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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there is no such child process. On some systems, a value of 0 indicates that there are processes still running. The status is returned in $? and ${^CHILD_ERROR_NATIVE}. If you say

use POSIX ":sys_wait_h";
#...
do {
    $kid = waitpid(-1, WNOHANG);
} while $kid > 0;

then you can do a non-blocking wait for all pending zombie processes. Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) syscalls. 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.)

Note that on some systems, a return value of -1 could mean that child processes are being automatically reaped. See 20 for details, and for other examples.

ISBN 9781906966027Perl Language Reference ManualSee the print edition