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GNU Info File | 1994-07-08 | 49.3 KB | 1,197 lines

This is Info file perl.info, produced by Makeinfo-1.55 from the input file perltexi. This file documents perl, Practical Extraction and Report Language, and was originally based on Larry Wall's unix-style man page for perl. GNU Texinfo version adapted by Jeff Kellem <composer@Beyond.Dreams.ORG>. Copyright (C) 1989, 1990, 1991, 1992, 1993 Larry Wall Texinfo version Copyright (C) 1990, 1991, 1993 Jeff Kellem Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Conditions for Using Perl" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English. File: perl.info, Node: Search and Replace Functions, Next: System Interaction, Prev: Input/Output, Up: Commands Search and Replace Functions ============================ m/PATTERN/gio /PATTERN/gio Searches a string for a pattern match, and returns true (1) or false (''). If no string is specified via the `=~' or `!~' operator, the `$_' string is searched. (The string specified with `=~' need not be an lvalue--it may be the result of an expression evaluation, but remember the `=~' binds rather tightly.) *Note Regular Expressions::, for more info. If `/' is the delimiter then the initial `m' is optional. With the `m' you can use any pair of non-alphanumeric characters as delimiters. This is particularly useful for matching Unix path names that contain `/'. If the final delimiter is followed by the optional letter `i', the matching is done in a case-insensitive manner. PATTERN may contain references to scalar variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated. (Note that `$)' and `$|' may not be interpolated because they look like end-of-string tests.) If you want such a pattern to be compiled only once, add an `o' after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won't change over the life of the script. If the PATTERN evaluates to a null string, the most recent successful regular expression is used instead. If used in a context that requires an array value, a pattern match returns an array consisting of the subexpressions matched by the parentheses in the pattern, i.e. `($1, $2, $3...)'. It does *NOT* actually set `$1', `$2', etc. in this case, nor does it set `$+', `$`', `$&' or `$''. If the match fails, a null array is returned. If the match succeeds, but there were no parentheses, an array value of (1) is returned. Examples: open(tty, '/dev/tty'); <tty> =~ /^y/i && do foo(); # do foo if desired if (/Version: *([0-9.]*)/) { $version = $1; } next if m#^/usr/spool/uucp#; # poor man's grep $arg = shift; while (<>) { print if /$arg/o; # compile only once } if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) This last example splits `$foo' into the first two words and the remainder of the line, and assigns those three fields to `$F1', `$F2' and `$Etc'. The conditional is true if any variables were assigned, i.e. if the pattern matched. The `g' modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In an array context, it returns a list of all the substrings matched by all the parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. In a scalar context, it iterates through the string, returning TRUE each time it matches, and FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point.) It presumes that you have not modified the string since the last match. Modifying the string between matches may result in undefined behavior. (You can actually get away with in-place modifications via `substr()' that do not change the length of the entire string. In general, however, you should be using `s///g' for such modifications.) Examples: # array context ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); # scalar context $/ = ""; $* = 1; while ($paragraph = <>) { while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { $sentences++; } } print "$sentences\n"; ?PATTERN? This is just like the `/pattern/' search, except that it matches only once between calls to the `reset' operator. This is a useful optimization when you only want to see the first occurrence of something in each file of a set of files, for instance. Only `??' patterns local to the current package are reset. s/PATTERN/REPLACEMENT/gieo Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (0). The `g' is optional, and if present, indicates that all occurrences of the pattern are to be replaced. The `i' is also optional, and if present, indicates that matching is to be done in a case-insensitive manner. The `e' is likewise optional, and if present, indicates that the replacement string is to be evaluated as an expression rather than just as a double-quoted string. Any non-alphanumeric delimiter may replace the slashes; if single quotes are used, no interpretation is done on the replacement string (the `e' modifier overrides this, however); if backquotes are used, the replacement string is a command to execute whose output will be used as the actual replacement text. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, e.g. `s(foo)(bar)' or `s<foo>/bar/'. If no string is specified via the `=~' or `!