1POSIX_SPAWN(3) Linux Programmer's Manual POSIX_SPAWN(3)
2
6 posix_spawn, posix_spawnp - spawn a process
7
9 #include <spawn.h>
10 int posix_spawn(pid_t *pid, const char *path,
12 const posix_spawn_file_actions_t *file_actions,
13 const posix_spawnattr_t *attrp,
14 char *const argv[], char *const envp[]);
15 int posix_spawnp(pid_t *pid, const char *file,
17 const posix_spawn_file_actions_t *file_actions,
18 const posix_spawnattr_t *attrp,
19 char *const argv[], char *const envp[]);
20
22 The posix_spawn() and posix_spawnp() functions are used to create a new
23 child process that executes a specified file. These functions were
24 specified by POSIX to provide a standardized method of creating new
25 processes on machines that lack the capability to support the fork(2)
26 system call. These machines are generally small, embedded systems
27 lacking MMU support.
28 The posix_spawn() and posix_spawnp() functions provide the functional‐
30 ity of a combined fork(2) and exec(3), with some optional housekeeping
31 steps in the child process before the exec(3). These functions are not
32 meant to replace the fork(2) and execve(2) system calls. In fact, they
33 provide only a subset of the functionality that can be achieved by
34 using the system calls.
35 The only difference between posix_spawn() and posix_spawnp() is the
37 manner in which they specify the file to be executed by the child
38 process. With posix_spawn(), the executable file is specified as a
39 pathname (which can be absolute or relative). With posix_spawnp(), the
40 executable file is specified as a simple filename; the system searches
41 for this file in the list of directories specified by PATH (in the same
42 way as for execvp(3)). For the remainder of this page, the discussion
43 is phrased in terms of posix_spawn(), with the understanding that
44 posix_spawnp() differs only on the point just described.
45 The remaining arguments to these two functions are as follows:
47 * The pid argument points to a buffer that is used to return the
49 process ID of the new child process.
50 * The file_actions argument points to a spawn file actions object that
52 specifies file-related actions to be performed in the child between
53 the fork(2) and exec(3) steps. This object is initialized and popu‐
54 lated before the posix_spawn() call using
55 posix_spawn_file_actions_init(3) and the
56 posix_spawn_file_actions_*() functions.
57 * The attrp argument points to an attributes objects that specifies
59 various attributes of the created child process. This object is
60 initialized and populated before the posix_spawn() call using
61 posix_spawnattr_init(3) and the posix_spawnattr_*() functions.
62 * The argv and envp arguments specify the argument list and environ‐
64 ment for the program that is executed in the child process, as for
65 execve(2).
66 Below, the functions are described in terms of a three-step process:
68 the fork() step, the pre-exec() step (executed in the child), and the
69 exec() step (executed in the child).
70 fork() step
72 Since glibc 2.24, the posix_spawn() function commences by calling
73 clone(2) with CLONE_VM and CLONE_VFORK flags. Older implementations
74 use fork(2), or possibly vfork(2) (see below).
75 The PID of the new child process is placed in *pid. The posix_spawn()
77 function then returns control to the parent process.
78 Subsequently, the parent can use one of the system calls described in
80 wait(2) to check the status of the child process. If the child fails
81 in any of the housekeeping steps described below, or fails to execute
82 the desired file, it exits with a status of 127.
83 Before glibc 2.24, the child process is created using vfork(2) instead
85 of fork(2) when either of the following is true:
86 * the spawn-flags element of the attributes object pointed to by attrp
88 contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or
89 * file_actions is NULL and the spawn-flags element of the attributes
91 object pointed to by attrp does not contain POSIX_SPAWN_SETSIGMASK,
92 POSIX_SPAWN_SETSIGDEF, POSIX_SPAWN_SETSCHEDPARAM,
93 POSIX_SPAWN_SETSCHEDULER, POSIX_SPAWN_SETPGROUP, or
94 POSIX_SPAWN_RESETIDS.
95 In other words, vfork(2) is used if the caller requests it, or if there
97 is no cleanup expected in the child before it exec(3)s the requested
98 file.
99 pre-exec() step: housekeeping
101 In between the fork() and the exec() steps, a child process may need to
102 perform a set of housekeeping actions. The posix_spawn() and
103 posix_spawnp() functions support a small, well-defined set of system
104 tasks that the child process can accomplish before it executes the exe‐
105 cutable file. These operations are controlled by the attributes object
106 pointed to by attrp and the file actions object pointed to by
107 file_actions. In the child, processing is done in the following
108 sequence:
109 1. Process attribute actions: signal mask, signal default handlers,
111 scheduling algorithm and parameters, process group, and effective
112 user and group IDs are changed as specified by the attributes object
113 pointed to by attrp.
114 2. File actions, as specified in the file_actions argument, are per‐
116 formed in the order that they were specified using calls to the
117 posix_spawn_file_actions_add*() functions.
118 3. File descriptors with the FD_CLOEXEC flag set are closed.
