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WAIT(3P) POSIX Programmer's Manual WAIT(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.
NAME
wait, waitpid - wait for a child process to stop or terminate
SYNOPSIS
#include <sys/wait.h>
pid_t wait(int *stat_loc);
pid_t waitpid(pid_t pid, int *stat_loc, int options);
DESCRIPTION
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() await-
ing 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. Oth-
erwise, 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 abso-
lute 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 con-
taining 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 argu-
ment 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() indi-
cate 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 WIF-
STOPPED(*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 WIFCONTIN-
UED(*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 WCON-
TINUED flags, or by a call to the wait() function, exactly one of the macros WIFEXITED(*stat_loc) and WIFSIG-
NALED(*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.
RETURN VALUE
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, (pid_t)-1 shall be returned, and errno set
to indicate the error.
ERRORS
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 speci-
fied 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.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
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 par-
ent. 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 IEEE Std 1003.1-2001 pre-
vents 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 IEEE Std 1003.1-2001, only the second type
is relevant. The name WUNTRACED was retained because its usage is the same, even though the name is not intuitively mean-
ingful 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
IEEE Std 1003.1-2001 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 fol-
low the conventions of other specifications in this volume of IEEE Std 1003.1-2001 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 indi-
cated 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 arbi-
trary 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 regard-
less 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 sig-
info_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 a process' SIGCHLD handler 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 asso-
ciated 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.
FUTURE DIRECTIONS
None.
SEE ALSO
exec(), exit(), fork(), waitid(), the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>, <sys/wait.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
right (C) 2001-2003 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.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 WAIT(3P)
