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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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
(see Section 2.13, Status Information) pertaining to one of the
caller's child processes. The wait() function obtains status
information for process termination from any child process. The
waitpid() function obtains status information for process
termination, and optionally process stop and/or continue, from a
specified subset of the child processes.
The wait() function shall cause the calling thread to become
blocked until status information generated by child process
termination is made available to the thread, or until delivery of
a signal whose action is either to execute a signal-catching
function or to terminate the process, or an error occurs. If
termination status information is available prior to the call to
wait(), return shall be immediate. If termination status
information is available for two or more child processes, the
order in which their status is reported is unspecified.
As described in Section 2.13, Status Information, the wait() and
waitpid() functions consume the status information they obtain.
The behavior when multiple threads are blocked in wait(),
waitid(), or waitpid() is described in Section 2.13, Status
Information.
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 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, 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.
As specified in Consequences of Process Termination, if the
calling process has SA_NOCLDWAIT set or has SIGCHLD set to
SIG_IGN, then the termination of a child process will not cause
status information to become available to a thread blocked in
wait(), waitid(), or waitpid(). Thus, a thread blocked in one of
the wait functions will remain blocked unless some other condition
causes the thread to resume execution (such as an [ECHILD] failure
due to no remaining children in the set of waited-for children).
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.1‐2017 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.1‐2017, 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.1‐2017 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.1‐2017 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.
Section 2.13, Status Information, exec(1p), exit(3p), fork(3p),
system(3p), waitid(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory
Synchronization, signal.h(0p), sys_wait.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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 2017 WAIT(3P)
Pages that refer to this page: signal.h(0p), sys_wait.h(0p), sh(1p), wait(1p), abort(3p), exit(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)
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