~' operator, the `$_' string is searched and modified. (The string specified with `=~' must be a scalar variable, an array element, or an assignment to one of those, i.e. an lvalue.) If the pattern contains a `$' that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you only want the pattern compiled once the first time the variable is interpolated, add an `o' at the end. If the PATTERN evaluates to a null string, the most recent successful regular expression is used instead. *Note Regular Expressions::, for more info. Examples: s/\bgreen\b/mauve/g; # don't change wintergreen $path =~ s|/usr/bin|/usr/local/bin|; s/Login: $foo/Login: $bar/; # run-time pattern ($foo = $bar) =~ s/bar/foo/; $_ = 'abc123xyz'; s/\d+/$&*2/e; # yields `abc246xyz' s/\d+/sprintf("%5d",$&)/e; # yields `abc 246xyz' s/\w/$& x 2/eg; # yields `aabbcc 224466xxyyzz' s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields (Note the use of `$' instead of `\' in the last example. *Note Regular Expressions::.) study(SCALAR) study SCALAR study Takes extra time to study SCALAR (`$_' if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribution of character frequencies in the string to be searched--you probably want to compare runtimes with and without it to see which runs faster. Those loops which scan for many short constant strings (including the constant parts of more complex patterns) will benefit most. You may have only one study active at a time--if you study a different scalar the first is "unstudied". (The way study works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the `k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this "rarest" character are examined.) For example, here is a loop which inserts index producing entries before any line containing a certain pattern: while (<>) { study; print ".IX foo\n" if /\bfoo\b/; print ".IX bar\n" if /\bbar\b/; print ".IX blurfl\n" if /\bblurfl\b/; ... print; } In searching for `/\bfoo\b/', only those locations in `$_' that contain `f' will be looked at, because `f' is rarer than `o'. In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place. Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval that to avoid recompiling all your patterns all the time. Together with undefining `$/' to input entire files as one record, this can be very fast, often faster than specialized programs like `fgrep'. The following scans a list of files (`@files') for a list of words (`@words'), and prints out the names of those files that contain a match: $search = 'while (<>) { study;'; foreach $word (@words) { $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; } $search .= "}"; @ARGV = @files; undef $/ eval $search; # this screams $/ = "\n"; # put back to normal input delim foreach $file (sort keys(%seen)) { print $file, "\n"; } tr/SEARCHLIST/REPLACEMENTLIST/cds y/SEARCHLIST/REPLACEMENTLIST/cds Translates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is specified via the `=~' or `!~' operator, the `$_' string is translated. (The string specified with `=~' must be a scalar variable, an array element, or an assignment to one of those, i.e. an lvalue.) For `sed' devotees, `y' is provided as a synonym for `tr'. If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g. `tr[A-Z][a-z]' or `tr(+-*/)/ABCD/'. If the `c' modifier is specified, the SEARCHLIST character set is complemented. If the `d' modifier is specified, any characters specified by SEARCHLIST that are not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some `tr' programs, which delete anything they find in the SEARCHLIST, period.) If the `s' modifier is specified, sequences of characters that were translated to the same character are squashed down to 1 instance of the character. If the `d' modifier was used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. The latter is useful for counting characters in a class, or for squashing character sequences in a class. Examples: $ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case $cnt = tr/*/*/; # count the stars in $_ $cnt = tr/0-9//; # count the digits in $_ tr/a-zA-Z//s; # bookkeeper -> bokeper ($HOST = $host) =~ tr/a-z/A-Z/; y/\001-@[-_{-\177/ /; # change non-alphas to space # (before the c & s modifiers) y/a-zA-Z/ /cs; # change non-alphas to single space # (version 3.0 patchlevel 40+) tr/\200-\377/\0-\177/; # delete 8th bit File: perl.info, Node: System Interaction, Next: Networking Functions, Prev: Search and Replace Functions, Up: Commands System Interaction ================== alarm(SECONDS) alarm SECONDS Arranges to have a `SIGALRM' delivered to this process after the specified number of seconds (minus 1, actually) have elapsed. Thus, `alarm(15)' will cause a `SIGALRM' at some point more than 14 seconds in the future. Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer. chroot(FILENAME) chroot FILENAME chroot Does the same as the system call of that name. If you don't know what it does, don't worry about it. If FILENAME is omitted, does `chroot' to `$_'. die(LIST) die LIST die Outside of an `eval', prints the value of LIST to `STDERR' and exits with the current value of `$!' (errno). As of version 3.0 patchlevel 27, `die' without LIST specified is equivalent to die 'Died'; If `$!' is 0, exits with the value of `($? >> 8)' (`command` status). If `($? >> 8)' is 0, exits with 255. Inside an `eval', the error message is stuffed into `$@' and the `eval' is terminated with the undefined value. Equivalent examples: die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; chdir '/usr/spool/news' || die "Can't cd to spool: $!