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WAIT(3P) POSIX Programmer's Manual WAIT(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
wait, waitpid — wait for a child process to stop or terminate
#include <sys/wait.h>
pid_t wait(int *stat_loc);
pid_t waitpid(pid_t pid, int *stat_loc, int options);
The wait() and waitpid() functions shall obtain status information
pertaining to one of the caller's child processes. Various options
permit status information to be obtained for child processes that
have terminated or stopped. If status information is available for
two or more child processes, the order in which their status is
reported is unspecified.
The wait() function shall suspend execution of the calling thread
until status information for one of the terminated child processes of
the calling process is available, or until delivery of a signal whose
action is either to execute a signal-catching function or to
terminate the process. If more than one thread is suspended in wait()
or waitpid() awaiting termination of the same process, exactly one
thread shall return the process status at the time of the target
process termination. If status information is available prior to the
call to wait(), return shall be immediate.
The waitpid() function shall be equivalent to wait() if the pid
argument is (pid_t)−1 and the options argument is 0. Otherwise, its
behavior shall be modified by the values of the pid and options
arguments.
The pid argument specifies a set of child processes for which status
is requested. The waitpid() function shall only return the status of
a child process from this set:
* If pid is equal to (pid_t)−1, status is requested for any child
process. In this respect, waitpid() is then equivalent to wait().
* If pid is greater than 0, it specifies the process ID of a single
child process for which status is requested.
* If pid is 0, status is requested for any child process whose
process group ID is equal to that of the calling process.
* If pid is less than (pid_t)−1, status is requested for any child
process whose process group ID is equal to the absolute value of
pid.
The options argument is constructed from the bitwise-inclusive OR of
zero or more of the following flags, defined in the <sys/wait.h>
header:
WCONTINUED The waitpid() function shall report the status of any
continued child process specified by pid whose status has
not been reported since it continued from a job control
stop.
WNOHANG The waitpid() function shall not suspend execution of the
calling thread if status is not immediately available for
one of the child processes specified by pid.
WUNTRACED The status of any child processes specified by pid that
are stopped, and whose status has not yet been reported
since they stopped, shall also be reported to the
requesting process.
If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to
SIG_IGN, and the process has no unwaited-for children that were
transformed into zombie processes, the calling thread shall block
until all of the children of the process containing the calling
thread terminate, and wait() and waitpid() shall fail and set errno
to [ECHILD].
If wait() or waitpid() return because the status of a child process
is available, these functions shall return a value equal to the
process ID of the child process. In this case, if the value of the
argument stat_loc is not a null pointer, information shall be stored
in the location pointed to by stat_loc. The value stored at the
location pointed to by stat_loc shall be 0 if and only if the status
returned is from a terminated child process that terminated by one of
the following means:
1. The process returned 0 from main().
2. The process called _exit() or exit() with a status argument of 0.
3. The process was terminated because the last thread in the process
terminated.
Regardless of its value, this information may be interpreted using
the following macros, which are defined in <sys/wait.h> and evaluate
to integral expressions; the stat_val argument is the integer value
pointed to by stat_loc.
WIFEXITED(stat_val)
Evaluates to a non-zero value if status was returned for a
child process that terminated normally.
WEXITSTATUS(stat_val)
If the value of WIFEXITED(stat_val) is non-zero, this macro
evaluates to the low-order 8 bits of the status argument that
the child process passed to _exit() or exit(), or the value the
child process returned from main().
WIFSIGNALED(stat_val)
Evaluates to a non-zero value if status was returned for a
child process that terminated due to the receipt of a signal
that was not caught (see <signal.h>).
WTERMSIG(stat_val)
If the value of WIFSIGNALED(stat_val) is non-zero, this macro
evaluates to the number of the signal that caused the
termination of the child process.
WIFSTOPPED(stat_val)
Evaluates to a non-zero value if status was returned for a
child process that is currently stopped.
WSTOPSIG(stat_val)
If the value of WIFSTOPPED(stat_val) is non-zero, this macro
evaluates to the number of the signal that caused the child
process to stop.
WIFCONTINUED(stat_val)
Evaluates to a non-zero value if status was returned for a
child process that has continued from a job control stop.
It is unspecified whether the status value returned by calls to
wait() or waitpid() for processes created by posix_spawn() or
posix_spawnp() can indicate a WIFSTOPPED(stat_val) before subsequent
calls to wait() or waitpid() indicate WIFEXITED(stat_val) as the
result of an error detected before the new process image starts
executing.
