myfile") or die "myfile: $!"; + $first = <$handle> + or return (); # Automatically closed here. + mung $first or die "mung failed"; # Or here. + return $first, <$handle> if $ALL; # Or here. + $first; # Or here. + } + +The filename that is passed to open will have leading and trailing +whitespace deleted. To open a file with arbitrary weird +characters in it, it's necessary to protect any leading and trailing +whitespace thusly: + + $file =~ s#^(\s)#./$1#; + open(FOO, "< $file\0"); + +If you want a "real" C open() (see L 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: ", ; + +See L for some details about mixing reading and writing. + +=item opendir DIRHANDLE,EXPR + +Opens a directory named EXPR for processing by readdir(), telldir(), +seekdir(), rewinddir(), and closedir(). Returns TRUE if successful. +DIRHANDLEs have their own namespace separate from FILEHANDLEs. + +=item ord EXPR + +=item ord + +Returns the numeric ascii value of the first character of EXPR. If +EXPR is omitted, uses $_. 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. + s A signed short value. + S An unsigned short value. + i A signed integer value. + I An unsigned integer value. + l A signed long value. + L An unsigned long value. + + n A short in "network" order. + N A long in "network" order. + v A short in "VAX" (little-endian) order. + V A long in "VAX" (little-endian) order. + + f A single-precision float in the native format. + d A double-precision float in the native format. + + p A pointer to a null-terminated string. + P A pointer to a structure (fixed-length string). + + u A uuencoded string. + + w A BER compressed integer. Bytes give an unsigned integer base + 128, most significant digit first, with as few digits as + possible, and with the bit 8 of each byte except the last set + to "1." + + x A null byte. + X Back up a byte. + @ Null fill to absolute position. + +Each letter may optionally be followed by a number which gives a repeat +count. With all types except "a", "A", "b", "B", "h", "H", and "P" the +pack function will gobble up that many values from the LIST. A * for the +repeat count means to use however many items are left. The "a" and "A" +types gobble just one value, but pack it as a string of length count, +padding with nulls or spaces as necessary. (When unpacking, "A" strips +trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B" +fields pack a string that many bits long. The "h" and "H" fields pack a +string that many nybbles long. The "P" packs a pointer to a structure of +the size indicated by the length. 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 $foo). + +Examples: + + $foo = pack("cccc",65,66,67,68); + # foo eq "ABCD" + $foo = pack("c4",65,66,67,68); + # same thing + + $foo = pack("ccxxcc",65,66,67,68); + # foo eq "AB\0\0CD" + + $foo = pack("s2",1,2); + # "\1\0\2\0" on little-endian + # "\0\1\0\2" on big-endian + + $foo = pack("a4","abcd","x","y","z"); + # "abcd" + + $foo = pack("aaaa","abcd","x","y","z"); + # "axyz" + + $foo = pack("a14","abcdefg"); + # "abcdefg\0\0\0\0\0\0\0" + + $foo = pack("i9pl", gmtime); + # a real struct tm (on my system anyway) + + sub bintodec { + unpack("N", pack("B32", substr("0" x 32 . shift, -32))); + } + +The same template may generally also be used in the unpack function. + +=item package NAMESPACE + +Declares the compilation unit as being in the given namespace. The scope +of the package declaration is from the declaration itself through the end of +the enclosing block (the same scope as the local() operator). All further +unqualified dynamic identifiers will be in this namespace. A package +statement affects only dynamic variables--including those you've used +local() on--but I lexical variables created with my(). Typically it +would be the first declaration in a file to be included by the C +or C operator. You can switch into a package in more than one place; +it influences merely which symbol table is used by the compiler for the +rest of that block. You can refer to variables and filehandles in other +packages by prefixing the identifier with the package name and a double +colon: C<$Package::Variable>. If the package name is null, the C