\n" If the value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended. Suppose you are running script "canasta". die "/etc/games is no good"; die "/etc/games is no good, stopped"; produce, respectively /etc/games is no good at canasta line 123. /etc/games is no good, stopped at canasta line 123. See also `exit'. exec(LIST) exec LIST If there is more than one argument in LIST, or if LIST is an array with more than one value, calls `execvp()' with the arguments in LIST. If there is only one scalar argument, the argument is checked for shell metacharacters. If there are any, the entire argument is passed to `/bin/sh -c' for parsing. If there are none, the argument is split into words and passed directly to `execvp()', which is more efficient. Note: `exec' (and `system') do not flush your output buffer, so you may need to set `$|' 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 by assigning that to a variable and putting the name of the variable in front of the LIST without a comma. (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 exit(EXPR) exit EXPR Evaluates EXPR and exits immediately with that value. Example: $ans = <STDIN>; exit 0 if $ans =~ /^[Xx]/; See also `die'. If EXPR is omitted, exits with 0 status. fork Does a `fork()' call. Returns the child pid to the parent process and 0 to the child process. Note: unflushed buffers remain unflushed in both processes, which means you may need to set `$|' to avoid duplicate output. getpwnam(NAME) getgrnam(NAME) gethostbyname(NAME) getnetbyname(NAME) getprotobyname(NAME) getpwuid(UID) getgrgid(GID) getservbyname(NAME,PROTO) gethostbyaddr(ADDR,ADDRTYPE) getnetbyaddr(ADDR,ADDRTYPE) getprotobynumber(NUMBER) getservbyport(PORT,PROTO) getpwent getgrent gethostent getnetent getprotoent getservent setpwent setgrent sethostent(STAYOPEN) setnetent(STAYOPEN) setprotoent(STAYOPEN) setservent(STAYOPEN) endpwent endgrent endhostent endnetent endprotoent endservent These routines perform the same functions as their counterparts in the system library. With an array 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 `getgr...' is a space separated list of the login names of the members of the group. For the `gethost...' functions, if the `h_errno' variable is supported in C, it will be returned to you via `$?' if the function call fails. The `@addrs' value returned by a successful call is a list of the raw addresses returned by the corresponding system library call. In the Internet domain, each address is four bytes long and you can unpack it by saying something like: ($a,$b,$c,$d) = unpack('C4',$addr[0]); getlogin Returns the current login from `/etc/utmp', if any. If null, use `getpwuid'. $login = getlogin || (getpwuid($<))[0] || "Somebody"; getpgrp(PID) getpgrp PID getpgrp Returns 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 `getpgrp(2)'. If PID is omitted, returns process group of current process. PID can be an expression. getppid Returns the process id of the parent process. getpriority(WHICH,WHO) Returns the current priority for a process, a process group, or a user. (See the `getpriority(2)' man page.) Will produce a fatal error if used on a machine that doesn't implement `getpriority(2)'. ioctl(FILEHANDLE,FUNCTION,SCALAR) Implements the `ioctl(2)' function. You'll probably have to say require "ioctl.ph"; # probably `/usr/local/lib/perl/ioctl.ph' first to get the correct function definitions. If `ioctl.ph' doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as `<sys/ioctl.h>'. (There is a perl script called `h2ph' that comes with the perl kit which may help you in this.) 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'; $sgttyb_t = "ccccs"; # 4 chars and a short if (ioctl(STDIN,$TIOCGETP,$sgttyb)) { @ary = unpack($sgttyb_t,$sgttyb); $ary[2] = 127; $sgttyb = pack($sgttyb_t,@ary); ioctl(STDIN,$TIOCSETP,$sgttyb) || die "Can't ioctl: $!"; } The return value of `ioctl' (and `fcntl') is as follows: if OS returns: perl returns: -1 undefined value 0 string "0 but true" anything else that number Thus perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system: ($retval = ioctl(...)) || ($retval = -1); printf "System returned %d\n", $retval; kill(LIST) 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; If the signal is negative, kills process groups instead of processes. (On System V, a negative *process* number will also kill process groups, but that's not portable.) You may use a signal name in quotes. 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)'. setpriority(WHICH,WHO,PRIORITY) Sets the current priority for a process, a process group, or a user. (See the `setpriority(2)' man page.) Will produce a fatal error if used on a machine that doesn't implement `setpriority(2)'. sleep(EXPR) sleep EXPR 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, since `sleep()' is often implemented using `alarm()'. syscall(LIST) syscall LIST Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, produces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. require 'syscall.ph'; # may need to run h2ph syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9); sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET) sysread(FILEHANDLE,SCALAR,LENGTH) Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the system call `read(2)'. It bypasses stdio, so mixing this with other kinds of reads may cause confusion. 