Manpage of WAIT

WAIT

Section: Linux Programmer's Manual (2)
Updated: 2016-07-17
Index
 

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);
                /* This is the glibc and POSIX interface; see
                   NOTES for information on the raw system call. */

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

waitid():

_XOPEN_SOURCE
    || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE
 

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).

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_RESTARTflag 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 calling process until one of its children terminates. The call wait(&wstatus)is equivalent to:
    waitpid(-1, &wstatus, 0);

The waitpid() system call suspends execution of the calling process until a child specified by pidargument has changed state. By default, waitpid() waits only for terminated children, but this behavior is modifiable via the optionsargument, as described below.

The value of pidcan 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 optionsis 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 tracedchildren 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.)

If wstatusis 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(wstatus)
returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().
WEXITSTATUS(wstatus)
returns the exit status of the child. This consists of the least significant 8 bits of the statusargument 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 be employed only if WIFEXITEDreturned true.
WIFSIGNALED(wstatus)
returns true if the child process was terminated by a signal.
WTERMSIG(wstatus)
returns the number of the signal that caused the child process to terminate. This macro should be employed only if WIFSIGNALEDreturned true.
WCOREDUMP(wstatus)
returns true if the child produced a core dump. This macro should be employed only if WIFSIGNALEDreturned 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(wstatus)
returns true if the child process was stopped by delivery of a signal; this is possible only if the call was done using WUNTRACEDor when the child is being traced (see ptrace(2)).
WSTOPSIG(wstatus)
returns the number of the signal which caused the child to stop. This macro should be employed only if WIFSTOPPEDreturned true.
WIFCONTINUED(wstatus)
(since Linux 2.6.10) returns true if the child process was resumed by delivery of SIGCONT.
 

waitid()

The waitid() system call (available since Linux 2.6.9) provides more precise control over which child state changes to wait for.

The idtypeand idarguments 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; idis 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_tstructure 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_codefield 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_DUMPED(child killed by signal, and dumped core); CLD_STOPPED(child stopped by signal); CLD_TRAPPED(traced child has trapped); or CLD_CONTINUED(child continued by SIGCONT).

If WNOHANGwas specified in optionsand there were no children in a waitable state, then waitid() returns 0 immediately and the state of the siginfo_tstructure pointed to by infopis unspecified. To distinguish this case from that where a child was in a waitable state, zero out the si_pidfield before the call and check for a nonzero 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; if WNOHANGwas specified and one or more child(ren) specified by pidexist, but have not yet changed state, then 0 is returned. On error, -1 is returned.

waitid(): returns 0 on success or if WNOHANGwas specified and no child(ren) specified by idhas yet changed state; on error, -1 is returned.

Each of these calls sets errnoto 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 idtypeand 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 SIGCHLDis set to SIG_IGN. See also the Linux Notes section about threads.)
EINTR
WNOHANGwas not set and an unblocked signal or a SIGCHLDwas caught; see signal(7).
EINVAL
The optionsargument was invalid.
 

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), which automatically performs a wait to remove the zombies.

POSIX.1-2001 specifies that if the disposition of SIGCHLDis set to SIG_IGNor the SA_NOCLDWAITflag 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 errnoset to ECHILD. (The original POSIX standard left the behavior of setting SIGCHLDto SIG_IGNunspecified. Note that even though the default disposition of SIGCHLDis "ignore", explicitly setting the disposition to SIG_IGNresults 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 SIGCHLDis being ignored, the call behaves just as though SIGCHLDwere 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 optionsare for use with children created using clone(2); they can also, since Linux 4.7, 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 SIGCHLDto its parent upon termination.) This option is ignored if __WALLis 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.

Since Linux 4.7, the __WALLflag is automatically implied if the child is being ptraced.  

C library/kernel differences

wait() is actually a library function that (in glibc) is implemented as a call to 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.  

BUGS

According to POSIX.1-2008, an application calling waitid() must ensure that infoppoints to a siginfo_tstructure (i.e., that it is a non-null pointer). On Linux, if infopis NULL, waitid() succeeds, and returns the process ID of the waited-for child. Applications should avoid relying on this inconsistent, nonstandard, and unnecessary feature.  

EXAMPLE

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.

The following shell session demonstrates the use of the program:

$ ./a.out &Child PID is 32360
[1] 32359
$ kill -STOP 32360stopped by signal 19
$ kill -CONT 32360continued
$ kill -TERM 32360killed by signal 15
[1]+  Done                    ./a.out
$
 

Program source

#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int
main(int argc, char *argv[])
{
    pid_t cpid, w;
    int wstatus;

    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, &wstatus, WUNTRACED | WCONTINUED);
            if (w == -1) {
                perror("waitpid");
                exit(EXIT_FAILURE);
            }

            if (WIFEXITED(wstatus)) {
                printf("exited, status=%d\n", WEXITSTATUS(wstatus));
            } else if (WIFSIGNALED(wstatus)) {
                printf("killed by signal %d\n", WTERMSIG(wstatus));
            } else if (WIFSTOPPED(wstatus)) {
                printf("stopped by signal %d\n", WSTOPSIG(wstatus));
            } else if (WIFCONTINUED(wstatus)) {
                printf("continued\n");
            }
        } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
        exit(EXIT_SUCCESS);
    }
}
 

SEE ALSO

_exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2), signal(2), wait4(2), pthread_create(3), credentials(7), signal(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
wait() and waitpid()
waitid()
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
Linux notes
C library/kernel differences
BUGS
EXAMPLE
Program source
SEE ALSO

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