posix_spawn(3) — Linux manual page


posix_spawn(3)          Library Functions Manual          posix_spawn(3)

NAME         top

       posix_spawn, posix_spawnp - spawn a process

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <spawn.h>

       int posix_spawn(pid_t *restrict pid, const char *restrict path,
                       const posix_spawn_file_actions_t *restrict file_actions,
                       const posix_spawnattr_t *restrict attrp,
                       char *const argv[restrict],
                       char *const envp[restrict]);
       int posix_spawnp(pid_t *restrict pid, const char *restrict file,
                       const posix_spawn_file_actions_t *restrict file_actions,
                       const posix_spawnattr_t *restrict attrp,
                       char *const argv[restrict],
                       char *const envp[restrict]);

DESCRIPTION         top

       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

       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_spawnp() differs only on the point just described.

       The remaining arguments to these two functions are as follows:

       pid    points to a buffer that is used to return the process ID
              of the new child process.

              points to a spawn file actions object that specifies file-
              related actions to be performed in the child between the
              fork(2) and exec(3) steps.  This object is initialized and
              populated before the posix_spawn() call using
              posix_spawn_file_actions_init(3) and the
              posix_spawn_file_actions_*() functions.

       attrp  points to an attributes objects that specifies various
              attributes of the created child process.  This object is
              initialized and populated before the posix_spawn() call
              using posix_spawnattr_init(3) and the posix_spawnattr_*()

       envp   specify the argument list and environment for the program
              that is executed in the child process, as for execve(2).

       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).

   fork() step
       Since glibc 2.24, the posix_spawn() function commences by calling
       clone(2) with CLONE_VM and CLONE_VFORK flags.  Older
       implementations use fork(2), or possibly vfork(2) (see below).

       The PID of the new child process is placed in *pid.  The
       posix_spawn() function then returns control to the parent

       Subsequently, the parent can use one of the system calls
       described in wait(2) to check the status of the child process.
       If the child fails in any of the housekeeping steps described
       below, or fails to execute the desired file, it exits with a
       status of 127.

       Before glibc 2.24, the child process is created using vfork(2)
       instead of fork(2) when either of the following is true:

       •  the spawn-flags element of the attributes object pointed to by
          attrp contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or

       •  file_actions is NULL and the spawn-flags element of the
          attributes object pointed to by attrp does not contain

       In other words, vfork(2) is used if the caller requests it, or if
       there is no cleanup expected in the child before it exec(3)s the
       requested file.

   pre-exec() step: housekeeping
       In between the fork() and the exec() steps, a child process may
       need to perform a set of housekeeping actions.  The posix_spawn()
       and posix_spawnp() functions support a small, well-defined set of
       system tasks that the child process can accomplish before it
       executes the executable file.  These operations are controlled by
       the attributes object pointed to by attrp and the file actions
       object pointed to by file_actions.  In the child, processing is
       done in the following sequence:

       (1)  Process attribute actions: signal mask, signal default
            handlers, scheduling algorithm and parameters, process
            group, and effective user and group IDs are changed as
            specified by the attributes object pointed to by attrp.

       (2)  File actions, as specified in the file_actions argument, are
            performed in the order that they were specified using calls
            to the posix_spawn_file_actions_add*() functions.

       (3)  File descriptors with the FD_CLOEXEC flag set are closed.

       All process attributes in the child, other than those affected by
       attributes specified in the object pointed to by attrp and the
       file actions in the object pointed to by file_actions, will be
       affected as though the child was created with fork(2) and it
       executed the program with execve(2).

       The process attributes actions are defined by the attributes
       object pointed to by attrp.  The spawn-flags attribute (set using
       posix_spawnattr_setflags(3)) controls the general actions that
       occur, and other attributes in the object specify values to be
       used during those actions.

       The effects of the flags that may be specified in spawn-flags are
       as follows:

              Set the signal mask to the signal set specified in the
              spawn-sigmask attribute of the object pointed to by attrp.
              If the POSIX_SPAWN_SETSIGMASK flag is not set, then the
              child inherits the parent's signal mask.

              Reset the disposition of all signals in the set specified
              in the spawn-sigdefault attribute of the object pointed to
              by attrp to the default.  For the treatment of the
              dispositions of signals not specified in the spawn-
              sigdefault attribute, or the treatment when
              POSIX_SPAWN_SETSIGDEF is not specified, see execve(2).

              If this flag is set, and the POSIX_SPAWN_SETSCHEDULER flag
              is not set, then set the scheduling parameters to the
              parameters specified in the spawn-schedparam attribute of
              the object pointed to by attrp.