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. syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET) syswrite(FILEHANDLE,SCALAR,LENGTH) Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call `write(2)'. It bypasses stdio, so mixing this with prints may cause confusion. Returns the number of bytes actually written, or `undef' if there was an error. An OFFSET may be specified to place the read data at some other place than the beginning of the string. system(LIST) system LIST Does exactly the same thing as `exec LIST' except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. The return value is the exit status of the program as returned by the `wait()' call. To get the actual exit value divide by 256. See also `exec'. times Returns a four-element array giving the user and system times, in seconds, for this process and the children of this process. ($user,$system,$cuser,$csystem) = times; umask(EXPR) umask EXPR umask Sets the umask for the process and returns the old one. If EXPR is omitted, merely returns current umask. wait Waits for a child process to terminate and returns the pid of the deceased process, or -1 if there are no child processes. The status is returned in `$?'. waitpid(PID,FLAGS) Waits for a particular child process to terminate and returns the pid of the deceased process or -1 if there are no such child process. The status is returns in `$?'. If you say require "sys/wait.ph"; ... waitpid(-1,&WNOHANG); then you can do a non-blocking wait for any process. Non-blocking wait is only available on machines supporting either the `waitpid(2)' or `wait4(2)' system calls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (*Perl* emulates the system call by remembering the status values of processes that have exited but have not been harvested by the *Perl* script yet.) warn(LIST) warn LIST Produces a message on `STDERR' just like `die', but doesn't exit. File: perl.info, Node: Networking Functions, Next: System V IPC, Prev: System Interaction, Up: Commands Networking Functions - Interprocess Communication ================================================= accept(NEWSOCKET,GENERICSOCKET) Does the same thing that the `accept' system call does. Returns true if it succeeded, false otherwise. *Note Interprocess Communication::, for an example. bind(SOCKET,NAME) Does the same thing that the `bind' system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the proper type for the socket. *Note Interprocess Communication::, for an example. connect(SOCKET,NAME) Does the same thing that the `connect' system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the proper type for the socket. *Note Interprocess Communication::, for an example. getpeername(SOCKET) Returns the packed *sockaddr* address of other end of the SOCKET connection. # An internet sockaddr $sockaddr = 'S n a4 x8'; $hersockaddr = getpeername(S); ($family, $port, $heraddr) = unpack($sockaddr,$hersockaddr); getsockname(SOCKET) Returns the packed *sockaddr* address of this end of the SOCKET connection. # An internet sockaddr $sockaddr = 'S n a4 x8'; $mysockaddr = getsockname(S); ($family, $port, $myaddr) = unpack($sockaddr,$mysockaddr); getsockopt(SOCKET,LEVEL,OPTNAME) Returns the socket option requested, or undefined if there is an error. listen(SOCKET,QUEUESIZE) Does the same thing that the `listen' system call does. Returns true if it succeeded, false otherwise. *Note Interprocess Communication::, for an example. 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. Returns the address of the sender, or 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. send(SOCKET,MSG,FLAGS,TO) 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. Returns the number of characters sent, or the undefined value if there is an error. 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. 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. 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 may need to run `h2ph' on `sys/socket.h' to get the proper values handy in a perl library file. Return true if successful. *Note Interprocess Communication::, for an example. 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. Return true if successful. File: perl.info, Node: System V IPC, Next: Time Functions, Prev: Networking Functions, Up: Commands Networking Functions - System V IPC =================================== msgctl(ID,CMD,ARG) Calls the System V IPC function `msgctl'. 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. msgget(KEY,FLAGS) Calls the System V IPC function `msgget'. Returns the message queue id, or the undefined value if there is an error. 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 `pack("L", $type)'. Returns true if successful, or false if there is an error. 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. 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. semget(KEY,NSEMS,SIZE,FLAGS) Calls the System V IPC function `semget'. Returns the semaphore id, or the undefined value if there is an error. 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 `pack("sss", $semnum, $semop, $semflag)'. 