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2012-05-19 12:06 @172.29.30.75 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
WAIT(2) Linux Programmer's Manual WAIT(2)
NAME
wait, waitpid - wait for process to change state
SYNOPSIS
#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);
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 terminated the child remains in a "zombie"
state (see NOTES below).
If a child has already changed state, then these calls return immediately. Other-
wise 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.
wait() and waitpid()
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:
< -1 meaning wait for any child process whose process group ID is equal to the
absolute value of pid.
-1 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.
> 0 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 signifi-
cant 16-8 bits of the status argument that the child specified in a call to
exit() or _exit() 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 deliv-
ery of SIGCONT.
waitid()
The waitid() system call (available since Linux 2.6.9) provides more precise con-
trol 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 fol-
lowing 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 imple-
mentations.)
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.
RETURN VALUE
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) speci-
fied 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.
ERRORS
ECHILD (for wait()) The calling process does not have any unwaited-for children.
ECHILD (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.)
EINTR WNOHANG was not set and an unblocked signal or a SIGCHLD was caught.
EINVAL The options argument was invalid.
NOTES
A child that terminates, but has not been waited for becomes a "zombie". The ker-
nel maintains a minimal set of information about the zombie process (PID, termina-
tion status, resource usage information) in order to allow the parent to later per-
form 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 pro-
cesses. 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 termi-
nate 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.
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.
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" chil-
dren 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.
EXAMPLE
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 wait-
pid(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);
}
}
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
SEE ALSO
_exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2), signal(2), wait4(2),
pthread_create(3), signal(7)
Linux 2004-11-11 WAIT(2)
Generated by $Id: phpMan.php,v 4.54 2007/08/21 09:05:22 chedong Exp $ Author: Che Dong
On Apache/2.2.3 (CentOS)
Under GNU General Public License
2012-05-19 12:06 @172.29.30.75 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)