+package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>. + +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. + +=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 +@ARGV array in the main program, and the @_ array in subroutines, just +like shift(). + +=item pos SCALAR + +=item pos + +Returns the offset of where the last C search left off for the variable +is in question ($_ is used when the variable is not specified). May be +modified to change that offset. Such modification will also influence +the 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 + 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 $_ to +STDOUT. To set the default output channel to something other than +STDOUT use the select operation. Note that, because print takes a +LIST, anything in the LIST is evaluated in a list context, and any +subroutine that you call will have one or more of its expressions +evaluated in a list context. Also be careful not to follow the print +keyword with a left parenthesis unless you want the corresponding right +parenthesis to terminate the arguments to the print--interpose a + or +put parentheses around all the arguments. + +Note that if you're storing FILEHANDLES in an array or other expression, +you will have to use a block returning its value instead: + + print { $files[$i] } "stuff\n"; + print { $OK ? STDOUT : STDERR } "stuff\n"; + +=item printf FILEHANDLE FORMAT, LIST + +=item printf FORMAT, LIST + +Equivalent to C. The first argument +of the list will be interpreted as the printf 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 printf() when a simple +print() would do. The print() 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. + +=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 qx/STRING/ + +=item qw/STRING/ + +Generalized quotes. See L. + +=item quotemeta EXPR + +=item quotemeta + +Returns the value of EXPR with 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 \Q escape in double-quoted strings. + +If EXPR is omitted, uses $_. + +=item rand EXPR + +=item rand + +Returns a random fractional number between 0 and the value of EXPR. +(EXPR should be positive.) If EXPR is omitted, returns a value between +0 and 1. Automatically calls srand() unless srand() has already been +called. See also srand(). + +(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, or +undef if there was an error. SCALAR will be grown or shrunk to the +length actually read. An OFFSET may be specified to place the read +data at some other place than the beginning of the string. This call +is actually implemented in terms of stdio's fread call. To get a true +read system call, see sysread(). + +=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 +directory. If there are no more entries, returns an undefined value in +a scalar context or a null list in a list context. + +If you're planning to filetest the return values out of a readdir(), you'd +better prepend the directory in question. Otherwise, because we didn't +chdir() there, it would have been testing the wrong file. + + opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; + @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); + closedir DIR; + +=item readlink EXPR + +=item readlink + +Returns the value of a symbolic link, if symbolic links are +implemented. If not, gives a fatal error. If there is some system +error, returns the undefined value and sets C<$!> (errno). If EXPR is +omitted, uses $_. + +=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 recvfrom(), so that it can returns the address of the +sender. Returns the undefined value if there's an error. SCALAR will +be grown or shrunk to the length actually read. Takes the same flags +as the system call of the same name. +See L for examples. + +=item redo LABEL + +=item redo + +The C command restarts the loop block without evaluating the +conditional again. The C 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; + } + +=item ref EXPR + +=item ref + +Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR +is not specified, $_ will be used. The value returned depends on the +type of thing the reference is a reference to. +Builtin types include: + + REF + SCALAR + ARRAY + HASH + CODE + GLOB + +If the referenced object has been blessed into a package, then that package +name is returned instead. You can think of ref() as a typeof() operator. + + if (ref($r) eq "HASH") { + print "r is a reference to a hash.\n"; + } + if (!ref ($r) { + print "r is not a reference at all.\n"; + } + +See also L. + +=item rename OLDNAME,NEWNAME + +Changes the name of a file. Returns 1 for success, 0 otherwise. Will +not work across file system boundaries. + +=item require EXPR + +=item require + +Demands some semantics specified by EXPR, or by $_ if EXPR is not +supplied. If EXPR is numeric, demands that the current version of Perl +(C<$]> or $PERL_VERSION) be equal or greater than EXPR. + +Otherwise, demands that a library file be included if it hasn't already +been included. The file is included via the do-FILE mechanism, which is +essentially just a variety of eval(). Has semantics similar to the following +subroutine: + + sub require { + local($filename) = @_; + return 1 if $INC{$filename}; + local($realfilename,$result); + ITER: { + foreach $prefix (@INC) { + $realfilename = "$prefix/$filename"; + if (-f $realfilename) { + $result = do $realfilename; + last ITER; + } + } + die "Can't find $filename in \@INC"; + } + die $@ if $@; + die "$filename did not return true value" unless $result; + $INC{$filename} = $realfilename; + $result; + } + +Note that the file will not be included twice under the same specified +name. The file must return TRUE as the last statement to indicate +successful execution of any initialization code, so it's customary to +end such a file with "1;" unless you're sure it'll return TRUE +otherwise. But it's better just to put the "C<1;>", in case you add more +statements. + +If EXPR is a bare word, 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. + +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 ?? searches so that they work again. The +expression is interpreted as a list of single characters (hyphens +allowed for ranges). All variables and arrays beginning with one of +those letters are reset to their pristine state. If the expression is +omitted, one-match searches (?pattern?) are reset to match again. Resets +only variables or searches in the current package. Always returns +1. Examples: + + reset 'X'; # reset all X variables + reset 'a-z'; # reset lower case variables + reset; # just reset ?? searches + +Resetting "A-Z" is not recommended because you'll wipe out your +ARGV and ENV arrays. Resets only package variables--lexical 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 LIST + +Returns from a subroutine or eval with the value specified. (Note that +in the absence of a return a subroutine or eval() 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 +elements of LIST, and returns a string value consisting of those bytes, +but in the opposite order. + + print reverse <>; # line tac, last line first + + undef $/; # for efficiency of <> + print scalar reverse <>; # byte tac, last line tsrif + +This operator is also handy for inverting a hash, although there are some +caveats. If a value is duplicated in the original hash, only one of those +can be represented as a key in the inverted hash. Also, this has to +unwind one hash and build a whole new one, which may take some time +on a large hash. + + %by_name = reverse %by_address; # Invert the hash + +=item rewinddir DIRHANDLE + +Sets the current position to the beginning of the directory for the +readdir() routine on DIRHANDLE. + +=item rindex STR,SUBSTR,POSITION + +=item rindex STR,SUBSTR + +Works just like index except that it returns the position of the LAST +occurrence of SUBSTR in STR. If POSITION is specified, returns the +last occurrence at or before that position. + +=item rmdir FILENAME + +=item rmdir + +Deletes the directory specified by FILENAME if it is empty. If it +succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If +FILENAME is omitted, uses $_. + +=item s/// + +The substitution operator. See L. + +=item scalar EXPR + +Forces EXPR to be interpreted in a scalar context and returns the value +of EXPR. + + @counts = ( scalar @a, scalar @b, scalar @c ); + +There is no equivalent operator to force an expression to +be interpolated in a list context because it's in practice never +needed. If you really wanted to do so, however, you could use +the construction C<@{[ (some expression) ]}>, but usually a simple +C<(some expression)> suffices. + +=item seek FILEHANDLE,POSITION,WHENCE + +Randomly positions the file pointer for FILEHANDLE, just like the fseek() +call of stdio. FILEHANDLE may be an expression whose value gives the name +of the filehandle. The values for WHENCE are 0 to set the file pointer to +POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF +plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for +this from POSIX module. Returns 1 upon success, 0 otherwise. + +On some systems you have to do a seek whenever you switch between reading +and writing. Amongst other things, this may have the effect of calling +stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving +the file pointer: + + seek(TEST,0,1); + +This is also useful for applications emulating C. Once you hit +EOF on your read, and then sleep for a while, you might have to stick in a +seek() to reset things. First the simple trick listed above to clear the +filepointer. The seek() doesn't change the current position, but it +I clear the end-of-file condition on the handle, so that the next +CFILEE> makes Perl try again to read something. We hope. + +If that doesn't work (some stdios are particularly cantankerous), then +you may need something more like this: + + for (;;) { + for ($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 readdir() routine on DIRHANDLE. POS +must be a value returned by telldir(). Has the same caveats about +possible directory compaction as the corresponding system library +routine. + +=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 fileno() and vec(), along these lines: + + $rin = $win = $ein = ''; + vec($rin,fileno(STDIN),1) = 1; + vec($win,fileno(STDOUT),1) = 1; + $ein = $rin | $win; + +If you want to select on many filehandles you might wish to write a +subroutine: + + sub fhbits { + local(@fhlist) = split(' ',$_[0]); + local($bits); + for (@fhlist) { + vec($bits,fileno($_),1) = 1; + } + $bits; + } + $rin = fhbits('STDIN TTY SOCK'); + +The usual idiom is: + + ($nfound,$timeleft) = + select($rout=$rin, $wout=$win, $eout=$ein, $timeout); + +or to block until something becomes ready just do this + + $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); + +Most systems do not bother to return anything useful in $timeleft, so +calling select() in a scalar context just returns $nfound. + +Any of the bit masks can also be undef. The timeout, if specified, is +in seconds, which may be fractional. Note: not all implementations are +capable of returning the $timeleft. If not, they always return +$timeleft equal to the supplied $timeout. + +You can effect a sleep of 250 milliseconds this way: + + select(undef, undef, undef, 0.25); + +B: Do not attempt to mix buffered I/O (like read() or EFHE) +with select(). 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 +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 semget KEY,NSEMS,FLAGS + +Calls the System V IPC function semget. Returns the semaphore id, or +the undefined value if there is an error. + +=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 $semnum of semaphore id $semid: + + $semop = pack("sss", $semnum, -1, 0); + die "Semaphore trouble: $!\n" unless semop($semid, $semop); + +To signal the semaphore, replace "-1" with "1". + +=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 sendto(). 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, 0 for the current +process. Will produce a fatal error if used on a machine that doesn't +implement setpgrp(2). If the arguments are omitted, it defaults to +0,0. Note that the POSIX version of setpgrp() does not accept any +arguments, so only setpgrp 0,0 is portable. + +=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 undef 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 +@ARGV array in the main program, and the @_ array in subroutines. +(This is determined lexically.) See also unshift(), push(), and pop(). +Shift() and unshift() do the same thing to the left end of an array +that pop() and push() do to the right end. + +=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. +Returns like ioctl: the undefined value for error, "0 but true" for +zero, or the actual return value otherwise. + +=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. + +=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 which will +hold the data read. When writing, if STRING is too long, only SIZE +bytes are used; if STRING is too short, nulls are written to fill out +SIZE bytes. Return TRUE if successful, or FALSE if there is an error. + +=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. + +=item sin EXPR + +=item sin + +Returns the sine of EXPR (expressed in radians). If EXPR is omitted, +returns sine of $_. + +For the inverse sine operation, you may use the POSIX::sin() +function, or use this relation: + + sub asin { atan2($_[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 by sending the process a SIGALRM. Returns the +number of seconds actually slept. You probably cannot mix alarm() and +sleep() calls, because sleep() is often implemented using alarm(). + +On some older systems, it may sleep up to a full second less than what +you requested, depending on how it counts seconds. Most modern systems +always sleep the full amount. + +For delays of finer granularity than one second, you may use Perl's +syscall() interface to access setitimer(2) if your system supports it, +or else see L below. + +See also the POSIX module's sigpause() 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 "use Socket;" 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. + +=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, sorts in standard string comparison order. If SUBNAME is +specified, it gives the name of a subroutine that returns an integer +less than, equal to, or greater than 0, depending on how the elements +of the array are to be ordered. (The C=E> and C +operators are extremely useful in such routines.) SUBNAME may be a +scalar variable name, in which case the value provides the name of the +subroutine to use. In place of a SUBNAME, you can provide a BLOCK as +an anonymous, in-line sort subroutine. + +In the interests of efficiency the normal calling code for subroutines is +bypassed, with the following effects: the subroutine may not be a +recursive subroutine, and the two elements to be compared are passed into +the subroutine not via @_ but as the package global variables $a and +$b (see example below). They are passed by reference, so don't +modify $a and $b. And don't try to declare them as lexicals either. + +You also cannot exit out of the sort block or subroutine using any of the +loop control operators described in L or with goto(). + +When C is in effect, C sorts LIST according to the +current collation locale. See L. + +Examples: + + # sort lexically + @articles = sort @files; + + # same thing, but with explicit sort routine + @articles = sort {$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 $a +and $b as lexicals. They are package globals. That means +if you're in the C
