1posix_spawn(3) Library Functions Manual posix_spawn(3)
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6 posix_spawn, posix_spawnp - spawn a process
7
9 Standard C library (libc, -lc)
10
12 #include <spawn.h>
13 int posix_spawn(pid_t *restrict pid, const char *restrict path,
15 const posix_spawn_file_actions_t *restrict file_actions,
16 const posix_spawnattr_t *restrict attrp,
17 char *const argv[restrict],
18 char *const envp[restrict]);
19 int posix_spawnp(pid_t *restrict pid, const char *restrict file,
20 const posix_spawn_file_actions_t *restrict file_actions,
21 const posix_spawnattr_t *restrict attrp,
22 char *const argv[restrict],
23 char *const envp[restrict]);
24
26 The posix_spawn() and posix_spawnp() functions are used to create a new
27 child process that executes a specified file. These functions were
28 specified by POSIX to provide a standardized method of creating new
29 processes on machines that lack the capability to support the fork(2)
30 system call. These machines are generally small, embedded systems
31 lacking MMU support.
32 The posix_spawn() and posix_spawnp() functions provide the functional‐
34 ity of a combined fork(2) and exec(3), with some optional housekeeping
35 steps in the child process before the exec(3). These functions are not
36 meant to replace the fork(2) and execve(2) system calls. In fact, they
37 provide only a subset of the functionality that can be achieved by us‐
38 ing the system calls.
39 The only difference between posix_spawn() and posix_spawnp() is the
41 manner in which they specify the file to be executed by the child
42 process. With posix_spawn(), the executable file is specified as a
43 pathname (which can be absolute or relative). With posix_spawnp(), the
44 executable file is specified as a simple filename; the system searches
45 for this file in the list of directories specified by PATH (in the same
46 way as for execvp(3)). For the remainder of this page, the discussion
47 is phrased in terms of posix_spawn(), with the understanding that
48 posix_spawnp() differs only on the point just described.
49 The remaining arguments to these two functions are as follows:
51 pid points to a buffer that is used to return the process ID of the
53 new child process.
54 file_actions
56 points to a spawn file actions object that specifies file-re‐
57 lated actions to be performed in the child between the fork(2)
58 and exec(3) steps. This object is initialized and populated be‐
59 fore the posix_spawn() call using posix_spawn_file_ac‐
60 tions_init(3) and the posix_spawn_file_actions_*() functions.
61 attrp points to an attributes objects that specifies various at‐
63 tributes of the created child process. This object is initial‐
64 ized and populated before the posix_spawn() call using
65 posix_spawnattr_init(3) and the posix_spawnattr_*() functions.
66 argv
68 envp specify the argument list and environment for the program that
69 is executed in the child process, as for execve(2).
70 Below, the functions are described in terms of a three-step process:
72 the fork() step, the pre-exec() step (executed in the child), and the
73 exec() step (executed in the child).
74 fork() step
76 Since glibc 2.24, the posix_spawn() function commences by calling
77 clone(2) with CLONE_VM and CLONE_VFORK flags. Older implementations
78 use fork(2), or possibly vfork(2) (see below).
79 The PID of the new child process is placed in *pid. The posix_spawn()
81 function then returns control to the parent process.
82 Subsequently, the parent can use one of the system calls described in
84 wait(2) to check the status of the child process. If the child fails
85 in any of the housekeeping steps described below, or fails to execute
86 the desired file, it exits with a status of 127.
87 Before glibc 2.24, the child process is created using vfork(2) instead
89 of fork(2) when either of the following is true:
90 • the spawn-flags element of the attributes object pointed to by attrp
92 contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or
93 • file_actions is NULL and the spawn-flags element of the attributes
95 object pointed to by attrp does not contain POSIX_SPAWN_SETSIGMASK,
96 POSIX_SPAWN_SETSIGDEF, POSIX_SPAWN_SETSCHEDPARAM,
97 POSIX_SPAWN_SETSCHEDULER, POSIX_SPAWN_SETPGROUP, or POSIX_SPAWN_RE‐
98 SETIDS.
99 In other words, vfork(2) is used if the caller requests it, or if there
101 is no cleanup expected in the child before it exec(3)s the requested
102 file.