It is unspecified whether the status value returned by calls to
wait() or waitpid() for processes created by posix_spawn() or
posix_spawnp() can indicate a WIFSIGNALED(stat_val) if a signal is
sent to the parent's process group after posix_spawn() or
posix_spawnp() is called.
If the information pointed to by stat_loc was stored by a call to
waitpid() that specified the WUNTRACED flag and did not specify the
WCONTINUED flag, exactly one of the macros WIFEXITED(*stat_loc),
WIFSIGNALED(*stat_loc), and WIFSTOPPED(*stat_loc) shall evaluate to a
non-zero value.
If the information pointed to by stat_loc was stored by a call to
waitpid() that specified the WUNTRACED and WCONTINUED flags, exactly
one of the macros WIFEXITED(*stat_loc), WIFSIGNALED(*stat_loc),
WIFSTOPPED(*stat_loc), and WIFCONTINUED(*stat_loc) shall evaluate to
a non-zero value.
If the information pointed to by stat_loc was stored by a call to
waitpid() that did not specify the WUNTRACED or WCONTINUED flags, or
by a call to the wait() function, exactly one of the macros
WIFEXITED(*stat_loc) and WIFSIGNALED(*stat_loc) shall evaluate to a
non-zero value.
If the information pointed to by stat_loc was stored by a call to
waitpid() that did not specify the WUNTRACED flag and specified the
WCONTINUED flag, or by a call to the wait() function, exactly one of
the macros WIFEXITED(*stat_loc), WIFSIGNALED(*stat_loc), and
WIFCONTINUED(*stat_loc) shall evaluate to a non-zero value.
If _POSIX_REALTIME_SIGNALS is defined, and the implementation queues
the SIGCHLD signal, then if wait() or waitpid() returns because the
status of a child process is available, any pending SIGCHLD signal
associated with the process ID of the child process shall be
discarded. Any other pending SIGCHLD signals shall remain pending.
Otherwise, if SIGCHLD is blocked, if wait() or waitpid() return
because the status of a child process is available, any pending
SIGCHLD signal shall be cleared unless the status of another child
process is available.
For all other conditions, it is unspecified whether child status will
be available when a SIGCHLD signal is delivered.
There may be additional implementation-defined circumstances under
which wait() or waitpid() report status. This shall not occur unless
the calling process or one of its child processes explicitly makes
use of a non-standard extension. In these cases the interpretation of
the reported status is implementation-defined.
If a parent process terminates without waiting for all of its child
processes to terminate, the remaining child processes shall be
assigned a new parent process ID corresponding to an implementation-
defined system process.
If wait() or waitpid() returns because the status of a child process
is available, these functions shall return a value equal to the
process ID of the child process for which status is reported. If
wait() or waitpid() returns due to the delivery of a signal to the
calling process, −1 shall be returned and errno set to [EINTR]. If
waitpid() was invoked with WNOHANG set in options, it has at least
one child process specified by pid for which status is not available,
and status is not available for any process specified by pid, 0 is
returned. Otherwise, −1 shall be returned, and errno set to indicate
the error.
The wait() function shall fail if:
ECHILD The calling process has no existing unwaited-for child
processes.
EINTR The function was interrupted by a signal. The value of the
location pointed to by stat_loc is undefined.
The waitpid() function shall fail if:
ECHILD 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.
EINTR The function was interrupted by a signal. The value of the
location pointed to by stat_loc is undefined.
EINVAL The options argument is not valid.
The following sections are informative.
Waiting for a Child Process and then Checking its Status
The following example demonstrates the use of waitpid(), fork(), and
the macros used to interpret the status value returned by waitpid()
(and wait()). The code segment creates a child process which does
some unspecified work. Meanwhile the parent loops performing calls to
waitpid() to monitor the status of the child. The loop terminates
when child termination is detected.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
...
pid_t child_pid, wpid;
int status;
child_pid = fork();
if (child_pid == −1) { /* fork() failed */
perror("fork");
exit(EXIT_FAILURE);
}
if (child_pid == 0) { /* This is the child */
/* Child does some work and then terminates */
...
