wait for process termination
int options, struct rusage
int *stat_loc, int options,
struct rusage *rusage);
int *stat_loc, int options);
function suspends execution of its calling process until
stat_loc information is available for a terminated
child process, or a signal is received. On return from a successful
wait() call, the stat_loc area
contains termination information about the process that exited as defined
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 pid parameter specifies the set of child processes for which to wait. If pid is -1, the call waits for any child process. If pid is 0, the call waits for any child process in the process group of the caller. If pid is greater than zero, the call waits for the process with process id pid. If pid is less than -1, the call waits for any process whose process group id equals the absolute value of pid.
The stat_loc 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
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
pid 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 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; see intro(2), System call restart.
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.
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() system call will fail and
return immediately if:
- The calling process has no existing unwaited-for child processes.
- The status or rusage argument points to an illegal address (may not be detected before the exit of a child process).
- Invalid or undefined flags are passed in the options argument.
waitpid() calls will fail and return immediately
- The process specified by pid does not exist or is not a child of the calling process, or the process group specified by pid does not exist or does not have any member process that is a child of the calling process.
waitpid() call will fail and return
- The options argument is not valid.
Any of these calls will fail and return immediately if:
- The call is interrupted by a caught signal or the signal does not have the
waitpid() functions are defined by POSIX;
wait4() are not
specified by POSIX. The
WCOREDUMP() macro and the
ability to restart a pending
wait() call are
extensions to the POSIX interface.
The include file
sigaction(2), exit(3), compat(5)
wait() function call appeared in
Version 6 AT&T UNIX.