103 pre-exec() step: housekeeping
105 In between the fork() and the exec() steps, a child process may need to
106 perform a set of housekeeping actions. The posix_spawn() and
107 posix_spawnp() functions support a small, well-defined set of system
108 tasks that the child process can accomplish before it executes the exe‐
109 cutable file. These operations are controlled by the attributes object
110 pointed to by attrp and the file actions object pointed to by file_ac‐
111 tions. In the child, processing is done in the following sequence:
112 (1) Process attribute actions: signal mask, signal default handlers,
114 scheduling algorithm and parameters, process group, and effective
115 user and group IDs are changed as specified by the attributes ob‐
116 ject pointed to by attrp.
117 (2) File actions, as specified in the file_actions argument, are per‐
119 formed in the order that they were specified using calls to the
120 posix_spawn_file_actions_add*() functions.
121 (3) File descriptors with the FD_CLOEXEC flag set are closed.
123 All process attributes in the child, other than those affected by at‐
125 tributes specified in the object pointed to by attrp and the file ac‐
126 tions in the object pointed to by file_actions, will be affected as
127 though the child was created with fork(2) and it executed the program
128 with execve(2).
129 The process attributes actions are defined by the attributes object
131 pointed to by attrp. The spawn-flags attribute (set using posix_spaw‐
132 nattr_setflags(3)) controls the general actions that occur, and other
133 attributes in the object specify values to be used during those ac‐
134 tions.
135 The effects of the flags that may be specified in spawn-flags are as
137 follows:
138 POSIX_SPAWN_SETSIGMASK
140 Set the signal mask to the signal set specified in the spawn-
141 sigmask attribute of the object pointed to by attrp. If the
142 POSIX_SPAWN_SETSIGMASK flag is not set, then the child inherits
143 the parent's signal mask.
144 POSIX_SPAWN_SETSIGDEF
146 Reset the disposition of all signals in the set specified in the
147 spawn-sigdefault attribute of the object pointed to by attrp to
148 the default. For the treatment of the dispositions of signals
149 not specified in the spawn-sigdefault attribute, or the treat‐
150 ment when POSIX_SPAWN_SETSIGDEF is not specified, see execve(2).
151 POSIX_SPAWN_SETSCHEDPARAM
153 If this flag is set, and the POSIX_SPAWN_SETSCHEDULER flag is
154 not set, then set the scheduling parameters to the parameters
155 specified in the spawn-schedparam attribute of the object
156 pointed to by attrp.
157 POSIX_SPAWN_SETSCHEDULER
159 Set the scheduling policy algorithm and parameters of the child,
160 as follows:
161 • The scheduling policy is set to the value specified in the
163 spawn-schedpolicy attribute of the object pointed to by at‐
164 trp.
165 • The scheduling parameters are set to the value specified in
167 the spawn-schedparam attribute of the object pointed to by
168 attrp (but see BUGS).
169 If the POSIX_SPAWN_SETSCHEDPARAM and POSIX_SPAWN_SETSCHEDPOLICY
171 flags are not specified, the child inherits the corresponding
172 scheduling attributes from the parent.
173 POSIX_SPAWN_RESETIDS
175 If this flag is set, reset the effective UID and GID to the real
176 UID and GID of the parent process. If this flag is not set,
177 then the child retains the effective UID and GID of the parent.
178 In either case, if the set-user-ID and set-group-ID permission
179 bits are enabled on the executable file, their effect will over‐
180 ride the setting of the effective UID and GID (se execve(2)).
