wait (2) - Linux Manuals
wait: wait for process to change state
NAME
wait, waitpid, waitid - wait for process to change state
SYNOPSIS
#include <sys/types.h>#include <sys/wait.h>
pid_t wait(int *wstatus);
pid_t waitpid(pid_t pid, int *wstatus, int options);
int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
waitid():
If a child has already changed state, then these calls return immediately.
Otherwise, they block until either a child changes state or
a signal handler interrupts the call (assuming that system calls
are not automatically restarted using the
SA_RESTART
flag of
sigaction(2)).
In the remainder of this page, a child whose state has changed
and which has not yet been waited upon by one of these system
calls is termed
waitable.
waitpid(-1, &wstatus, 0);
The
waitpid()
system call suspends execution of the calling thread until a
child specified by
pid
argument has changed state.
By default,
waitpid()
waits only for terminated children, but this behavior is modifiable
via the
options
argument, as described below.
The value of
pid
can be:
The value of
options
is an OR of zero or more of the following constants:
(For Linux-only options, see below.)
If
wstatus
is not NULL,
wait()
and
waitpid()
store status information in the int to which it points.
This integer can be inspected with the following macros (which
take the integer itself as an argument, not a pointer to it,
as is done in
wait()
and
waitpid()!):
The
idtype
and
id
arguments select the child(ren) to wait for, as follows:
The child state changes to wait for are specified by ORing
one or more of the following flags in
options:
The following flags may additionally be ORed in
options:
Upon successful return,
waitid()
fills in the following fields of the
siginfo_t
structure pointed to by
infop:
If
WNOHANG
was specified in
options
and there were no children in a waitable state, then
waitid()
returns 0 immediately and
the state of the
siginfo_t
structure pointed to by
infop
depends on the implementation.
To (portably) distinguish this case from that where a child was in a
waitable state, zero out the
si_pid
field before the call and check for a nonzero value in this field
after the call returns.
POSIX.1-2008 Technical Corrigendum 1 (2013) adds the requirement that when
WNOHANG
is specified in
options
and there were no children in a waitable state, then
waitid()
should zero out the
si_pid
and
si_signo
fields of the structure.
On Linux and other implementations that adhere to this requirement,
it is not necessary to zero out the
si_pid
field before calling
waitid().
However,
not all implementations follow the POSIX.1 specification on this point.
waitpid():
on success, returns the process ID of the child whose state has changed;
if
WNOHANG
was specified and one or more child(ren) specified by
pid
exist, but have not yet changed state, then 0 is returned.
On error, -1 is returned.
waitid():
returns 0 on success or
if
WNOHANG
was specified and no child(ren) specified by
id
has yet changed state;
on error, -1 is returned.
Each of these calls sets
errno
to an appropriate value in the case of an error.
POSIX.1-2001 specifies that if the disposition of
SIGCHLD
is set to
SIG_IGN
or the
SA_NOCLDWAIT
flag is set for
SIGCHLD
(see
sigaction(2)),
then children that terminate do not become zombies and a call to
wait()
or
waitpid()
will block until all children have terminated, and then fail with
errno
set to
ECHILD.
(The original POSIX standard left the behavior of setting
SIGCHLD
to
SIG_IGN
unspecified.
Note that even though the default disposition of
SIGCHLD
is "ignore", explicitly setting the disposition to
SIG_IGN
results in different treatment of zombie process children.)
Linux 2.6 conforms to the POSIX requirements.
However, Linux 2.4 (and earlier) does not:
if a
wait()
or
waitpid()
call is made while
SIGCHLD
is being ignored, the call behaves just as though
SIGCHLD
were not being ignored, that is, the call blocks until the next child
terminates and then returns the process ID and status of that child.
The following Linux-specific
options
are for use with children created using
clone(2);
they can also, since Linux 4.7,
be used with
waitid():
Since Linux 4.7,
the
__WALL
flag is automatically implied if the child is being ptraced.
On some architectures, there is no
waitpid()
system call;
instead, this interface is implemented via a C library
wrapper function that calls
wait4(2).
The raw
waitid()
system call takes a fifth argument, of type
struct rusage *.
If this argument is non-NULL,
then it is used to return resource usage information about the child,
in the same manner as
wait4(2).
See
getrusage(2)
for details.
The following shell session demonstrates the use of the program:
$ ./a.out &
Child PID is 32360
[1] 32359
$ kill -STOP 32360
stopped by signal 19
$ kill -CONT 32360
continued
$ kill -TERM 32360
killed by signal 15
[1]+ Done ./a.out
$
int
main(int argc, char *argv[])
{
Glibc 2.25 and earlier:
DESCRIPTION
All of these system calls are used to wait for state changes
in a child of the calling process, and obtain information
about the child whose state has changed.
A state change is considered to be: the child terminated;
the child was stopped by a signal; or the child was resumed by a signal.
In the case of a terminated child, performing a wait allows
the system to release the resources associated with the child;
if a wait is not performed, then the terminated child remains in
a "zombie" state (see NOTES below).
wait() and waitpid()
The
wait()
system call suspends execution of the calling thread until one of its
children terminates.
The call
wait(&wstatus)
is equivalent to:
waitid()
The
waitid()
system call (available since Linux 2.6.9) provides more precise
control over which child state changes to wait for.
RETURN VALUE
wait():
on success, returns the process ID of the terminated child;
on error, -1 is returned.
ERRORS
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
A child that terminates, but has not been waited for becomes a "zombie".
The kernel maintains a minimal set of information about the zombie
process (PID, termination status, resource usage information)
in order to allow the parent to later perform a wait to obtain
information about the child.
As long as a zombie is not removed from the system via a wait,
it will consume a slot in the kernel process table, and if
this table fills, it will not be possible to create further processes.
If a parent process terminates, then its "zombie" children (if any)
are adopted by
init(1),
(or by the nearest "subreaper" process as defined through the use of the
prctl(2)
PR_SET_CHILD_SUBREAPER
operation);
init(1)
automatically performs a wait to remove the zombies.
Linux notes
In the Linux kernel, a kernel-scheduled thread is not a distinct
construct from a process.
Instead, a thread is simply a process
that is created using the Linux-unique
clone(2)
system call; other routines such as the portable
pthread_create(3)
call are implemented using
clone(2).
Before Linux 2.4, a thread was just a special case of a process,
and as a consequence one thread could not wait on the children
of another thread, even when the latter belongs to the same thread group.
However, POSIX prescribes such functionality, and since Linux 2.4
a thread can, and by default will, wait on children of other threads
in the same thread group.
C library/kernel differences
wait()
is actually a library function that (in glibc) is implemented as a call to
wait4(2).
BUGS
According to POSIX.1-2008, an application calling
waitid()
must ensure that
infop
points to a
siginfo_t
structure (i.e., that it is a non-null pointer).
On Linux, if
infop
is NULL,
waitid()
succeeds, and returns the process ID of the waited-for child.
Applications should avoid relying on this inconsistent,
nonstandard, and unnecessary feature.
EXAMPLES
The following program demonstrates the use of
fork(2)
and
waitpid().
The program creates a child process.
If no command-line argument is supplied to the program,
then the child suspends its execution using
pause(2),
to allow the user to send signals to the child.
Otherwise, if a command-line argument is supplied,
then the child exits immediately,
using the integer supplied on the command line as the exit status.
The parent process executes a loop that monitors the child using
waitpid(),
and uses the W*() macros described above to analyze the wait status value.
Program source
#include <sys/wait.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>