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 defined below.
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 options:
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 SIGCHLD.
__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:
WIFSIGNALED(status)
True if the process terminated due to receipt of a signal.
WIFSTOPPED(status)
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:
WEXITSTATUS(status)
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.
WTERMSIG(status)
If WIFSIGNALED(status) is true, evaluates to the number of the signal that caused the termination of the process.
WCOREDUMP(status)
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 received.
WSTOPSIG(status)
If WIFSTOPPED(status) is true, evaluates to the number of the signal that caused the process to stop.