path: root/man3p/wait.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "WAIT" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" wait 
.SH NAME
wait, waitpid \- wait for a child process to stop or terminate
.SH SYNOPSIS
.LP
\fB#include <sys/wait.h>
.br
.sp
pid_t wait(int *\fP\fIstat_loc\fP\fB);
.br
pid_t waitpid(pid_t\fP \fIpid\fP\fB, int *\fP\fIstat_loc\fP\fB, int\fP
\fIoptions\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIwait\fP() and \fIwaitpid\fP() 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.
.LP
The \fIwait\fP() 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 \fIwait\fP() or \fIwaitpid\fP()
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 \fIwait\fP(), return shall be immediate.
.LP
The \fIwaitpid\fP() function shall be equivalent to \fIwait\fP() if
the \fIpid\fP argument is (\fBpid_t\fP)-1 and the
\fIoptions\fP argument is 0. Otherwise, its behavior shall be modified
by the values of the \fIpid\fP and \fIoptions\fP
arguments.
.LP
The \fIpid\fP argument specifies a set of child processes for which
\fIstatus\fP is requested. The \fIwaitpid\fP() function
shall only return the status of a child process from this set:
.IP " *" 3
If \fIpid\fP is equal to (\fBpid_t\fP)-1, \fIstatus\fP is requested
for any child process. In this respect, \fIwaitpid\fP()
is then equivalent to \fIwait\fP().
.LP
.IP " *" 3
If \fIpid\fP is greater than 0, it specifies the process ID of a single
child process for which \fIstatus\fP is requested.
.LP
.IP " *" 3
If \fIpid\fP is 0, \fIstatus\fP is requested for any child process
whose process group ID is equal to that of the calling
process.
.LP
.IP " *" 3
If \fIpid\fP is less than (\fBpid_t\fP)-1, \fIstatus\fP is requested
for any child process whose process group ID is equal to
the absolute value of \fIpid\fP.
.LP
.LP
The \fIoptions\fP argument is constructed from the bitwise-inclusive
OR of zero or more of the following flags, defined in the
\fI<sys/wait.h>\fP header:
.TP 7
WCONTINUED
The \fIwaitpid\fP() function shall report the status of any continued
child process specified by \fIpid\fP whose status has not
been reported since it continued from a job control stop. 
.TP 7
WNOHANG
The \fIwaitpid\fP() function shall not suspend execution of the calling
thread if \fIstatus\fP is not immediately available
for one of the child processes specified by \fIpid\fP.
.TP 7
WUNTRACED
The status of any child processes specified by \fIpid\fP that are
stopped, and whose status has not yet been reported since
they stopped, shall also be reported to the requesting process.
.sp
.LP
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 \fIwait\fP() and \fIwaitpid\fP() shall fail
and set \fIerrno\fP to [ECHILD]. 
.LP
If \fIwait\fP() or \fIwaitpid\fP() 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 \fIstat_loc\fP is not a null
pointer, information shall be stored in the location pointed to by
\fIstat_loc\fP. The value stored at the location pointed to by
\fIstat_loc\fP shall be 0 if and only if the status returned is from
a terminated child process that terminated by one of the
following means:
.IP " 1." 4
The process returned 0 from \fImain\fP().
.LP
.IP " 2." 4
The process called \fI_exit\fP() or \fIexit\fP()
with a \fIstatus\fP argument of 0.
.LP
.IP " 3." 4
The process was terminated because the last thread in the process
terminated.
.LP
.LP
Regardless of its value, this information may be interpreted using
the following macros, which are defined in \fI<sys/wait.h>\fP and
evaluate to integral expressions; the \fIstat_val\fP argument is
the integer value pointed to by \fIstat_loc\fP.
.TP 7
WIFEXITED(\fIstat_val\fP)
.sp
Evaluates to a non-zero value if \fIstatus\fP was returned for a child
process that terminated normally.