181 POSIX_SPAWN_SETPGROUP
183 Set the process group to the value specified in the spawn-pgroup
184 attribute of the object pointed to by attrp. If the spawn-
185 pgroup attribute has the value 0, the child's process group ID
186 is made the same as its process ID. If the POSIX_SPAWN_SETP‐
187 GROUP flag is not set, the child inherits the parent's process
188 group ID.
189 POSIX_SPAWN_USEVFORK
191 Since glibc 2.24, this flag has no effect. On older implementa‐
192 tions, setting this flag forces the fork() step to use vfork(2)
193 instead of fork(2). The _GNU_SOURCE feature test macro must be
194 defined to obtain the definition of this constant.
195 POSIX_SPAWN_SETSID (since glibc 2.26)
197 If this flag is set, the child process shall create a new ses‐
198 sion and become the session leader. The child process shall
199 also become the process group leader of the new process group in
200 the session (see setsid(2)). The _GNU_SOURCE feature test macro
201 must be defined to obtain the definition of this constant.
202 If attrp is NULL, then the default behaviors described above for each
204 flag apply.
205 The file_actions argument specifies a sequence of file operations that
207 are performed in the child process after the general processing de‐
208 scribed above, and before it performs the exec(3). If file_actions is
209 NULL, then no special action is taken, and standard exec(3) semantics
210 apply—file descriptors open before the exec remain open in the new
211 process, except those for which the FD_CLOEXEC flag has been set. File
212 locks remain in place.
213 If file_actions is not NULL, then it contains an ordered set of re‐
215 quests to open(2), close(2), and dup2(2) files. These requests are
216 added to the file_actions by posix_spawn_file_actions_addopen(3),
217 posix_spawn_file_actions_addclose(3), and posix_spawn_file_actions_ad‐
218 ddup2(3). The requested operations are performed in the order they
219 were added to file_actions.
220 If any of the housekeeping actions fails (due to bogus values being
222 passed or other reasons why signal handling, process scheduling,
223 process group ID functions, and file descriptor operations might fail),
224 the child process exits with exit value 127.
225 exec() step
227 Once the child has successfully forked and performed all requested pre-
228 exec steps, the child runs the requested executable.
229 The child process takes its environment from the envp argument, which
231 is interpreted as if it had been passed to execve(2). The arguments to
232 the created process come from the argv argument, which is processed as
233 for execve(2).
234
236 Upon successful completion, posix_spawn() and posix_spawnp() place the
237 PID of the child process in pid, and return 0. If there is an error
238 during the fork() step, then no child is created, the contents of *pid
239 are unspecified, and these functions return an error number as de‐
240 scribed below.
241 Even when these functions return a success status, the child process
243 may still fail for a plethora of reasons related to its pre-exec() ini‐
244 tialization. In addition, the exec(3) may fail. In all of these
245 cases, the child process will exit with the exit value of 127.
246
248 The posix_spawn() and posix_spawnp() functions fail only in the case
249 where the underlying fork(2), vfork(2), or clone(2) call fails; in
250 these cases, these functions return an error number, which will be one
251 of the errors described for fork(2), vfork(2), or clone(2).
252 In addition, these functions fail if:
254 ENOSYS Function not supported on this system.
256
258 POSIX.1-2008.
259
261 glibc 2.2. POSIX.1-2001.
262
264 The housekeeping activities in the child are controlled by the objects
265 pointed to by attrp (for non-file actions) and file_actions In POSIX
266 parlance, the posix_spawnattr_t and posix_spawn_file_actions_t data
267 types are referred to as objects, and their elements are not specified
268 by name. Portable programs should initialize these objects using only
269 the POSIX-specified functions. (In other words, although these objects
270 may be implemented as structures containing fields, portable programs
271 must avoid dependence on such implementation details.)
272 According to POSIX, it is unspecified whether fork handlers established
274 with pthread_atfork(3) are called when posix_spawn() is invoked. Since
275 glibc 2.24, the fork handlers are not executed in any case. On older
276 implementations, fork handlers are called only if the child is created
277 using fork(2).
