wait for process termination
wpid, int *status,
options, struct rusage
wpid, int *status,
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,
if non-zero, is filled in with termination information about the process
that exited (see below).
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
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
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
The status parameter is defined below. The options argument is the bitwise OR of zero or more of the following values:
- Causes status to be reported for stopped child processes that have been
continued by receipt of a
- Indicates that the call should not block if there are no processes that wish to report status.
- If set, children of the current process that are stopped due to a
SIGSTOPsignal 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).
WNOHANG option is
specified and no processes wish to report status,
returns a process ID of 0.
call is identical to
wait4() with an
rusage value of zero. The older
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) value:
- 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
- True if the process terminated normally by a call to _exit(2) or exit(3).
- 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
WUNTRACEDoption 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:
WIFEXITED(status) is true, evaluates to the low-order 8 bits of the argument passed to _exit(2) or exit(3) by the child.
WIFSIGNALED(status) is true, evaluates to the number of the signal that caused the termination of the process.
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 received.
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
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 information, see
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.
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.
fail and return immediately if:
- The calling process has no existing unwaited-for child processes.
- No status from the terminated child process is available because the
calling process has asked the system to discard such status by ignoring
SIGCHLDor setting the flag
SA_NOCLDWAITfor that signal.
- The status or rusage arguments point to an illegal address. (May not be detected before exit of a child process.)
- The call was interrupted by a caught signal, or the signal did not have
wait4() will fail and return immediately if:
- The process specified by the wpid argument does not exist or is not a child of the calling process.
- Invalid or undefined flags were passed in the options argument.
_exit(2), sigaction(2), exit(3)
waitpid() functions conform to IEEE
Std 1003.1-2008 (“POSIX.1”).
wait3() are not specified by POSIX. The
WCOREDUMP() macro and the ability to restart a
wait() call are extensions to that
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
established in 4.2BSD. The
function calls first appeared in 4.3BSD-Reno.