WAIT(2) BSD System Calls Manual WAIT(2)NAME
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 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 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. 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 call‐
er. 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 options. 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() 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 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; 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.
ERRORSWait() 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 underfined flags were passed in the options
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‐
SEE ALSOexit(3), sigaction(2)HISTORY
A wait() function call appeared in Version 6 AT&T UNIX.
4th Berkeley Distribution April 19, 1994 4th Berkeley Distribution