} else { /* This is the parent */
do {
wpid = waitpid(child_pid, &status, WUNTRACED
#ifdef WCONTINUED /* Not all implementations support this */
| WCONTINUED
#endif
);
if (wpid == −1) {
perror("waitpid");
exit(EXIT_FAILURE);
}
if (WIFEXITED(status)) {
printf("child exited, status=%d\n", WEXITSTATUS(status));
} else if (WIFSIGNALED(status)) {
printf("child killed (signal %d)\n", WTERMSIG(status));
} else if (WIFSTOPPED(status)) {
printf("child stopped (signal %d)\n", WSTOPSIG(status));
#ifdef WIFCONTINUED /* Not all implementations support this */
} else if (WIFCONTINUED(status)) {
printf("child continued\n");
#endif
} else { /* Non-standard case -- may never happen */
printf("Unexpected status (0x%x)\n", status);
}
} while (!WIFEXITED(status) && !WIFSIGNALED(status));
}
Waiting for a Child Process in a Signal Handler for SIGCHLD
The following example demonstrates how to use waitpid() in a signal
handler for SIGCHLD without passing −1 as the pid argument. (See the
APPLICATION USAGE section below for the reasons why passing a pid of
−1 is not recommended.) The method used here relies on the standard
behavior of waitpid() when SIGCHLD is blocked. On historical non-
conforming systems, the status of some child processes might not be
reported.
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#define CHILDREN 10
static void
handle_sigchld(int signum, siginfo_t *sinfo, void *unused)
{
int sav_errno = errno;
int status;
/*
* Obtain status information for the child which
* caused the SIGCHLD signal and write its exit code
* to stdout.
*/
if (sinfo->si_code != CLD_EXITED)
{
static char msg[] = "wrong si_code\n";
write(2, msg, sizeof msg − 1);
}
else if (waitpid(sinfo->si_pid, &status, 0) == −1)
{
static char msg[] = "waitpid() failed\n";
write(2, msg, sizeof msg − 1);
}
else if (!WIFEXITED(status))
{
static char msg[] = "WIFEXITED was false\n";
write(2, msg, sizeof msg − 1);
}
else
{
int code = WEXITSTATUS(status);
char buf[2];
buf[0] = '0' + code;
buf[1] = '\n';
write(1, buf, 2);
}
errno = sav_errno;
}
int
main(void)
{
int i;
pid_t pid;
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = handle_sigchld;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGCHLD, &sa, NULL) == −1)
{
perror("sigaction");
exit(EXIT_FAILURE);
}
for (i = 0; i < CHILDREN; i++)
{
switch (pid = fork())
{
case −1:
perror("fork");
exit(EXIT_FAILURE);
case 0:
sleep(2);
_exit(i);
}
}
/* Wait for all the SIGCHLD signals, then terminate on SIGALRM */
alarm(3);
for (;;)
pause();
return 0; /* NOTREACHED */
}
Calls to wait() will collect information about any child process.
This may result in interactions with other interfaces that may be
waiting for their own children (such as by use of system()). For
this and other reasons it is recommended that portable applications
not use wait(), but instead use waitpid(). For these same reasons,
the use of waitpid() with a pid argument of −1, and the use of
waitid() with the idtype argument set to P_ALL, are also not
recommended for portable applications.
A call to the wait() or waitpid() function only returns status on an
immediate child process of the calling process; that is, a child that
was produced by a single fork() call (perhaps followed by an exec or
other function calls) from the parent. If a child produces
grandchildren by further use of fork(), none of those grandchildren
nor any of their descendants affect the behavior of a wait() from the
original parent process. Nothing in this volume of POSIX.1‐2008
prevents an implementation from providing extensions that permit a
process to get status from a grandchild or any other process, but a
process that does not use such extensions must be guaranteed to see
status from only its direct children.
The waitpid() function is provided for three reasons:
1. To support job control
2. To permit a non-blocking version of the wait() function
3. To permit a library routine, such as system() or pclose(), to
wait for its children without interfering with other terminated
children for which the process has not waited
The first two of these facilities are based on the wait3() function
provided by 4.3 BSD. The function uses the options argument, which is
equivalent to an argument to wait3(). The WUNTRACED flag is used
only in conjunction with job control on systems supporting job
control. Its name comes from 4.3 BSD and refers to the fact that
there are two types of stopped processes in that implementation:
processes being traced via the ptrace() debugging facility and
(untraced) processes stopped by job control signals. Since ptrace()
is not part of this volume of POSIX.1‐2008, only the second type is
relevant. The name WUNTRACED was retained because its usage is the
same, even though the name is not intuitively meaningful in this
context.
The third reason for the waitpid() function is to permit independent
sections of a process to spawn and wait for children without
interfering with each other. For example, the following problem
occurs in developing a portable shell, or command interpreter:
stream = popen("/bin/true");
(void) system("sleep 100");
(void) pclose(stream);
On all historical implementations, the final pclose() fails to reap
the wait() status of the popen().
The status values are retrieved by macros, rather than given as
specific bit encodings as they are in most historical implementations
(and thus expected by existing programs). This was necessary to
eliminate a limitation on the number of signals an implementation can
support that was inherent in the traditional encodings. This volume
of POSIX.1‐2008 does require that a status value of zero corresponds
to a process calling _exit(0), as this is the most common encoding
expected by existing programs. Some of the macro names were adopted
from 4.3 BSD.
