wait, waitpid, waitid — wait for process to change state
#include <sys/types.h> #include <sys/wait.h>
pid_t
wait( |
int * | status) ; |
pid_t
waitpid( |
pid_t | pid, |
int * | status, | |
int | options) ; |
int
waitid( |
idtype_t | idtype, |
id_t | id, | |
siginfo_t * | infop, | |
int | options) ; |
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 terminated the child remains in a "zombie" state (see NOTES below).
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
.
The wait
() system call
suspends execution of the current process until one of its
children terminates. The call wait(&status)
is
equivalent to:
waitpid(−1, &status, 0);
The waitpid
() system call
suspends execution of the current process until a child
specified by pid
argument has changed state. By default, waitpid
() waits only for terminated
children, but this behaviour is modifiable via the
options
argument,
as described below.
The value of pid
can be:
meaning wait for any child process whose process
group ID is equal to the absolute value of pid
.
meaning wait for any child process.
0
meaning wait for any child process whose process group ID is equal to that of the calling process.
meaning wait for the child whose process ID is
equal to the value of pid
.
The value of options
is an OR of zero or
more of the following constants:
WNOHANG
return immediately if no child has exited.
WUNTRACED
also return if a child has stopped (but not traced
via ptrace(2)). Status
for traced
children which have stopped is provided even if this
option is not specified.
WCONTINUED
(Since Linux 2.6.10) also return if a stopped
child has been resumed by delivery of SIGCONT
.
(For Linux-only options, see below.)
The WUNTRACED
and
WCONTINUED
options are only
effective if the SA_NOCLDSTOP
flag has not been set for the SIGCHLD
signal (see sigaction(2)).
If status
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
()!):
WIFEXITED
(status
)returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().
WEXITSTATUS
(status
)returns the exit status of the child. This
consists of the least significant 8 bits of the
status
argument that the child specified in a call to
exit(3) or _exit(2) or as the
argument for a return statement in main(). This macro
should only be employed if WIFEXITED
returned true.
WIFSIGNALED
(status
)returns true if the child process was terminated by a signal.
WTERMSIG
(status
)returns the number of the signal that caused the
child process to terminate. This macro should only be
employed if WIFSIGNALED
returned true.
WCOREDUMP
(status
)returns true if the child produced a core dump.
This macro should only be employed if WIFSIGNALED
returned true. This
macro is not specified in POSIX.1-2001 and is not
available on some Unix implementations (e.g., AIX,
SunOS). Only use this enclosed in #ifdef WCOREDUMP
... #endif.
WIFSTOPPED
(status
)returns true if the child process was stopped by
delivery of a signal; this is only possible if the
call was done using WUNTRACED
or when the child is
being traced (see ptrace(2)).
WSTOPSIG
(status
)returns the number of the signal which caused the
child to stop. This macro should only be employed if
WIFSTOPPED
returned
true.
WIFCONTINUED
(status
)(Since Linux 2.6.10) returns true if the child
process was resumed by delivery of SIGCONT
.
The waitid
() system call
(available since Linux 2.6.9) provides more precise control
over which child state changes to wait for.
The idtype
and
id
arguments select
the child(ren) to wait for, as follows:
idtype
== P_PID
Wait for the child whose process ID matches
id
.
idtype
== P_PGID
Wait for any child whose process group ID matches
id
.
idtype
== P_ALL
Wait for any child; id
is ignored.
The child state changes to wait for are specified by
ORing one or more of the following flags in options
:
WEXITED
Wait for children that have terminated.
WSTOPPED
Wait for children that have been stopped by delivery of a signal.
WCONTINUED
Wait for (previously stopped) children that have
been resumed by delivery of SIGCONT
.
The following flags may additionally be ORed in
options
:
WNOHANG
As for waitpid
().
WNOWAIT
Leave the child in a waitable state; a later wait call can be used to again retrieve the child status information.
Upon successful return, waitid
() fills in the following fields of
the siginfo_t
structure pointed to by infop
:
si_pid
The process ID of the child.
si_uid
The real user ID of the child. (This field is not set on most other implementations.)
si_signo
Always set to SIGCHLD
.
si_status
Either the exit status of the child, as given to
_exit(2) (or
exit(3)), or the
signal that caused the child to terminate, stop, or
continue. The si_code
field can be
used to determine how to interpret this field.
si_code
Set to one of: CLD_EXITED
(child called _exit(2));
CLD_KILLED
(child
killed by signal); CLD_STOPPED
(child stopped by
signal); or CLD_CONTINUED
(child continued by
SIGCONT
).
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
is
unspecified. To 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 non-zero value in this field after the
call returns.
wait
(): on success, returns
the process ID of the terminated child; on error, −1 is
returned.
waitpid
(): on success,
returns the process ID of the child whose state has changed;
on error, −1 is returned; if WNOHANG
was specified and no child(ren)
specified by pid
has
yet changed state, then 0 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.
(for wait
()) The
calling process does not have any unwaited-for
children.
(for waitpid
() or
waitid
()) The process
specified by pid
(waitpid
()) or idtype
and id
(waitid
()) does not exist or is not a
child of the calling process. (This can happen for
one's own child if the action for SIGCHLD is set to
SIG_IGN. See also the LINUX NOTES section about
threads.)
WNOHANG
was not set
and an unblocked signal or a SIGCHLD
was caught.
The options
argument was invalid.
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(8), which automatically performs a wait to remove the zombies.
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 behaviour of setting SIGCHLD
to SIG_IGN
unspecified.) Linux 2.6 conforms to
this specification. 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.
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.
The following Linux specific options
are for use with
children created using clone(2); they cannot be
used with waitid
():
__WCLONE
Wait for "clone" children only. If omitted then wait
for "non-clone" children only. (A "clone" child is one
which delivers no signal, or a signal other than
SIGCHLD
to its parent
upon termination.) This option is ignored if
__WALL
is also
specified.
__WALL
(Since Linux 2.4) Wait for all children, regardless of type ("clone" or "non-clone").
__WNOTHREAD
(Since Linux 2.4) Do not wait for children of other threads in the same thread group. This was the default before Linux 2.4.
The following program demonstrates the use of fork(2) and waitpid(2). 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(2), and uses the W*() macros described above to analyse the wait status value.
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 $ #include <sys/wait.h> #include <stdlib.h> #include <unistd.h> #include <stdio.h> int main(int argc, char *argv[]) { pid_t cpid, w; int status; cpid = fork(); if (cpid == -1) { perror("fork"); exit(EXIT_FAILURE); } if (cpid == 0) { /* Code executed by child */ printf("Child PID is %ld\n", (long) getpid()); if (argc == 1) pause(); /* Wait for signals */ _exit(atoi(argv[1])); } else { /* Code executed by parent */ do { w = waitpid(cpid, &status, WUNTRACED | WCONTINUED); if (w == -1) { perror("waitpid"); exit(EXIT_FAILURE); } if (WIFEXITED(status)) { printf("exited, status=%d\n", WEXITSTATUS(status)); } else if (WIFSIGNALED(status)) { printf("killed by signal %d\n", WTERMSIG(status)); } else if (WIFSTOPPED(status)) { printf("stopped by signal %d\n", WSTOPSIG(status)); } else if (WIFCONTINUED(status)) { printf("continued\n"); } } while (!WIFEXITED(status) && !WIFSIGNALED(status)); exit(EXIT_SUCCESS); } }
_exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2), signal(2), wait4(2), pthread_create(3), signal(7)
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