POSIX_SPAWN(3)            Linux Programmer's Manual           POSIX_SPAWN(3)

NAME         top

       posix_spawn, posix_spawnp - spawn a process

SYNOPSIS         top

       #include <spawn.h>

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

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

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 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

       The remaining arguments to these two functions are as follows:

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

       *  The file_actions argument 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.

       *  The attrp argument 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_*() functions.

       *  The argv and envp arguments 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
       The posix_spawn() function commences by calling 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 process.

       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.

       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(2) and the exec(3), 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

       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

               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

               *  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

               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.

       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_addopen(3),
       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 127.

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

       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 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         top

       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:

       ENOSYS Function not supported on this system.

VERSIONS         top

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

CONFORMING TO         top

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

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 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

       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

EXAMPLE         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 objects.

           $ ./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 <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;

       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: %ld\n", (long) 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_addclose(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, 

COLOPHON         top

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GNU                              2016-10-08                   POSIX_SPAWN(3)