Programming Perl, 4th Edition

waitpid

waitpid PID, FLAGS

This function waits for a particular child process to terminate and returns the PID when the process is dead, –1 if there are no child processes, or 0 if the FLAGS specify nonblocking and the process isn’t quite dead yet. The status of any dead process is returned in $?, as described under system. To get valid flag values, you’ll need to import the “:sys_wait_h” import tag group from the POSIX module. Here’s an example that does a nonblocking wait for all pending zombie processes.

use POSIX ":sys_wait_h";
do {
    $kid = waitpid(–1,&WNOHANG);
} until $kid == –1;

On systems that implement neither the waitpid(2) nor wait4(2) syscall, FLAGS may be specified only as 0. In other words, you can wait for a specific PID there, but you can’t do so in nonblocking mode.

On some systems, a return value of –1 could mean that child processes are being automatically reaped because you set $SIG{CHLD} = "IGNORE".