120 All process attributes in the child, other than those affected by
122 attributes specified in the object pointed to by attrp and the file
123 actions in the object pointed to by file_actions, will be affected as
124 though the child was created with fork(2) and it executed the program
125 with execve(2).
126 The process attributes actions are defined by the attributes object
128 pointed to by attrp. The spawn-flags attribute (set using posix_spaw‐
129 nattr_setflags(3)) controls the general actions that occur, and other
130 attributes in the object specify values to be used during those
131 actions.
132 The effects of the flags that may be specified in spawn-flags are as
134 follows:
135 POSIX_SPAWN_SETSIGMASK
137 Set the signal mask to the signal set specified in the spawn-
138 sigmask attribute of the object pointed to by attrp. If the
139 POSIX_SPAWN_SETSIGMASK flag is not set, then the child inherits
140 the parent's signal mask.
141 POSIX_SPAWN_SETSIGDEF
143 Reset the disposition of all signals in the set specified in the
144 spawn-sigdefault attribute of the object pointed to by attrp to
145 the default. For the treatment of the dispositions of signals
146 not specified in the spawn-sigdefault attribute, or the treat‐
147 ment when POSIX_SPAWN_SETSIGDEF is not specified, see execve(2).
148 POSIX_SPAWN_SETSCHEDPARAM
150 If this flag is set, and the POSIX_SPAWN_SETSCHEDULER flag is
151 not set, then set the scheduling parameters to the parameters
152 specified in the spawn-schedparam attribute of the object
153 pointed to by attrp.
154 POSIX_SPAWN_SETSCHEDULER
156 Set the scheduling policy algorithm and parameters of the child,
157 as follows:
158 * The scheduling policy is set to the value specified in the
160 spawn-schedpolicy attribute of the object pointed to by
161 attrp.
162 * The scheduling parameters are set to the value specified in
164 the spawn-schedparam attribute of the object pointed to by
165 attrp (but see BUGS).
166 If the POSIX_SPAWN_SETSCHEDPARAM and POSIX_SPAWN_SETSCHEDPOLICY
168 flags are not specified, the child inherits the corresponding
169 scheduling attributes from the parent.
170 POSIX_SPAWN_RESETIDS
172 If this flag is set, reset the effective UID and GID to the real
173 UID and GID of the parent process. If this flag is not set,
174 then the child retains the effective UID and GID of the parent.
175 In either case, if the set-user-ID and set-group-ID permission
176 bits are enabled on the executable file, their effect will over‐
177 ride the setting of the effective UID and GID (se execve(2)).
178 POSIX_SPAWN_SETPGROUP
180 Set the process group to the value specified in the spawn-pgroup
181 attribute of the object pointed to by attrp. If the spawn-
182 pgroup attribute has the value 0, the child's process group ID
183 is made the same as its process ID. If the POSIX_SPAWN_SETP‐
184 GROUP flag is not set, the child inherits the parent's process
185 group ID.
186 POSIX_SPAWN_USEVFORK
188 Since glibc 2.24, this flag has no effect. On older implementa‐
189 tions, setting this flag forces the fork() step to use vfork(2)
190 instead of fork(2). The _GNU_SOURCE feature test macro must be
191 defined to obtain the definition of this constant.
192 POSIX_SPAWN_SETSID (since glibc 2.26)
194 If this flag is set, the child process shall create a new ses‐
195 sion and become the session leader. The child process shall
196 also become the process group leader of the new process group in
197 the session (see setsid(2)). The _GNU_SOURCE feature test macro
198 must be defined to obtain the definition of this constant.
199 If attrp is NULL, then the default behaviors described above for each
201 flag apply.
202 The file_actions argument specifies a sequence of file operations that
204 are performed in the child process after the general processing
205 described above, and before it performs the exec(3). If file_actions
206 is NULL, then no special action is taken, and standard exec(3) seman‐
207 tics apply--file descriptors open before the exec remain open in the
208 new process, except those for which the FD_CLOEXEC flag has been set.
209 File locks remain in place.
210 If file_actions is not NULL, then it contains an ordered set of
212 requests to open(2), close(2), and dup2(2) files. These requests are
213 added to the file_actions by posix_spawn_file_actions_addopen(3),
214 posix_spawn_file_actions_addclose(3), and
215 posix_spawn_file_actions_adddup2(3). The requested operations are per‐
216 formed in the order they were added to file_actions.
217 If any of the housekeeping actions fails (due to bogus values being
219 passed or other reasons why signal handling, process scheduling,
220 process group ID functions, and file descriptor operations might fail),
221 the child process exits with exit value 127.
222 exec() step
224 Once the child has successfully forked and performed all requested pre-
225 exec steps, the child runs the requested executable.
226 The child process takes its environment from the envp argument, which
228 is interpreted as if it had been passed to execve(2). The arguments to
229 the created process come from the argv argument, which is processed as
230 for execve(2).
231
233 Upon successful completion, posix_spawn() and posix_spawnp() place the
234 PID of the child process in pid, and return 0. If there is an error
235 during the fork() step, then no child is created, the contents of *pid
236 are unspecified, and these functions return an error number as
237 described below.
