The waitpid() function will suspend execution of the calling thread until status information for one of its terminated child processes is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If more than one thread is suspended in waitpid() or wait(2) awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination. If status information is available prior to the call to waitpid(), return will be immediate.

The pid argument specifies a set of child processes for which status is requested, as follows:

• If pid is equal to (pid_t)-1, status is requested for any child process.

  If pid is greater than (pid_t)0, it specifies the process ID of the child process for which status is requested.

• If pid is equal to (pid_t)0 status is requested for any child process whose process group ID is equal to that of the calling process.
• If pid is less than (pid_t)-1, status is requested for any child process whose process group ID is equal to the absolute value of pid.

If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN and the process has no unwaited children that were transformed into zombie processes, it will block until all of its children terminate, and waitpid() will fail and set errno to ECHILD.

If waitpid() returns because the status of a child process is available, then that status may be evaluated with the macros defined by wstat(3xfn) If the calling process had specified a non-zero value of stat_loc, the status of the child process will be stored in the location pointed to by stat_loc.

The options argument is constructed from the bitwise inclusive OR of zero or more of the following flags, defined in the header <sys/wait.h>:

WCONTINUED

The status of any continued child process specified by pid, whose status has not been reported since it continued, is also reported to the calling process.

WNOHANG

waitpid() will not suspend execution of the calling process if status is not immediately available for one of the child processes specified by pid.

WNOWAIT

Keep the process whose status is returned in stat_loc in a waitable state. The process may be waited for again with identical results.

WUNTRACED

The status of any child processes specified by pid that are stopped, and whose status has not yet been reported since they stopped, is also reported to the calling process.

If waitpid() returns because the status of a child process is available, it returns a value equal to the process ID of the child process for which status is reported. If waitpid() returns due to the delivery of a signal to the calling process, -1 is returned and errno is set to EINTR. If waitpid() was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, then 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The waitpid() function will fail if:

ECHILD

The process or process group specified by pid does not exist or is not a child of the calling process or can never be in the states specified by options.

EINTR

The waitpid() function was interrupted due to the receipt of a signal sent by the calling process.

EINVAL

An invalid value was specified for options.

With options equal to 0 and pid equal to (pid_t)-1, waitpid() is identical to wait(2).

See attributes(5) for descriptions of the following attributes:

intro(2), exec(2), exit(2), fork(2), pause(2), ptrace(2), sigaction(2), wait(2), signal(3C), attributes(5), siginfo(3HEAD), wstat(3xfn)