pcntl_waitpid

Description

Suspends execution of the current process until a child as specified by the process_id argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function.

If a child as requested by process_id has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed. Please see your system's waitpid(2) man page for specific details as to how waitpid works on your system.

Parameters

process_id

The value of process_id can be one of the following:

< -1	wait for any child process whose process group ID is equal to the absolute value of process_id.
-1	wait for any child process; this is the same behaviour that the pcntl_wait() function exhibits.
0	wait for any child process whose process group ID is equal to that of the calling process.
> 0	wait for the child whose process ID is equal to the value of process_id.

Note

Specifying -1 as the process_id is equivalent to the functionality pcntl_wait() provides (minus flags).

status

pcntl_waitpid() will store status information in the status parameter which can be evaluated using the following functions: pcntl_wifexited(), pcntl_wifstopped(), pcntl_wifsignaled(), pcntl_wexitstatus(), pcntl_wtermsig() and pcntl_wstopsig().

flags

The value of flags is the value of zero or more of the following two global constants OR'ed together:

WNOHANG	return immediately if no child has exited.
WUNTRACED	return for children which are stopped, and whose status has not been reported.

resource_usage

The resource_usage parameter is set to an associative array containing resource usage statistics from the child process. For information on the contents see getrusage().

This is only supported if the wait4(2) system call is available on the system; otherwise the parameter is left untouched. If no child process was reaped, that is if the function returns 0 or -1, the parameter is set to an empty array.

Return Values

pcntl_waitpid() returns the process ID of the child which exited, -1 on error or zero if WNOHANG was used and no child was available

Examples

pcntl_waitpid() example

<?php
function checkChild(int $pid, bool $wait): void
{
    $flags = ($wait ? 0 : WNOHANG);
    $deadPid = pcntl_waitpid($pid, $status, $flags, $rusage);
    if ($deadPid === -1) {
        $errNo = pcntl_get_last_error();
        $errMsg = pcntl_strerror($errNo);
        print "PARENT pcntl_waitpid FAILED ({$errNo}): {$errMsg}\n";
    } elseif ($deadPid === 0) {
        print "PARENT pcntl_waitpid: No exited children.\n";
    } elseif (pcntl_wifexited($status)) {
        print "PARENT pcntl_waitpid: Child PID {$deadPid} exited with status: "
            . pcntl_wexitstatus($status) . "\n";
        var_dump($rusage);
    } else {
        print "PARENT pcntl_waitpid: Child PID {$deadPid} was terminated by signal: "
            . pcntl_wtermsig($status) . "\n";
    }
}

$parentPid = getmypid();
$forkedPid = pcntl_fork();
if ($forkedPid == -1) {
    die('Could not fork');
} else if ($forkedPid) {
    // Parent process
    print "PARENT {$parentPid} has forked {$forkedPid}\n";

    print "PARENT is checking for exited children\n";
    checkChild($forkedPid, false);

    // Prevents children remaining as a "zombie process"
    print "PARENT is waiting for a child to exit\n";
    checkChild($forkedPid, true);

    print "PARENT is checking for exited children\n";
    checkChild($forkedPid, false);

    print "PARENT is exiting\n";
} else {
    // Child process
    $pid = getmypid();
    print "CHILD {$pid} is sleeping\n";
    sleep(10);
    print "CHILD is exiting\n";
    exit(7);
}

See Also

• pcntl_fork()
• pcntl_signal()
• pcntl_wait()
• pcntl_wifexited()
• pcntl_wifstopped()
• pcntl_wifsignaled()
• pcntl_wexitstatus()
• pcntl_wtermsig()
• pcntl_wstopsig()