              Set the scheduling policy algorithm and parameters of the
              child, as follows:

              •  The scheduling policy is set to the value specified in
                 the spawn-schedpolicy attribute of the object pointed
                 to by attrp.

              •  The scheduling parameters are set to the value
                 specified in the spawn-schedparam attribute of the
                 object pointed to by attrp (but see BUGS).

              If the POSIX_SPAWN_SETSCHEDPARAM and
              POSIX_SPAWN_SETSCHEDPOLICY flags are not specified, the
              child inherits the corresponding scheduling attributes
              from the parent.

              If this flag is set, reset the effective UID and GID to
              the real UID and GID of the parent process.  If this flag
              is not set, then the child retains the effective UID and
              GID of the parent.  In either case, if the set-user-ID and
              set-group-ID permission bits are enabled on the executable
              file, their effect will override the setting of the
              effective UID and GID (se execve(2)).

              Set the process group to the value specified in the spawn-
              pgroup attribute of the object pointed to by attrp.  If
              the spawn-pgroup attribute has the value 0, the child's
              process group ID is made the same as its process ID.  If
              the POSIX_SPAWN_SETPGROUP flag is not set, the child
              inherits the parent's process group ID.

              Since glibc 2.24, this flag has no effect.  On older
              implementations, setting this flag forces the fork() step
              to use vfork(2) instead of fork(2).  The _GNU_SOURCE
              feature test macro must be defined to obtain the
              definition of this constant.

       POSIX_SPAWN_SETSID (since glibc 2.26)
              If this flag is set, the child process shall create a new
              session and become the session leader.  The child process
              shall also become the process group leader of the new
              process group in the session (see setsid(2)).  The
              _GNU_SOURCE feature test macro must be defined to obtain
              the definition of this constant.

       If attrp is NULL, then the default behaviors described above for
       each flag apply.

       The file_actions argument specifies a sequence of file operations
       that are performed in the child process after the general
       processing described above, and before it performs the exec(3).
       If file_actions is NULL, then no special action is taken, and
       standard exec(3) semantics apply—file descriptors open before the
       exec remain open in the new process, except those for which the
       FD_CLOEXEC flag has been set.  File locks remain in place.

       If file_actions is not NULL, then it contains an ordered set of
       requests to open(2), close(2), and dup2(2) files.  These requests
       are added to the file_actions by
       posix_spawn_file_actions_addclose(3), and
       posix_spawn_file_actions_adddup2(3).  The requested operations
       are performed in the order they were added to file_actions.

       If any of the housekeeping actions fails (due to bogus values
       being passed or other reasons why signal handling, process
       scheduling, process group ID functions, and file descriptor
       operations might fail), the child process exits with exit value

   exec() step
       Once the child has successfully forked and performed all
       requested pre-exec steps, the child runs the requested

       The child process takes its environment from the envp argument,
       which is interpreted as if it had been passed to execve(2).  The
       arguments to the created process come from the argv argument,
       which is processed as for execve(2).

RETURN VALUE         top

       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 during the fork() step, 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         top

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

       In addition, these functions fail if:

       ENOSYS Function not supported on this system.

STANDARDS         top


HISTORY         top

       glibc 2.2.  POSIX.1-2001.

NOTES         top

       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 is unspecified whether fork handlers
       established with pthread_atfork(3) are called when posix_spawn()
       is invoked.  Since glibc 2.24, the fork handlers are not executed
       in any case.  On older implementations, 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.

BUGS         top

       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.

EXAMPLES         top

       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

           $ ./a.out date
           PID of child: 7634
           Tue Feb  1 19:47:50 CEST 2011
           Child status: exited, status=0

       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.

           $ ./a.out -c date
           PID of child: 7636
           date: write error: Bad file descriptor
           Child status: exited, status=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).

           $ ./a.out -s sleep 60 &
           [1] 7637
           $ PID of child: 7638

       $ kill 7638
       $ kill -KILL 7638
       $ Child status: killed by signal 9
       [1]+  Done                    ./a.out -s sleep 60

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

           $ ./a.out xxxxx
           PID of child: 10190
           Child status: exited, status=127

   Program source

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

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

                   file_actionsp = &file_actions;

               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");

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

                   attrp = &attr;

           /* 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: %jd\n", (intmax_t) child_pid);

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

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

               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)) {
           } while (!WIFEXITED(status) && !WIFSIGNALED(status));


SEE ALSO         top

       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_init(3), posix_spawnattr_destroy(3),
       posix_spawnattr_getflags(3), posix_spawnattr_getpgroup(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_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 

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Linux man-pages 6.9.1          2024-06-15                 posix_spawn(3)

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