.TP 7
WEXITSTATUS(\fIstat_val\fP)
.sp
If the value of WIFEXITED(\fIstat_val\fP) is non-zero, this macro
evaluates to the low-order 8 bits of the \fIstatus\fP argument
that the child process passed to \fI_exit\fP() or \fIexit\fP(), or
the value the child process returned from \fImain\fP().
.TP 7
WIFSIGNALED(\fIstat_val\fP)
.sp
Evaluates to a non-zero value if \fIstatus\fP was returned for a child
process that terminated due to the receipt of a signal that
was not caught (see \fI<signal.h>\fP).
.TP 7
WTERMSIG(\fIstat_val\fP)
.sp
If the value of WIFSIGNALED(\fIstat_val\fP) is non-zero, this macro
evaluates to the number of the signal that caused the
termination of the child process.
.TP 7
WIFSTOPPED(\fIstat_val\fP)
.sp
Evaluates to a non-zero value if \fIstatus\fP was returned for a child
process that is currently stopped.
.TP 7
WSTOPSIG(\fIstat_val\fP)
.sp
If the value of WIFSTOPPED(\fIstat_val\fP) is non-zero, this macro
evaluates to the number of the signal that caused the child
process to stop.
.TP 7
WIFCONTINUED(\fIstat_val\fP)
.sp
Evaluates to a non-zero value if \fIstatus\fP was returned for a child
process that has continued from a job control stop. 
.sp
.LP
It is unspecified whether the \fIstatus\fP value returned by calls
to \fIwait\fP() or \fIwaitpid\fP() for processes created by
\fIposix_spawn\fP() or \fIposix_spawnp\fP() can indicate a WIFSTOPPED(\fIstat_val\fP)
before subsequent calls to
\fIwait\fP() or \fIwaitpid\fP() indicate WIFEXITED(\fIstat_val\fP)
as the result of an error detected before the new process
image starts executing.
.LP
It is unspecified whether the \fIstatus\fP value returned by calls
to \fIwait\fP() or \fIwaitpid\fP() for processes created
by \fIposix_spawn\fP() or \fIposix_spawnp\fP() can indicate a WIFSIGNALED(\fIstat_val\fP)
if a signal is sent to the
parent's process group after \fIposix_spawn\fP() or \fIposix_spawnp\fP()
is called. 
.LP
If the information pointed to by \fIstat_loc\fP was stored by a call
to \fIwaitpid\fP() that specified the WUNTRACED flag
\ and did not specify the WCONTINUED flag,  exactly one of the
macros WIFEXITED(*\fIstat_loc\fP), WIFSIGNALED(*\fIstat_loc\fP), and
WIFSTOPPED(*\fIstat_loc\fP) shall evaluate to a non-zero
value.
.LP
If the information pointed to by \fIstat_loc\fP was stored by a call
to \fIwaitpid\fP() that specified the WUNTRACED   \ and
WCONTINUED  flags, exactly one of the macros
WIFEXITED(*\fIstat_loc\fP), WIFSIGNALED(*\fIstat_loc\fP), WIFSTOPPED(*\fIstat_loc\fP),
\ and
WIFCONTINUED(*\fIstat_loc\fP)  shall evaluate to a non-zero
value.
.LP
If the information pointed to by \fIstat_loc\fP was stored by a call
to \fIwaitpid\fP() that did not specify the WUNTRACED
\ or WCONTINUED  flags, or by a call to the \fIwait\fP()
function, exactly one of the macros WIFEXITED(*\fIstat_loc\fP) and
WIFSIGNALED(*\fIstat_loc\fP) shall evaluate to a non-zero
value.
.LP
If the information pointed to by \fIstat_loc\fP was stored by a call
to \fIwaitpid\fP() that did not specify the WUNTRACED
flag  
\ and specified the WCONTINUED flag,  or by a call to the
\fIwait\fP() function, exactly one of the macros WIFEXITED(*\fIstat_loc\fP),
WIFSIGNALED(*\fIstat_loc\fP),   \ and
WIFCONTINUED(*\fIstat_loc\fP)  shall evaluate to a non-zero
value.
