# posix_spawn

## 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.
    


## STANDARDS



           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);
           }
    


* * *