wait() and waitpid()¶

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:

< -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.)

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()!):

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 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 be employed only if WIFEXITED returned 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 WIFSIGNALED returned true.

WCOREDUMP(wstatus)

returns true if the child produced a core dump. This macro should be employed only 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). Therefore, enclose its use inside #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 WUNTRACED or 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 WIFSTOPPED returned true.

WIFCONTINUED(wstatus)

(since Linux 2.6.10) returns true if the child process was resumed by delivery of SIGCONT.

waitid()¶

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_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 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.

Linux notes¶

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():

__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.

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

C library/kernel differences¶

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.

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);
    }
}