posix_spawn(3)



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

NAME
       posix_spawn, posix_spawnp - spawn a process

SYNOPSIS
       #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
       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  functional-
       ity  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  us-
       ing 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_spawnp() differs only on the point just described.

       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 popu-
          lated  before  the  posix_spawn()  call  using  posix_spawn_file_ac-
          tions_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 environ-
          ment 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 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.

       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  POSIX_SPAWN_SETSIGMASK,
          POSIX_SPAWN_SETSIGDEF,                    POSIX_SPAWN_SETSCHEDPARAM,
          POSIX_SPAWN_SETSCHEDULER, POSIX_SPAWN_SETPGROUP, or  POSIX_SPAWN_RE-
          SETIDS.

       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 exe-
       cutable file.  These operations are controlled by the attributes object
       pointed to by attrp and the file actions object pointed to by  file_ac-
       tions.  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  per-
          formed  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  at-
       tributes  specified  in the object pointed to by attrp and the file ac-
       tions 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_spaw-
       nattr_setflags(3)) controls the general actions that occur,  and  other
       attributes  in  the  object  specify values to be used during those ac-
       tions.

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

       POSIX_SPAWN_SETSIGMASK
              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.

       POSIX_SPAWN_SETSIGDEF
              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 treat-
              ment when POSIX_SPAWN_SETSIGDEF is not specified, see execve(2).

       POSIX_SPAWN_SETSCHEDPARAM
              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.

       POSIX_SPAWN_SETSCHEDULER
              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 at-
                 trp.

              *  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.

       POSIX_SPAWN_RESETIDS
              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 over-
              ride the setting of the effective UID and GID (se execve(2)).

       POSIX_SPAWN_SETPGROUP
              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_SETP-
              GROUP  flag  is not set, the child inherits the parent's process
              group ID.

       POSIX_SPAWN_USEVFORK
              Since glibc 2.24, this flag has no effect.  On older implementa-
              tions,  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 ses-
              sion 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 de-
       scribed 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  re-
       quests  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_ad-
       ddup2(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
       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 de-
       scribed 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() ini-
       tialization.  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), 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.

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

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

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 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 function-
       ality can be obtained by specifying the path argument  as  one  of  the
       files in the caller's /proc/self/fd directory.

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.

EXAMPLES
       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 at-
       tributes 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 sig-
       nal  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;

       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

COLOPHON
       This page is part of release 5.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                               2020-04-11                    POSIX_SPAWN(3)

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