WAIT(2) System Calls Manual WAIT(2)
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() 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. 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 parameter is defined below.
The options parameter contains the bitwise OR of any of the following
WNOHANG This option is used to indicate that the call should not block
if there are no processes that wish to report status.
WUNTRACED If this 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.
WALTSIG If this option is specified, the call will wait only for
processes that are configured to post a signal other than
SIGCHLD when they exit. If WALTSIG is not specified, the call
will wait only for processes that are configured to post
__WCLONE This is an alias for WALTSIG. It is provided for
compatibility with the Linux clone(2) API.
WALLSIG If this option is specified, the call will wait for all
children regardless of what exit signal they post.
__WALL This is an alias for WALLSIG. It is provided for
compatibility with the Linux clone(2) API .
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, 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. Note that these macros expect the status value itself, not a
pointer to the status value. One of the first three macros will evaluate
to a non-zero (true) value:
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 is pending, the call
may be interrupted or restarted when the signal-catching routine returns,
depending on the options in effect for the signal; see intro(2), System
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.
In addition, wait3(), wait4(), and waitpid() will fail and return
[EINVAL] An invalid value was specified for options.
The wait() and waitpid() functions conform to IEEE Std 1003.1-1990
(``POSIX.1''); the wait3() function conforms to X/Open Portability Guide
Issue 4 (``XPG4''); wait4() is an extension. The WCOREDUMP() macro and
the ability to restart a pending wait() call are extensions to the POSIX
A wait() function call appeared in Version 2 AT&T UNIX.
NetBSD 6.1.5 April 3, 2010 NetBSD 6.1.5