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, $semop); To signal the semaphore, replace `-1' with `1'. shmctl(ID,CMD,ARG) Calls the System V IPC function `shmctl'. If CMD is &IPC_STAT, then ARG must be a variable which will hold the returned shmid_ds structure. Returns like `ioctl': the undefined value for error, "0 but true" for zero, or the actual return value otherwise. shmget(KEY,SIZE,FLAGS) Calls the System V IPC function `shmget'. Returns the shared memory segment id, or the undefined value if there is an error. shmread(ID,VAR,POS,SIZE) shmwrite(ID,STRING,POS,SIZE) Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable which will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Returns true if successful, or false if there is an error. File: perl.info, Node: Time Functions, Next: DBM Functions, Prev: System V IPC, Up: Commands Time Related Functions ====================== gmtime(EXPR) gmtime EXPR gmtime Converts a time as returned by the `time' function to a 9-element array with the time analyzed for the Greenwich timezone. Typically used as follows: ($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'. If EXPR is omitted, does `gmtime(time)'. localtime(EXPR) localtime EXPR localtime Converts a time as returned by the time function to a 9-element array with the time analyzed for the local timezone. 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'. If EXPR is omitted, does `localtime(time)'. time Returns the number of non-leap seconds since 00:00:00 UTC, January 1, 1970. Suitable for feeding to `gmtime()' and `localtime()'. File: perl.info, Node: DBM Functions, Next: Flow Control Functions, Prev: Time Functions, Up: Commands DBM Functions ============= dbmclose(ASSOC_ARRAY) dbmclose ASSOC_ARRAY Breaks the binding between a dbm file and an associative array. The values remaining in the associative array are meaningless unless you happen to want to know what was in the cache for the dbm file. This function is only useful if you have ndbm. dbmopen(ASSOC,DBNAME,MODE) This binds a dbm or ndbm file to an associative array. ASSOC is the name of the associative array. (Unlike normal open, the first argument is *NOT* a filehandle, even though it looks like one). DBNAME is the name of the database (without the `.dir' or `.pag' extension). If the database does not exist, it is created with protection specified by MODE (as modified by the umask). If your system only supports the older dbm functions, you may perform only one `dbmopen' in your program. If your system has neither dbm nor ndbm, calling `dbmopen' produces a fatal error. Values assigned to the associative array prior to the `dbmopen' are lost. A certain number of values from the dbm file are cached in memory. By default this number is 64, but you can increase it by preallocating that number of garbage entries in the associative array before the `dbmopen'. You can flush the cache if necessary with the reset command. If you don't have write access to the dbm file, you can only read associative array variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy array 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); File: perl.info, Node: Flow Control Functions, Next: Perl Library Functions, Prev: DBM Functions, Up: Commands Flow Control Functions ====================== do BLOCK 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.) goto LABEL Finds the statement labeled with LABEL and resumes execution there. Currently you may only go to statements in the main body of the program that are not nested inside a `do {}' construct. This statement is not implemented very efficiently, and is here only to make the *sed*-to-*perl* translator easier. I may change its semantics at any time, consistent with support for translated `sed' scripts. Use it at your own risk. Better yet, don't use it at all. last LABEL last The `last' command is like the `break' statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The `continue' block, if any, is not executed: line: while (<STDIN>) { last line if /^$/; # exit when done with header ... } next LABEL next The `next' command is like the `continue' statement in C; it starts the next iteration of the loop: line: while (<STDIN>) { next line if /^#/; # discard comments ... } Note that if there were a `continue' block on the above, it would get executed even on discarded lines. If the LABEL is omitted, the command refers to the innermost enclosing loop. redo LABEL redo The `redo' command restarts the loop block without evaluating the conditional again. The `continue' 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 (<STDIN>) { while (s|({.*}.*){.*}|$1 |) {} s|{.*}| |; if (s|{.*| |) { $front = $_; while (<STDIN>) { if (/}/) { # end of comment? s|^|$front{|; redo line; } } } print; } File: perl.info, Node: Perl Library Functions, Next: Subroutine Functions, Prev: Flow Control Functions, Up: Commands Perl Library Functions ====================== require(EXPR) require EXPR require Includes the library file specified by EXPR, or by `$_' if EXPR is not supplied. Has semantics similar to the following subroutine: sub require { local($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. 