.LP
If _POSIX_REALTIME_SIGNALS is defined, and the implementation queues
the SIGCHLD signal, then if \fIwait\fP() or
\fIwaitpid\fP() 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.
.LP
Otherwise, if SIGCHLD is blocked, if \fIwait\fP() or \fIwaitpid\fP()
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.
.LP
For all other conditions, it is unspecified whether child \fIstatus\fP
will be available when a SIGCHLD signal is
delivered.
.LP
There may be additional implementation-defined circumstances under
which \fIwait\fP() or \fIwaitpid\fP() report \fIstatus\fP.
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 \fIstatus\fP is implementation-defined.
.LP
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. 
.SH RETURN VALUE
.LP
If \fIwait\fP() or \fIwaitpid\fP() 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 \fIstatus\fP
is reported. If \fIwait\fP() or \fIwaitpid\fP()
returns due to the delivery of a signal to the calling process, -1
shall be returned and \fIerrno\fP set to [EINTR]. If
\fIwaitpid\fP() was invoked with WNOHANG set in \fIoptions\fP, it
has at least one child process specified by \fIpid\fP for
which \fIstatus\fP is not available, and \fIstatus\fP is not available
for any process specified by \fIpid\fP, 0 is returned.
Otherwise, (\fBpid_t\fP)-1 shall be returned, and \fIerrno\fP set
to indicate the error.
.SH ERRORS
.LP
The \fIwait\fP() function shall fail if:
.TP 7
.B ECHILD
The calling process has no existing unwaited-for child processes.
.TP 7
.B EINTR
The function was interrupted by a signal. The value of the location
pointed to by \fIstat_loc\fP is undefined.
.sp
.LP
The \fIwaitpid\fP() function shall fail if:
.TP 7
.B ECHILD
The process specified by \fIpid\fP does not exist or is not a child
of the calling process, or the process group specified by
\fIpid\fP does not exist or does not have any member process that
is a child of the calling process.
.TP 7
.B EINTR
The function was interrupted by a signal. The value of the location
pointed to by \fIstat_loc\fP is undefined.
.TP 7
.B EINVAL
The \fIoptions\fP argument is not valid.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
A call to the \fIwait\fP() or \fIwaitpid\fP() function only returns
\fIstatus\fP on an immediate child process of the calling
process; that is, a child that was produced by a single \fIfork\fP()
call (perhaps followed
by an \fIexec\fP or other function calls) from the parent. If a child
produces grandchildren
by further use of \fIfork\fP(), none of those grandchildren nor any
of their descendants
affect the behavior of a \fIwait\fP() 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 \fIstatus\fP from a grandchild or any other
process, but a process that does not use such extensions must be guaranteed
to see \fIstatus\fP from only its direct children.
.LP
The \fIwaitpid\fP() function is provided for three reasons:
.IP " 1." 4
To support job control
.LP
.IP " 2." 4
To permit a non-blocking version of the \fIwait\fP() function
.LP
.IP " 3." 4
To permit a library routine, such as \fIsystem\fP() or \fIpclose\fP(),
to wait for its children without interfering with other terminated
children for
which the process has not waited
.LP
.LP
The first two of these facilities are based on the \fIwait3\fP() function
provided by 4.3 BSD. The function uses the
\fIoptions\fP argument, which is equivalent to an argument to \fIwait3\fP().
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 \fIptrace\fP() debugging facility and (untraced)
processes stopped by job control signals. Since \fIptrace\fP() 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.
.LP
The third reason for the \fIwaitpid\fP() 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:
.sp
.RS
.nf

\fBstream = popen("/bin/true");
(void) system("sleep 100");
(void) pclose(stream);
\fP
.fi
.RE
.LP
On all historical implementations, the final \fIpclose\fP() fails
to reap the
\fIwait\fP() \fIstatus\fP of the \fIpopen\fP().