278 There is no "posix_fspawn" function (i.e., a function that is to
280 posix_spawn() as fexecve(3) is to execve(2)). However, this function‐
281 ality can be obtained by specifying the path argument as one of the
282 files in the caller's /proc/self/fd directory.
283
285 POSIX.1 says that when POSIX_SPAWN_SETSCHEDULER is specified in spawn-
286 flags, then the POSIX_SPAWN_SETSCHEDPARAM (if present) is ignored.
287 However, before glibc 2.14, calls to posix_spawn() failed with an error
288 if POSIX_SPAWN_SETSCHEDULER was specified without also specifying
289 POSIX_SPAWN_SETSCHEDPARAM.
290
292 The program below demonstrates the use of various functions in the
293 POSIX spawn API. The program accepts command-line attributes that can
294 be used to create file actions and attributes objects. The remaining
295 command-line arguments are used as the executable name and command-line
296 arguments of the program that is executed in the child.
297 In the first run, the date(1) command is executed in the child, and the
299 posix_spawn() call employs no file actions or attributes objects.
300 $ ./a.out date
302 PID of child: 7634
303 Tue Feb 1 19:47:50 CEST 2011
304 Child status: exited, status=0
305 In the next run, the -c command-line option is used to create a file
307 actions object that closes standard output in the child. Consequently,
308 date(1) fails when trying to perform output and exits with a status of
309 1.
310 $ ./a.out -c date
312 PID of child: 7636
313 date: write error: Bad file descriptor
314 Child status: exited, status=1
315 In the next run, the -s command-line option is used to create an at‐
317 tributes object that specifies that all (blockable) signals in the
318 child should be blocked. Consequently, trying to kill child with the
319 default signal sent by kill(1) (i.e., SIGTERM) fails, because that sig‐
320 nal is blocked. Therefore, to kill the child, SIGKILL is necessary
321 (SIGKILL can't be blocked).
322 $ ./a.out -s sleep 60 &
324 [1] 7637
325 $ PID of child: 7638
326 $ kill 7638
328 $ kill -KILL 7638
329 $ Child status: killed by signal 9
330 [1]+ Done ./a.out -s sleep 60
331 When we try to execute a nonexistent command in the child, the exec(3)
333 fails and the child exits with a status of 127.
334 $ ./a.out xxxxx
336 PID of child: 10190
337 Child status: exited, status=127
338 Program source
340 #include <errno.h>
342 #include <spawn.h>
343 #include <stdint.h>
344 #include <stdio.h>
345 #include <stdlib.h>
346 #include <string.h>
347 #include <unistd.h>
348 #include <wait.h>
349 #define errExit(msg) do { perror(msg); \
351 exit(EXIT_FAILURE); } while (0)
352 #define errExitEN(en, msg) \
354 do { errno = en; perror(msg); \
355 exit(EXIT_FAILURE); } while (0)
356 char **environ;
358 int
360 main(int argc, char *argv[])
361 {
362 pid_t child_pid;
363 int s, opt, status;
364 sigset_t mask;
365 posix_spawnattr_t attr;
366 posix_spawnattr_t *attrp;
367 posix_spawn_file_actions_t file_actions;
368 posix_spawn_file_actions_t *file_actionsp;
369 /* Parse command-line options, which can be used to specify an
371 attributes object and file actions object for the child. */
372 attrp = NULL;
374 file_actionsp = NULL;
375 while ((opt = getopt(argc, argv, "sc")) != -1) {
377 switch (opt) {
378 case 'c': /* -c: close standard output in child */
379 /* Create a file actions object and add a "close"
