WAIT(3C) Standard C Library Functions WAIT(3C)

NAME

wait - wait for child process to stop or terminate

SYNOPSIS

#include <sys/types.h>
#include <sys/wait.h>

pid_t wait(int *stat_loc);

DESCRIPTION

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(), waitpid(3C), or
waitid(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 can be evaluated with the macros described on the
wait.h(3HEAD) manual page.


In the following, status is the object pointed to by stat_loc:

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 7 bits of the
argument that the child process passed to _exit(); see
exit(2).

o If the child process terminated due to a signal, the high
order 8 bits of status will be 0 and the low order 7bits
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.h(3HEAD) and
wait.h(3HEAD).


One instance of a SIGCHLD signal is queued for each child process
whose status has changed. If wait() returns because the status of a
child process is available, any pending SIGCHLD signal associated
with the process ID of that child process is discarded. Any other
pending SIGCHLD signals remain pending.


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 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 inheriting the child
processes; see Intro(2).

RETURN VALUES

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 error.

ERRORS

The wait() function will fail if:

ECHILD
The calling process has no existing unwaited-for child
processes.


EINTR
The function was interrupted by a signal.

USAGE

Since wait() blocks on a stopped child, a calling process wanting to
see the return results of such a call should use waitpid(3C) or
waitid(2) instead of wait(). The wait() function is implemented as a
call to waitpid(-1, stat_loc, 0).

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Standard |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+

SEE ALSO

Intro(2), exec(2), exit(2), fork(2), pause(2), waitid(2), ptrace(3C),
signal(3C), waitpid(3C), signal.h(3HEAD), wait.h(3HEAD),
attributes(7)

June 9, 2004 WAIT(3C)