These macros syntactically operate on an arbitrary integer value. The
behavior is undefined unless that value is one stored by a successful
call to wait() or waitpid() in the location pointed to by the
stat_loc argument. An early proposal attempted to make this clearer
by specifying each argument as *stat_loc rather than stat_val.
However, that did not follow the conventions of other specifications
in this volume of POSIX.1‐2008 or traditional usage. It also could
have implied that the argument to the macro must literally be
*stat_loc; in fact, that value can be stored or passed as an argument
to other functions before being interpreted by these macros.
The extension that affects wait() and waitpid() and is common in
historical implementations is the ptrace() function. It is called by
a child process and causes that child to stop and return a status
that appears identical to the status indicated by WIFSTOPPED. The
status of ptrace() children is traditionally returned regardless of
the WUNTRACED flag (or by the wait() function). Most applications do
not need to concern themselves with such extensions because they have
control over what extensions they or their children use. However,
applications, such as command interpreters, that invoke arbitrary
processes may see this behavior when those arbitrary processes misuse
such extensions.
Implementations that support core file creation or other
implementation-defined actions on termination of some processes
traditionally provide a bit in the status returned by wait() to
indicate that such actions have occurred.
Allowing the wait() family of functions to discard a pending SIGCHLD
signal that is associated with a successfully waited-for child
process puts them into the sigwait() and sigwaitinfo() category with
respect to SIGCHLD.
This definition allows implementations to treat a pending SIGCHLD
signal as accepted by the process in wait(), with the same meaning of
``accepted'' as when that word is applied to the sigwait() family of
functions.
Allowing the wait() family of functions to behave this way permits an
implementation to be able to deal precisely with SIGCHLD signals.
In particular, an implementation that does accept (discard) the
SIGCHLD signal can make the following guarantees regardless of the
queuing depth of signals in general (the list of waitable children
can hold the SIGCHLD queue):
1. If a SIGCHLD signal handler is established via sigaction()
without the SA_RESETHAND flag, SIGCHLD signals can be accurately
counted; that is, exactly one SIGCHLD signal will be delivered to
or accepted by the process for every child process that
terminates.
2. A single wait() issued from a SIGCHLD signal handler can be
guaranteed to return immediately with status information for a
child process.
3. When SA_SIGINFO is requested, the SIGCHLD signal handler can be
guaranteed to receive a non-null pointer to a siginfo_t structure
that describes a child process for which a wait via waitpid() or
waitid() will not block or fail.
4. The system() function will not cause the SIGCHLD handler of a
process to be called as a result of the fork()/exec executed
within system() because system() will accept the SIGCHLD signal
when it performs a waitpid() for its child process. This is a
desirable behavior of system() so that it can be used in a
library without causing side-effects to the application linked
with the library.
An implementation that does not permit the wait() family of functions
to accept (discard) a pending SIGCHLD signal associated with a
successfully waited-for child, cannot make the guarantees described
above for the following reasons:
Guarantee #1
Although it might be assumed that reliable queuing of all
SIGCHLD signals generated by the system can make this
guarantee, the counter-example is the case of a process that
blocks SIGCHLD and performs an indefinite loop of fork()/wait()
operations. If the implementation supports queued signals, then
eventually the system will run out of memory for the queue. The
guarantee cannot be made because there must be some limit to
the depth of queuing.
Guarantees #2 and #3
These cannot be guaranteed unless the wait() family of
functions accepts the SIGCHLD signal. Otherwise, a
fork()/wait() executed while SIGCHLD is blocked (as in the
system() function) will result in an invocation of the handler
when SIGCHLD is unblocked, after the process has disappeared.
Guarantee #4
Although possible to make this guarantee, system() would have
to set the SIGCHLD handler to SIG_DFL so that the SIGCHLD
signal generated by its fork() would be discarded (the SIGCHLD
default action is to be ignored), then restore it to its
previous setting. This would have the undesirable side-effect
of discarding all SIGCHLD signals pending to the process.
None.
exec(1p), exit(3p), fork(3p), system(3p), waitid(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
Synchronization, signal.h(0p), sys_wait.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 WAIT(3P)
Pages that refer to this page: signal.h(0p), sys_wait.h(0p), sh(1p), wait(1p), abort(3p), _Exit(3p), getrusage(3p), kill(3p), pclose(3p), popen(3p), posix_spawn(3p), pthread_join(3p), sigaction(3p), sighold(3p), system(3p), times(3p), waitid(3p), waitpid(3p)