381 action to it. */
382 s = posix_spawn_file_actions_init(&file_actions);
384 if (s != 0)
385 errExitEN(s, "posix_spawn_file_actions_init");
386 s = posix_spawn_file_actions_addclose(&file_actions,
388 STDOUT_FILENO);
389 if (s != 0)
390 errExitEN(s, "posix_spawn_file_actions_addclose");
391 file_actionsp = &file_actions;
393 break;
394 case 's': /* -s: block all signals in child */
396 /* Create an attributes object and add a "set signal mask"
398 action to it. */
399 s = posix_spawnattr_init(&attr);
401 if (s != 0)
402 errExitEN(s, "posix_spawnattr_init");
403 s = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK);
404 if (s != 0)
405 errExitEN(s, "posix_spawnattr_setflags");
406 sigfillset(&mask);
408 s = posix_spawnattr_setsigmask(&attr, &mask);
409 if (s != 0)
410 errExitEN(s, "posix_spawnattr_setsigmask");
411 attrp = &attr;
413 break;
414 }
415 }
416 /* Spawn the child. The name of the program to execute and the
418 command-line arguments are taken from the command-line arguments
419 of this program. The environment of the program execed in the
420 child is made the same as the parent's environment. */
421 s = posix_spawnp(&child_pid, argv[optind], file_actionsp, attrp,
423 &argv[optind], environ);
424 if (s != 0)
425 errExitEN(s, "posix_spawn");
426 /* Destroy any objects that we created earlier. */
428 if (attrp != NULL) {
430 s = posix_spawnattr_destroy(attrp);
431 if (s != 0)
432 errExitEN(s, "posix_spawnattr_destroy");
433 }
434 if (file_actionsp != NULL) {
436 s = posix_spawn_file_actions_destroy(file_actionsp);
437 if (s != 0)
438 errExitEN(s, "posix_spawn_file_actions_destroy");
439 }
440 printf("PID of child: %jd\n", (intmax_t) child_pid);
442 /* Monitor status of the child until it terminates. */
444 do {
446 s = waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
447 if (s == -1)
448 errExit("waitpid");
449 printf("Child status: ");
451 if (WIFEXITED(status)) {
452 printf("exited, status=%d\n", WEXITSTATUS(status));
453 } else if (WIFSIGNALED(status)) {
454 printf("killed by signal %d\n", WTERMSIG(status));
455 } else if (WIFSTOPPED(status)) {
456 printf("stopped by signal %d\n", WSTOPSIG(status));
457 } else if (WIFCONTINUED(status)) {
458 printf("continued\n");
459 }
460 } while (!WIFEXITED(status) && !WIFSIGNALED(status));
461 exit(EXIT_SUCCESS);
463 }
464
466 close(2), dup2(2), execl(2), execlp(2), fork(2), open(2),
467 sched_setparam(2), sched_setscheduler(2), setpgid(2), setuid(2),
468 sigaction(2), sigprocmask(2), posix_spawn_file_actions_addclose(3),
469 posix_spawn_file_actions_adddup2(3),
470 posix_spawn_file_actions_addopen(3),
471 posix_spawn_file_actions_destroy(3), posix_spawn_file_actions_init(3),
472 posix_spawnattr_destroy(3), posix_spawnattr_getflags(3),
473 posix_spawnattr_getpgroup(3), posix_spawnattr_getschedparam(3),
474 posix_spawnattr_getschedpolicy(3), posix_spawnattr_getsigdefault(3),
475 posix_spawnattr_getsigmask(3), posix_spawnattr_init(3),
476 posix_spawnattr_setflags(3), posix_spawnattr_setpgroup(3),
477 posix_spawnattr_setschedparam(3), posix_spawnattr_setschedpolicy(3),
478 posix_spawnattr_setsigdefault(3), posix_spawnattr_setsigmask(3),
479 pthread_atfork(3), <spawn.h>, Base Definitions volume of POSIX.1-2001,
480 http://www.opengroup.org/unix/online.html
481Linux man-pages 6.04 2023-03-30 posix_spawn(3)