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Manual page for wait(2)

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

wait() suspends the calling process until one of its immediate children terminates or until a child that is being traced stops because it has received a signal. The wait() function will return prematurely if a signal is received. If any unawaited process stopped or terminated prior to the call on wait(), return is 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 had specified a non-zero value for stat_loc, the status of the child process will be stored in the location pointed to by stat_loc. It may be evaluated with the macros described on wstat.5 In the following, status is the object pointed to by stat_loc:

: 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.
: 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(); see exit.2
: 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.3c

If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN, and the process has no unwaited for 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 wait() returns because the status of a child process is available, then that status may be evaluated with the macros defined by wstat.5

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. This means the initialization process inherits 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

wait() will fail if one or both of the following is true:

ECHILD: The calling process has no existing unwaited-for child processes.
EINTR: The function was interrupted by a signal.

ATTRIBUTES

See attributes.5 for descriptions of the following attributes:

+---------------+-------------------+
|ATTRIBUTE TYPE |  ATTRIBUTE VALUE  |
+---------------+-------------------+
|MT-Level       | Async-Signal-Safe |
+---------------+-------------------+

NOTES

Since wait() will block on a stopped child, if the calling process wishes to see the return results of such a wait, it should use waitid.2 or waitpid.2 instead of wait().

Last modified 07/October/97