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() 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.
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 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.
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 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
IEEE Std 1003.1-2001 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
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 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
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 follow 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 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 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 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.
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, Copyright (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.opengroup.org/unix/online.html .