DESCRIPTION

The posix_spawn() and posix_spawnp() functions are used to create a new child process that executes a specified file. These functions were specified by POSIX to provide a standardized method of creating new processes on machines that lack the capability to support the fork(2) system call. These machines are generally small, embedded systems lacking MMU support.

The posix_spawn() and posix_spawnp() functions provide the functionality of a combined fork(2) and exec(3), with some optional housekeeping steps in the child process before the exec(3). These functions are not meant to replace the fork(2) and execve(2) system calls. In fact, they provide only a subset of the functionality that can be achieved by using the system calls.

The only difference between posix_spawn() and posix_spawnp() is the manner in which they specify the file to be executed by the child process. With posix_spawn(), the executable file is specified as a pathname (which can be absolute or relative). With posix_spawnp(), the executable file is specified as a simple filename; the system searches for this file in the list of directories specified by PATH (in the same way as for execvp(3)). For the remainder of this page, the discussion is phrased in terms of posix_spawn(), with the understanding that posix_spawn() differs only on the point just described.

The remaining arguments to these two functions are as follows:

Below, the functions are described in terms of a three-step process: the fork() step, the pre-exec() step (executed in the child), and the exec() step (executed in the child).

RETURN VALUE

Upon successful completion, posix_spawn() and posix_spawnp() place the PID of the child process in pid, and return 0. If there is an error before or during the fork(2), then no child is created, the contents of *pid are unspecified, and these functions return an error number as described below.

Even when these functions return a success status, the child process may still fail for a plethora of reasons related to its pre- exec() initialization. In addition, the exec(3) may fail. In all of these cases, the child process will exit with the exit value of 127.

ERRORS

The posix_spawn() and posix_spawnp() functions fail only in the case where the underlying fork(2) or vfork(2) call fails; in these cases, these functions return an error number, which will be one of the errors described for fork(2) or vfork(2).

In addition, these functions fail if:

NOTES

The housekeeping activities in the child are controlled by the objects pointed to by attrp (for non-file actions) and file_actions In POSIX parlance, the posix_spawnattr_t and posix_spawn_file_actions_t data types are referred to as objects, and their elements are not specified by name. Portable programs should initialize these objects using only the POSIX-specified functions. (In other words, although these objects may be implemented as structures containing fields, portable programs must avoid dependence on such implementation details.)

According to POSIX, it unspecified whether fork handlers established with pthread_atfork(3) are called when posix_spawn() is invoked. On glibc, fork handlers are called only if the child is created using fork(2).

There is no "posix_fspawn" function (i.e., a function that is to posix_spawn() as fexecve(3) is to execve(2)). However, this functionality can be obtained by specifying the path argument as one of the files in the caller's /proc/self/fd directory.

VERSIONS

The posix_spawn() and posix_spawnp() functions are available since glibc 2.2.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.

BUGS

POSIX.1 says that when POSIX_SPAWN_SETSCHEDULER is specified in spawn-flags, then the POSIX_SPAWN_SETSCHEDPARAM (if present) is ignored. However, before glibc 2.14, calls to posix_spawn() failed with an error if POSIX_SPAWN_SETSCHEDULER was specified without also specifying POSIX_SPAWN_SETSCHEDPARAM.

EXAMPLE

The program below demonstrates the use of various functions in the POSIX spawn API. The program accepts command-line attributes that can be used to create file actions and attributes objects. The remaining command-line arguments are used as the executable name and command-line arguments of the program that is executed in the child.

In the first run, the date(1) command is executed in the child, and the posix_spawn() call employs no file actions or attributes objects.

In the next run, the −c command-line option is used to create a file actions object that closes standard output in the child. Consequently, date(1) fails when trying to perform output and exits with a status of 1.

In the next run, the −s command-line option is used to create an attributes object that specifies that all (blockable) signals in the child should be blocked. Consequently, trying to kill child with the default signal sent by kill(1) (i.e., SIGTERM) fails, because that signal is blocked. Therefore, to kill the child, SIGKILL is necessary (SIGKILL can't be blocked).

When we try to execute a nonexistent command in the child, the exec(3) fails and the child exits with a status of 127.