.LP
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 \fIstatus\fP value of zero corresponds to a process
calling \fI_exit\fP(0), as this is the most common encoding
expected by existing programs. Some of the macro names were adopted
from 4.3 BSD.
.LP
These macros syntactically operate on an arbitrary integer value.
The behavior is undefined unless that value is one stored by a
successful call to \fIwait\fP() or \fIwaitpid\fP() in the location
pointed to by the \fIstat_loc\fP argument. An early proposal
attempted to make this clearer by specifying each argument as *\fIstat_loc\fP
rather than \fIstat_val\fP. 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 *\fIstat_loc\fP;
in fact, that value can be stored or passed as an
argument to other functions before being interpreted by these macros.
.LP
The extension that affects \fIwait\fP() and \fIwaitpid\fP() and is
common in historical implementations is the \fIptrace\fP()
function. It is called by a child process and causes that child to
stop and return a \fIstatus\fP that appears identical to the
\fIstatus\fP indicated by WIFSTOPPED. The \fIstatus\fP of \fIptrace\fP()
children is traditionally returned regardless of the
WUNTRACED flag (or by the \fIwait\fP() 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.
.LP
Implementations that support \fBcore\fP file creation or other implementation-defined
actions on termination of some processes
traditionally provide a bit in the \fIstatus\fP returned by \fIwait\fP()
to indicate that such actions have occurred.
.LP
Allowing the \fIwait\fP() family of functions to discard a pending
SIGCHLD signal that is associated with a successfully
waited-for child process puts them into the \fIsigwait\fP() and \fIsigwaitinfo\fP()
category with respect to SIGCHLD.
.LP
This definition allows implementations to treat a pending SIGCHLD
signal as accepted by the process in \fIwait\fP(), with the
same meaning of "accepted" as when that word is applied to the \fIsigwait\fP()
family of
functions.
.LP
Allowing the \fIwait\fP() family of functions to behave this way permits
an implementation to be able to deal precisely with
SIGCHLD signals.
.LP
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):
.IP " 1." 4
If a SIGCHLD signal handler is established via \fIsigaction\fP() 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.
.LP
.IP " 2." 4
A single \fIwait\fP() issued from a SIGCHLD signal handler can be
guaranteed to return immediately with status information for
a child process.
.LP
.IP " 3." 4
When SA_SIGINFO is requested, the SIGCHLD signal handler can be guaranteed
to receive a non-NULL pointer to a \fBsiginfo_t\fP
structure that describes a child process for which a wait via \fIwaitpid\fP()
or \fIwaitid\fP() will not block or fail.
.LP
.IP " 4." 4
The \fIsystem\fP() function will not cause a process' SIGCHLD handler
to be called as a
result of the \fIfork\fP()/ \fIexec\fP executed within
\fIsystem\fP() because \fIsystem\fP() will accept
the SIGCHLD signal when it performs a \fIwaitpid\fP() for its child
process. This is a desirable behavior of \fIsystem\fP() so that it
can be used in a library without causing side effects to the application
linked with the library.
.LP
.LP
An implementation that does not permit the \fIwait\fP() 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:
.TP 7
Guarantee #1
.sp
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 \fIfork\fP()/ \fIwait\fP() 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.
.TP 7
Guarantees #2 and #3
.sp
These cannot be guaranteed unless the \fIwait\fP() family of functions
accepts the SIGCHLD signal. Otherwise, a \fIfork\fP()/ \fIwait\fP()
executed while SIGCHLD is blocked (as in the \fIsystem\fP() function)
will result in an invocation of the handler when SIGCHLD is unblocked,
after the process has disappeared.
.TP 7
Guarantee #4
.sp
Although possible to make this guarantee, \fIsystem\fP() would have
to set the SIGCHLD
handler to SIG_DFL so that the SIGCHLD signal generated by its \fIfork\fP()
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.
.sp
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIexec\fP() , \fIexit\fP() , \fIfork\fP() , \fIwaitid\fP() , the
Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<signal.h>\fP, \fI<sys/wait.h>\fP
.SH 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 .