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wait, waitpid, wait4, wait3 — wait for process termination
Standard C Library (libc, -lc)
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
signal is received. On return from a successful wait() call, the status
area contains termination information about the process that exited as
The wait4() system 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 argument specifies the set of child processes for which to wait.
If wpid is -1, the call waits for any child process. If wpid is 0, 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 argument is defined below.
The options argument contains the bitwise OR of any of the following
options. The WCONTINUED option indicates that children of the current
process that have continued from a job control stop, by receiving a
SIGCONT signal, should also have their status reported. The WNOHANG
option is used to indicate that the call should not block if there are no
processes that wish to report status. If the WUNTRACED option is set,
children of the current process that are stopped due to a SIGTTIN,
SIGTTOU, SIGTSTP, or SIGSTOP signal also have their status reported. The
WSTOPPED option is an alias for WUNTRACED. The WNOWAIT option keeps the
process whose status is returned in a waitable state. The process may be
waited for again after this call completes.
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 or continued processes).
When the WNOHANG option is specified and no processes wish to report
status, wait4() returns a process id of 0.
The waitpid() function is identical to wait4() with an rusage value of
zero. The older wait3() call is the same as wait4() with a wpid value of
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
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:
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
termination 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
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 wait() calls are pending, the call
may be interrupted or restarted when the signal-catching routine returns,
depending on the options in effect for the signal; see discussion of
SA_RESTART in sigaction(2).
The implementation queues one SIGCHLD signal for each child process whose
status has changed, if wait() returns because the status of a child
process is available, the pending SIGCHLD signal associated with the
process ID of the child process will be discarded. Any other pending
SIGCHLD signals remain pending.
If SIGCHLD is blocked, wait() returns because the status of a child
process is available, the pending SIGCHLD signal will be cleared unless
another status of the child process is available.
If wait() returns due to a stopped, continued, 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
If wait4(), wait3(), or waitpid() returns due to a stopped, continued, 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, continued 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.
The wait() function will fail and return immediately if:
[ECHILD] The calling process has no existing unwaited-for child
[ECHILD] 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
[EFAULT] The status or rusage argument points 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.
_exit(2), ptrace(2), sigaction(2), exit(3), siginfo(3)
The wait() and waitpid() functions are defined by POSIX; wait4() and
wait3() are not specified by POSIX. The WCOREDUMP() macro and the
ability to restart a pending wait() call are extensions to the POSIX
The wait() function appeared in Version 6 AT&T UNIX.