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Waits for a particular child process pid to terminate and returns the pid when the process is dead, or -1 if there are no child processes or if the flags specify non-blocking and the process isn't dead yet. The status of the dead process is returned in $? . To get valid flag values say this:

use POSIX "sys_wait_h";
On systems that implement neither the waitpid(2) nor the wait4(2) system call, flags may be specified only as 0 . In other words, you can wait for a specific pid , but you can't do it in non-blocking mode.