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WAIT(2) System Calls Manual WAIT(2)

wait, waitpid, wait4, wait3, WCOREDUMP, WEXITSTATUS, WIFCONTINUED, WIFEXITED, WIFSIGNALED, WIFSTOPPED, WSTOPSIG, WTERMSIG — wait for process termination

#include <sys/wait.h>

pid_t
wait(int *status);

pid_t
waitpid(pid_t wpid, int *status, int options);

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

pid_t
wait3(int *status, int options, struct rusage *rusage);

pid_t
wait4(pid_t wpid, int *status, int options, struct rusage *rusage);

The () function suspends execution of its calling process until status information is available for a terminated child process, or a signal is received. On return from a successful wait() call, the status area, if non-zero, is filled in with termination information about the process that exited (see below).

The () call provides a more general interface for programs that need to wait for certain child processes, that need resource utilization statistics accumulated by child processes, or that require options. The other wait functions are implemented using wait4().

The wpid parameter specifies the set of child processes for which to wait. The following symbolic constants are currently defined in <sys/wait.h>:

#define WAIT_ANY        (-1)    /* any process */
#define WAIT_MYPGRP     0       /* any process in my process group */

If wpid is set to WAIT_ANY, the call waits for any child process. If wpid is set to WAIT_MYPGRP, the call waits for any child process in the process group of the caller. If wpid is greater than zero, the call waits for the process with process ID wpid. If wpid is less than -1, the call waits for any process whose process group ID equals the absolute value of wpid.

The status parameter is defined below. The options argument is the bitwise OR of zero or more of the following values:

Causes status to be reported for stopped child processes that have been continued by receipt of a SIGCONT signal.
Indicates that the call should not block if there are no processes that wish to report status.
If set, children of the current process that are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal also have their status reported.

If rusage is non-zero, a summary of the resources used by the terminated process and all its children is returned (this information is currently not available for stopped processes).

When the WNOHANG option is specified and no processes wish to report status, () returns a process ID of 0.

The () call is identical to wait4() with an rusage value of zero. The older () call is the same as wait4() with a wpid value of -1.

The following macros may be used to test the manner of exit of the process. One of the first three macros will evaluate to a non-zero (true) value:

(status)
True if the process has not terminated, and has continued after a job control stop. This macro can be true only if the wait call specified the WCONTINUED option.
(status)
True if the process terminated normally by a call to _exit(2) or exit(3).
(status)
True if the process terminated due to receipt of a signal.
(status)
True if the process has not terminated, but has stopped and can be restarted. This macro can be true only if the wait call specified the WUNTRACED option or if the child process is being traced (see ptrace(2)).

Depending on the values of those macros, the following macros produce the remaining status information about the child process:

(status)
If (status) is true, evaluates to the low-order 8 bits of the argument passed to _exit(2) or exit(3) by the child.
(status)
If WIFSIGNALED(status) is true, evaluates to the number of the signal that caused the termination of the process.
(status)
If WIFSIGNALED(status) is true, evaluates as true if the termination of the process was accompanied by the creation of a core file containing an image of the process when the signal was received.
(status)
If WIFSTOPPED(status) is true, evaluates to the number of the signal that caused the process to stop.

See sigaction(2) for a list of termination signals. A status of 0 indicates normal termination.

If a parent process terminates without waiting for all of its child processes to terminate, the remaining child processes are assigned the parent process 1 ID (the init process ID).

If a signal is caught while any of the () calls is pending, the call may be interrupted or restarted when the signal-catching routine returns, depending on the options in effect for the signal; for further information, see siginterrupt(3).

If wait() returns due to a stopped or terminated child process, the process ID of the child is returned to the calling process. Otherwise, a value of -1 is returned and errno is set to indicate the error.

If wait4(), wait3() or waitpid() returns due to a stopped or terminated child process, the process ID of the child is returned to the calling process. If there are no children not previously awaited, -1 is returned with errno set to [ECHILD]. Otherwise, if WNOHANG is specified and there are no stopped or exited children, 0 is returned. If an error is detected or a caught signal aborts the call, a value of -1 is returned and errno is set to indicate the error.

wait(), waitpid(), wait3(), and wait4() will fail and return immediately if:

[]
The calling process has no existing unwaited-for child processes.
[]
No status from the terminated child process is available because the calling process has asked the system to discard such status by ignoring the signal SIGCHLD or setting the flag SA_NOCLDWAIT for that signal.
[]
The status or rusage arguments point to an illegal address. (May not be detected before exit of a child process.)
[]
The call was interrupted by a caught signal, or the signal did not have the SA_RESTART flag set.

waitpid() and wait4() will fail and return immediately if:

[]
The process specified by the wpid argument does not exist or is not a child of the calling process.
[]
Invalid or undefined flags were passed in the options argument.

_exit(2), sigaction(2), waitid(2), exit(3)

The wait() and waitpid() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

wait4() and wait3() are not specified by POSIX. The WCOREDUMP() macro and the ability to restart a pending wait() call are extensions to that specification.

A wait() system call first appeared in Version 1 AT&T UNIX. The status argument is accepted since Version 2 AT&T UNIX. A wait3() system call first appeared in 4.0BSD, but the final calling convention was only established in 4.2BSD. The wait4() and waitpid() function calls first appeared in 4.3BSD-Reno.

December 19, 2022 OpenBSD-current