WAIT(2) System Calls WAIT(2)
NAME
wait, waitpid, wait4, wait3 - wait for process termination
SYNOPSIS
#include <sys/types.h>
#include <sys/wait.h>
pid_t wait (union wait *status);
#include <sys/time.h>
#include <sys/resource.h>
pid_t waitpid (pid_t wpid, union wait *status, int options);
pid_t wait3 (union wait *status, int options, struct rusage *rusage);
pid_t wait4 (pid_t wpid, union wait *status, int options, struct rusage
*rusage);
DESCRIPTION
The wait function suspends execution of its calling process until sta���
tus 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
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 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. The WNOHANG option is
used to indicate that the call should not block if there are no pro‐
cesses 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 termi‐
nated 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. One of the first three macros will evaluate to a non-zero
(true) value:
WIFEXITED(status)
True if the process terminated normally by a call to
_exit(2), exit(3), rexit(3), or a GS/OS quit call.
WIFSIGNALED(status)
True if the process terminated due to receipt of a sig‐
nal.
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), exit(3),
rexit(3), or a GS/OS Quit call 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 cre‐
ation of a core file containing an image of the process
when the signal was received. This feature is not avail���
able under GNO.
WSTOPSIG(status)
If WIFSTOPPED(status) is true, evaluates to the number of
the signal that caused the process to stop.
NOTES
See signal(2) for a list of termination signals. A status of 0 indi‐
cates normal termination.
Some Unix uses of these functions expect status to be a pointer to an
int rather than to a union wait. It is safe to use a cast under such
circumstances.
If a parent process terminates without waiting for all of its child
processes to terminate, the remaining child processes are inherited by
the Kernel Null Process (pid zero).
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 re‐
turns, depending on the options in effect for the signal; see intro(2),
System call restart.
BUGS
Currently, only wait is implemented in the GNO kernel. There is a min‐
imal version of waitpid in the libraries, but that version of waitpid
ignores its options parameter (making it impossible to make a non-
blocking wait). It may also give unexpected results when there is more
than one child task being waited upon.
wait4 and wait3 are not implemented.
RETURN VALUES
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 re‐
turned with errno set to ECHILD. Otherwise, if WNOHANG is specified
and there are no stopped or exited children, 0 is returned. If an er‐
ror is detected or a caught signal aborts the call, a value of -1 is
returned and errno is set to indicate the error.
ERRORS
Wait will fail and return immediately if:
ECHILD The calling process has no existing unwaited-for child
processes.
EFAULT The status or rusage arguments point to an illegal ad‐
dress. (May not be detected before exit of a child
process.)
EINTR The call was interrupted by a caught signal, or the sig‐
nal did not have the SA_RESTART flag set.
STANDARDS
The wait and waitpid functions are defined by POSIX; wait4 and wait3
are not specified by POSIX. The WCOREDUMP macro and the ability to
restart a pending wait call are extensions to the POSIX interface.
SEE ALSO
_exit(2), execve(2), signal(2), exit(3) rexit(3)
HISTORY
A wait3 function call appeared in Version 6 AT&T UNIX.
GNO 19 January 1997 WAIT(2)
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