#include <spawn.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <wait.h>
#include <errno.h>

#define errExit(msg)    do { perror(msg); \
                             exit(EXIT_FAILURE); } while (0)

#define errExitEN(en, msg) \
                        do { errno = en; perror(msg); \
                             exit(EXIT_FAILURE); } while (0)

char **environ;

int
main(int argc, char *argv[])
{
    pid_t child_pid;
    int s, opt, status;
    sigset_t mask;
    posix_spawnattr_t attr;
    posix_spawnattr_t *attrp;
    posix_spawn_file_actions_t file_actions;
    posix_spawn_file_actions_t *file_actionsp;

    /* Parse command−line options, which can be used to specify an
       attributes object and file actions object for the child. */

    attrp = NULL;
    file_actionsp = NULL;

    while ((opt = getopt(argc, argv, "sc")) != −1) {
        switch (opt) {
        case 'c':       /* −c: close standard output in child */

            /* Create a file actions object and add a "close"
               action to it */

            s = posix_spawn_file_actions_init(&file_actions);
            if (s != 0)
                errExitEN(s, "posix_spawn_file_actions_init");

            s = posix_spawn_file_actions_addclose(&file_actions,
                                                  STDOUT_FILENO);
            if (s != 0)
                errExitEN(s, "posix_spawn_file_actions_addclose");

            file_actionsp = &file_actions;
            break;

        case 's':       /* −s: block all signals in child */

            /* Create an attributes object and add a "set signal mask"
               action to it */

            s = posix_spawnattr_init(&attr);
            if (s != 0)
                errExitEN(s, "posix_spawnattr_init");
            s = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK);
            if (s != 0)
                errExitEN(s, "posix_spawnattr_setflags");

            sigfillset(&mask);
            s = posix_spawnattr_setsigmask(&attr, &mask);
            if (s != 0)
                errExitEN(s, "posix_spawnattr_setsigmask");

            attrp = &attr;
            break;
        }
    }

    /* Spawn the child. The name of the program to execute and the
       command−line arguments are taken from the command−line arguments
       of this program. The environment of the program execed in the
       child is made the same as the parent's environment. */

    s = posix_spawnp(&child_pid, argv[optind], file_actionsp, attrp,
                     &argv[optind], environ);
    if (s != 0)
        errExitEN(s, "posix_spawn");

    /* Destroy any objects that we created earlier */

    if (attrp != NULL) {
        s = posix_spawnattr_destroy(attrp);
        if (s != 0)
            errExitEN(s, "posix_spawnattr_destroy");
    }

    if (file_actionsp != NULL) {
        s = posix_spawn_file_actions_destroy(file_actionsp);
        if (s != 0)
            errExitEN(s, "posix_spawn_file_actions_destroy");
    }

    printf("PID of child: %ld\n", (long) child_pid);

    /* Monitor status of the child until it terminates */

    do {
        s = waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
        if (s == −1)
            errExit("waitpid");

        printf("Child status: ");
        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);
}

SEE ALSO

close(2), dup2(2), execl(2), execlp(2), fork(2), open(2), sched_setparam(2), sched_setscheduler(2), setpgid(2), setuid(2), sigaction(2), sigprocmask(2), posix_spawn_file_actions_addclose(3), posix_spawn_file_actions_adddup2(3), posix_spawn_file_actions_addopen(3), posix_spawn_file_actions_destroy(3), posix_spawn_file_actions_init(3), posix_spawnattr_destroy(3), posix_spawnattr_getflags(3), posix_spawnattr_getpgroup(3), posix_spawnattr_getschedparam(3), posix_spawnattr_getschedpolicy(3), posix_spawnattr_getsigdefault(3), posix_spawnattr_getsigmask(3), posix_spawnattr_init(3), posix_spawnattr_setflags(3), posix_spawnattr_setpgroup(3), posix_spawnattr_setschedparam(3), posix_spawnattr_setschedpolicy(3), posix_spawnattr_setsigdefault(3), posix_spawnattr_setsigmask(3), pthread_atfork(3), <spawn.h>, Base Definitions volume of POSIX.1-2001, http://www.opengroup.org/unix/online.html