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 `-I' libraries if the file isn't in the current directory (*note "INC" array: Predefined Names., for more info). It's the same, however, in that it does reparse the file every time you call it, so if you are going to use the file inside a loop you might prefer to use `-P' and `#include', at the expense of a little more startup time. (The main problem with `#include' is that cpp doesn't grok `#' comments--a workaround is to use `;#' for standalone comments.) Note that the following are *NOT* equivalent: do $foo; # eval a file do $foo(); # call a subroutine Note that inclusion of library routines is better done with the `require' operator. File: perl.info, Node: Subroutine Functions, Next: Variable Functions, Prev: Perl Library Functions, Up: Commands Subroutine Functions ==================== caller(EXPR) caller Returns the context of the current subroutine call: ($package,$filename,$line) = caller; With EXPR, 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. do SUBROUTINE (LIST) Executes a SUBROUTINE declared by a `sub' declaration, and returns the value of the last expression evaluated in SUBROUTINE. If there is no subroutine by that name, produces a fatal error. (You may use the `defined' operator to determine if a subroutine exists.) If you pass arrays as part of LIST you may wish to pass the length of the array in front of each array. (*Note Subroutines::.) The parentheses are required to avoid confusion with the `do EXPR' form. SUBROUTINE may also be a single scalar variable, in which case the name of the subroutine to execute is take from the variable. As an alternate (and preferred) form, you may call a subroutine by prefixing the name with an ampersand: `&foo(@args)'. If you aren't passing any arguments, you don't have to use parentheses. If you omit the parentheses, no `@_' array is passed to the subroutine. The `&' form is also used to specify subroutines to the `defined' and `undef' operators. if (defined &$var) { &$var($parm); undef &$var; } local(LIST) Declares the listed variables to be local to the enclosing block, subroutine, `eval' or `do'. All the listed elements must be legal lvalues. This operator works by saving the current values of those variables in LIST on a hidden stack and restoring them upon exiting the block, subroutine or `eval'. This means that called subroutines can also reference the local variable, but not the global one. The LIST may be assigned to if desired, which allows you to initialize your local variables. (If no initializer is given for a particular variable, it is created with an undefined value.) Commonly this is used to name the parameters to a subroutine. Examples: sub RANGEVAL { local($min, $max, $thunk) = @_; local($result) = ''; local($i); # Presumably $thunk makes reference to $i for ($i = $min; $i < $max; $i++) { $result .= eval $thunk; } $result; } if ($sw eq '-v') { # init local array with global array local(@ARGV) = @ARGV; unshift(@ARGV,'echo'); system @ARGV; } # @ARGV restored # temporarily add to digits associative array if ($base12) { # (NOTE: not claiming this is efficient!) local(%digits) = (%digits,'t',10,'e',11); do parse_num(); } Note that `local()' is a run-time command, and so gets executed every time through a loop, using up more stack storage each time until it's all released at once when the loop is exited. return LIST Returns from a subroutine with the value specified. (Note that a subroutine can automatically return the value of the last expression evaluated. That's the preferred method--use of an explicit `return' is a bit slower.) wantarray Returns true if the context of the currently executing subroutine is looking for an array value. Returns false if the context is looking for a scalar. return wantarray ? () : undef; File: perl.info, Node: Variable Functions, Next: Miscellaneous Functions, Prev: Subroutine Functions, Up: Commands Variable Functions ================== defined(EXPR) defined EXPR Returns a boolean value saying whether the lvalue EXPR has a real value or not. Many operations return the undefined value under exceptional conditions, such as end of file, uninitialized variable, system error and such. This function allows you to distinguish between an undefined null string and a defined null string with operations that might return a real null string, in particular referencing elements of an array. You may also check to see if arrays or subroutines exist. Use on predefined variables is not guaranteed to produce intuitive results. 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"; } See also `undef'. reset(EXPR) reset EXPR reset Generally used in a `continue' 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. Only resets 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 Note: resetting `A-Z' is not recommended since you'll wipe out your `ARGV' and `ENV' arrays. The use of reset on dbm associative arrays does not change the dbm file. (It does, however, flush any entries cached by perl, which may be useful if you are sharing the dbm file. Then again, maybe not.) scalar(EXPR) Forces EXPR to be interpreted in a scalar context and returns the value of EXPR. undef(EXPR) undef EXPR undef Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an entire array, or a subroutine name (using `&'). (`undef' will probably not do what you expect on most predefined variables or dbm array values.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine. Examples: undef $foo; undef $bar{'blurfl'}; undef @ary; undef %assoc; undef &mysub; return (wantarray ? () : undef) if $they_blew_it;