WAIT(2) FreeBSD 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 sig-
nal 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 uti-
lization 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 indi-
cates that children of the current process that have continued from a job
control stop, by receiving a SIGCONT signal, should also have their sta-
tus 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.
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() 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 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 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 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.
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 avail-
able 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.
[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)
The wait() and waitpid() functions are defined by POSIX; wait4() and
wait3() are not specified by POSIX. The WCOREDUMP() macro and the abil-
ity to restart a pending wait() call are extensions to the POSIX inter-
The wait() function appeared in Version 6 AT&T UNIX.
FreeBSD 6.1 April 19, 1994 FreeBSD 6.1