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As you fork new processes and they eventually die, you need to wait for the child processes to exit cleanly to ensure they do not remain as zombies within the process table Child processes send the SIGCHLD signal to the parent when they exit, but unless the signal is caught, or the processes are otherwise acknowledged, they remain within the process table They are called zombies because they have completed execution but have not been cleared from the table In order to acknowledge the completion of the child process, you need to use one of the two available functions, wait and waitpid Both functions block the parent process until the child process (or processes) has exited cleanly This should not cause problems if the functions are used as part of a signal handler, or if they are called as the last function within a parent that knows its children should have exited, probably because it sent a suitable signal
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wait waitpid PID, FLAGS
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The wait function simply waits for a child process to terminate It s usually used within a signal handler to automatically reap child processes as they die:
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$SIG{CHLD} = sub { wait };
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This should guarantee that the child process completes correctly The other alternative is to use the waitpid, which enables you to wait for a specific process ID and condition Valid flags are defined in the POSIX module, and they are summarized here in Table 14-2 Of course, there are times when you specifically want to wait for your children to exit cleanly
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Interprocess Communication
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WIFEXITED WIFSIGNALED WNOHANG WSTOPSIG WTERMSIG WUNTRACED Table 14-2
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Description
Wait for processes that have exited Wait for processes that received a signal Non-blocking wait Wait for processes that received STOP signal Wait for processes that received TERM signal
PROGRAMMING WITH PERL
Wait for processes stopped by signals
Flags for waitpid
Communicating with Children
It s possible to do one-way communication between a parent and its children using the | and | methods to the open command However, this is a one-way transfer, and the fork is implied by the open command, which reduces your flexibility somewhat A better solution is to use the pipe function to create a pair of filehandles
pipe READHANDLE, WRITEHANDLE
Information written to WRITEHANDLE is immediately available on READHANDLE on a simple first in, first out (FIFO) basis Since a forked process inherits open filehandles from the parent, you can use a pair of filehandles for communicating between the child and parent and for reading from and writing to the corresponding filehandle The following example creates a new subprocess, which accepts calculations that are then evaluated by eval to produce a result
use IO::Handle; pipe(PARENTREAD, PARENTWRITE); pipe(CHILDREAD, CHILDWRITE); PARENTWRITE->autoflush(1); CHILDWRITE->autoflush(1); if ($child = fork) { close CHILDTREAD; # Parent code # We don't need these in the parent
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close PARENTWRITE; print CHILDWRITE "34+56;\n"; chomp($result = <PARENTREAD>); print "Got a value of $result from child\n"; close PARENTREAD; close CHILDWRITE; waitpid($child,0); } else { close PARENTREAD; # We don't need these in the child close CHILDWRITE; chomp($calculation = <CHILDREAD>); print "Got $calculation\n"; $result = eval "$calculation"; print PARENTWRITE "$result\n"; close CHILDREAD; close PARENTWRITE; exit; }
You can see that the calculation is sent to CHILDWRITE, which is then read by the child from CHILDREAD The result is then calculated and sent back to the parent via PARENTWRITE, where the parent reads the result from PARENTREAD Note that you must use newlines as terminators when communicating between the parent and the child to identify the end of the communication You could have used any string (see Data Transfer in 12), but newlines are the natural choice, since it s what you use elsewhere Another alternative is to use sockets, and you saw many examples of this in 12 There is, however, one trick particularly relevant to communication between parents and children This is the socketpair function, which is only supported on a small number of platforms It works in a similar way to pipe, except that you can use just two filehandles to communicate between the two processes Here s another version of the preceding example, this time using socketpair:
use IO::Handle; use Socket; socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) or die "socketpair failed: $!"; PARENT->autoflush(1); CHILD->autoflush(1);
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