package, it's + + @articles = sort {$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 $x[1] is less than $x[2] and +sometimes saying the opposite, for example) the Perl interpreter will +probably crash and dump core. This is entirely due to and dependent +upon your system's qsort(3) library routine; this routine often avoids +sanity checks in the interest of speed. + +=item splice ARRAY,OFFSET,LENGTH,LIST + +=item splice ARRAY,OFFSET,LENGTH + +=item splice ARRAY,OFFSET + +Removes the elements designated by OFFSET and LENGTH from an array, and +replaces them with the elements of LIST, if any. Returns the elements +removed from the array. The array grows or shrinks as necessary. If +LENGTH is omitted, removes everything from OFFSET onward. The +following equivalences hold (assuming C<$[ == 0>): + + push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y) + pop(@a) splice(@a,-1) + shift(@a) splice(@a,0,1) + unshift(@a,$x,$y) splice(@a,0,0,$x,$y) + $a[$x] = $y splice(@a,$x,1,$y); + +Example, assuming array lengths are passed before arrays: + + sub aeq { # compare two list values + local(@a) = splice(@_,0,shift); + local(@b) = splice(@_,0,shift); + return 0 unless @a == @b; # same len? + while (@a) { + return 0 if pop(@a) ne pop(@b); + } + return 1; + } + if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } + +=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. + +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 EXPR is omitted, splits the $_ string. If PATTERN is also omitted, +splits on whitespace (after skipping any leading whitespace). Anything +matching PATTERN is taken to be a delimiter separating the fields. (Note +that the delimiter may be longer than one character.) If LIMIT is +specified and is not negative, splits into no more than that many fields +(though it may split into fewer). If LIMIT is unspecified, trailing null +fields are stripped (which potential users of pop() would do well to +remember). If LIMIT is negative, it is treated as if an arbitrarily large +LIMIT had been specified. + +A pattern matching the null string (not to be confused with +a null pattern C, which is just one member of the set of patterns +matching a null string) will split the value of EXPR into separate +characters at each point it matches that way. For example: + + print join(':', split(/ */, 'hi there')); + +produces the output 'h:i:t:h:e:r:e'. + +The LIMIT parameter can be used to split a line partially + + ($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 $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*/m, $header); + +The pattern C may be replaced with an expression to specify +patterns that vary at runtime. (To do runtime compilation only once, +use C.) + +As a special case, specifying a PATTERN of space (C<' '>) will split on +white space just as split with no arguments does. Thus, split(' ') can +be used to emulate B's default behavior, whereas C +will give you as many null initial fields as there are leading spaces. +A split on /\s+/ is like a split(' ') except that any leading +whitespace produces a null first field. A split with no arguments +really does a C internally. + +Example: + + open(passwd, '/etc/passwd'); + while () { + ($login, $passwd, $uid, $gid, $gcos, + $home, $shell) = split(/:/); + ... + } + +(Note that $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 printf conventions of the C +language. See L or L on your system for details. +(The * character for an indirectly specified length is not +supported, but you can get the same effect by interpolating a variable +into the pattern.) 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. +Some C libraries' implementations of sprintf() can +dump core when fed ludicrous arguments. + +=item sqrt EXPR + +=item sqrt + +Return the square root of EXPR. If EXPR is omitted, returns square +root of $_. + +=item srand EXPR + +=item srand + +Sets the random number seed for the C operator. If EXPR is +omitted, uses a semi-random value based on the current time and process +ID, among other things. In versions of Perl prior to 5.004 the default +seed was just the current time(). This isn't a particularly good seed, +so many old programs supply their own seed value (often C