System Calls wait(2)
wait - wait for child process to stop or terminate
pid_t wait(int *stat_loc);
The wait() 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 wait() or waitpid(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 wait(),
return will be immediate.
If wait() returns because the status of a child process is
available, it returns the process ID of the child process.
If the calling process specified a non-zero value for
stat_loc, the status of the child process is stored in the
location pointed to by stat_loc. That status may be
evaluated with the macros described on the wstat(3XFN)
In the following, status is the object pointed to by
o If the child process stopped, the high order 8 bits of
status will contain the number of the signal that
caused the process to stop and the low order 8 bits
will be set equal to WSTOPFLG.
o If the child process terminated due to an _exit()
call, the low order 8 bits of status will be 0 and the
high order 8 bits will contain the low order 8 bits of
the argument that the child process passed to _exit();
o If the child process terminated due to a signal, the
high order 8 bits of status will be 0 and the low
order 8 bits will contain the number of the signal
that caused the termination. In addition, if WCOREFLG
is set, a "core image" will have been produced; see
signal(3HEAD) and wstat(3XFN).
If the calling process has SA_NOCLDWAIT set or has SIGCHLD
set to SIG_IGN, and the process has no unwaited children
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System Calls wait(2)
that were transformed into zombie processes, it will block
until all of its children terminate, and wait() will fail
and set errno to ECHILD.
If a parent process terminates without waiting for its child
processes to terminate, the parent process ID of each child
process is set to 1, with the initialization process inher-
iting the child processes; see intro(2).
When wait() returns due to a terminated child process, the
process ID of the child is returned to the calling process.
Otherwise, -1 is returned and errno is set to indicate the
The wait() function will fail if:
The calling process has no existing unwaited-for child
EINTR The function was interrupted by a signal.
Since wait() blocks on a stopped child, a calling process
wishing to see the return results of such a call should use
waitid(2) or waitpid(2) instead of wait().
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | Async-Signal-Safe |
intro(2), exec(2), exit(2), fork(2), pause(2), ptrace(2),
waitid(2), waitpid(2), signal(3C), attributes(5),
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