wait —
waitpid,
wait4,
wait3
wait for process termination
SYNOPSIS
#include <sys/types.h> #include <sys/wait.h> pid_twait(int *status);
pid_twaitpid(pid_t wpid, int *status, int options);
#include <sys/time.h> #include <sys/resource.h> pid_twait3(int *status, int options, struct rusage *rusage);
pid_twait4(pid_t wpid, int *status, int options, struct rusage *rusage);
DESCRIPTION
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, if non-zero, is filled in with termination information about the
process that exited (see 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.
The following symbolic constants are currently defined in
‹sys/wait.h›:
#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
WAIT_MYPGRP,
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:
WCONTINUED
Causes status to be reported for stopped child processes that have been
continued by receipt of a
SIGCONT
signal.
WNOHANG
Indicates that the call should not block if there are no processes that wish
to report status.
WUNTRACED
If 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 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:
WIFCONTINUED(status);
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
WCONTINUED
option.
WIFEXITED(status);
True if the process terminated normally by a call to
_exit(2)
or
exit(3).
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.
NOTES
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
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; for further information, see
siginterrupt(3).
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 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.
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 address.
(May not be detected before exit of a child process.)
[EINTR]
The call was interrupted by a caught signal, or the signal did not have the
SA_RESTART
flag set.
[EINVAL]
Invalid or undefined flags were passed in the
options
argument.
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.
HISTORY
A
wait();
function call appeared in
Version 2 AT&T UNIX.