WAIT(2) BSD System Calls Manual WAIT(2)
wait, waitpid, wait4, wait3 -- wait for process termination
waitpid(pid_t wpid, int *status, int options);
wait3(int *status, int options, struct rusage *rusage);
wait4(pid_t wpid, int *status, int options, struct rusage *rusage);
The wait() function suspends execution of its calling process until
status information is available for a terminated child process, or a sig-
nal 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 wait4() call provides a more general interface for programs that need
to wait for certain child processes, that need resource utilization sta-
tistics 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
#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 bit-
wise OR of zero or more of the following values:
WCONTINUED Causes status to be reported for stopped child processes that
have been continued by receipt of a SIGCONT signal.
WNOHANG Indicates that the call should not block if there are no pro-
cesses that wish to report status.
WUNTRACED 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 sta-
tus, wait4() returns a process ID of 0.
The waitpid() call is identical to wait4() with an rusage value of zero.
The older wait3() 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 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.
True if the process terminated normally by a call to _exit(2) or
True if the process terminated due to receipt of a signal.
True if the process has not terminated, but has stopped and can
be restarted. This macro can be true only if the wait call spec-
ified 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:
If WIFEXITED(status) is true, evaluates to the low-order 8 bits
of the argument passed to _exit(2) or exit(3) by the child.
If WIFSIGNALED(status) is true, evaluates to the number of the
signal that caused the termination of the process.
If WIFSIGNALED(status) is true, evaluates as true if the termina-
tion of the process was accompanied by the creation of a core
file containing an image of the process when the signal was
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 indi-
cates normal termination.
If a parent process terminates without waiting for all of its child pro-
cesses to terminate, the remaining child processes are assigned the par-
ent process 1 ID (the init process ID).
If a signal is caught while any of the wait() 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 informa-
tion, 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() will fail and return immediately if:
[ECHILD] The calling process has no existing unwaited-for child
[EFAULT] The status or rusage arguments point to an illegal
address. (May not be detected before exit of a child
[EINTR] The call was interrupted by a caught signal, or the
signal did not have the SA_RESTART flag set.
[EINVAL] Invalid or undefined flags were passed in the options
_exit(2), sigaction(2), exit(3)
The wait() and waitpid() functions conform to IEEE Std 1003.1-2008
wait4() and wait3() are not specified by POSIX. The WCOREDUMP() macro
and the ability to restart a pending wait() call are extensions to that
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 4BSD, but the final calling convention was only estab-
lished in 4.2BSD. The wait4() and waitpid() function calls first
appeared in 4.3BSD-Reno.
BSD March 28, 2017 BSD