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NAME

       posix_spawn, posix_spawnp - spawn a process

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       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_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.

       file_actions
              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_*() functions.

       argv
       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 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_RESETIDS.

       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:

       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
              treatment 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 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.

       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 override  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_SETPGROUP 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 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_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

       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

       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.

STANDARDS

       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 functionality 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 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;

       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: %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)
                   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