238 Even when these functions return a success status, the child process
240 may still fail for a plethora of reasons related to its pre-exec() ini‐
241 tialization. In addition, the exec(3) may fail. In all of these
242 cases, the child process will exit with the exit value of 127.
243
245 The posix_spawn() and posix_spawnp() functions fail only in the case
246 where the underlying fork(2), vfork(2) or clone(2) call fails; in
247 these cases, these functions return an error number, which will be one
248 of the errors described for fork(2), vfork(2) or clone(2).
249 In addition, these functions fail if:
251 ENOSYS Function not supported on this system.
253
255 The posix_spawn() and posix_spawnp() functions are available since
256 glibc 2.2.
257
259 POSIX.1-2001, POSIX.1-2008.
260
262 The housekeeping activities in the child are controlled by the objects
263 pointed to by attrp (for non-file actions) and file_actions In POSIX
264 parlance, the posix_spawnattr_t and posix_spawn_file_actions_t data
265 types are referred to as objects, and their elements are not specified
266 by name. Portable programs should initialize these objects using only
267 the POSIX-specified functions. (In other words, although these objects
268 may be implemented as structures containing fields, portable programs
269 must avoid dependence on such implementation details.)
270 According to POSIX, it is unspecified whether fork handlers established
272 with pthread_atfork(3) are called when posix_spawn() is invoked. Since
273 glibc 2.24, the fork handlers are not executed in any case. On older
274 implementations, fork handlers are called only if the child is created
275 using fork(2).
276 There is no "posix_fspawn" function (i.e., a function that is to
278 posix_spawn() as fexecve(3) is to execve(2)). However, this function‐
279 ality can be obtained by specifying the path argument as one of the
280 files in the caller's /proc/self/fd directory.
281
283 POSIX.1 says that when POSIX_SPAWN_SETSCHEDULER is specified in spawn-
284 flags, then the POSIX_SPAWN_SETSCHEDPARAM (if present) is ignored.
285 However, before glibc 2.14, calls to posix_spawn() failed with an error
286 if POSIX_SPAWN_SETSCHEDULER was specified without also specifying
287 POSIX_SPAWN_SETSCHEDPARAM.
288
290 The program below demonstrates the use of various functions in the
291 POSIX spawn API. The program accepts command-line attributes that can
292 be used to create file actions and attributes objects. The remaining
293 command-line arguments are used as the executable name and command-line
294 arguments of the program that is executed in the child.
295 In the first run, the date(1) command is executed in the child, and the
297 posix_spawn() call employs no file actions or attributes objects.
298 $ ./a.out date
300 PID of child: 7634
301 Tue Feb 1 19:47:50 CEST 2011
302 Child status: exited, status=0
303 In the next run, the -c command-line option is used to create a file
305 actions object that closes standard output in the child. Consequently,
306 date(1) fails when trying to perform output and exits with a status of
307 1.
308 $ ./a.out -c date
310 PID of child: 7636
311 date: write error: Bad file descriptor
312 Child status: exited, status=1
313 In the next run, the -s command-line option is used to create an
315 attributes object that specifies that all (blockable) signals in the
316 child should be blocked. Consequently, trying to kill child with the
317 default signal sent by kill(1) (i.e., SIGTERM) fails, because that sig‐
318 nal is blocked. Therefore, to kill the child, SIGKILL is necessary
319 (SIGKILL can't be blocked).
320 $ ./a.out -s sleep 60 &
322 [1] 7637
323 $ PID of child: 7638
324 $ kill 7638
326 $ kill -KILL 7638
327 $ Child status: killed by signal 9
328 [1]+ Done ./a.out -s sleep 60
329 When we try to execute a nonexistent command in the child, the exec(3)
331 fails and the child exits with a status of 127.
332 $ ./a.out xxxxx
334 PID of child: 10190
335 Child status: exited, status=127
336 Program source
338 #include <spawn.h>
340 #include <stdio.h>
341 #include <unistd.h>
342 #include <stdlib.h>
343 #include <string.h>
344 #include <wait.h>
345 #include <errno.h>
346 #define errExit(msg) do { perror(msg); \
348 exit(EXIT_FAILURE); } while (0)
349 #define errExitEN(en, msg) \
351 do { errno = en; perror(msg); \
352 exit(EXIT_FAILURE); } while (0)
353 char **environ;
355 int
357 main(int argc, char *argv[])
358 {
359 pid_t child_pid;
360 int s, opt, status;
361 sigset_t mask;
362 posix_spawnattr_t attr;
363 posix_spawnattr_t *attrp;
364 posix_spawn_file_actions_t file_actions;
365 posix_spawn_file_actions_t *file_actionsp;
366 /* Parse command-line options, which can be used to specify an
368 attributes object and file actions object for the child. */
369 attrp = NULL;
371 file_actionsp = NULL;
372 while ((opt = getopt(argc, argv, "sc")) != -1) {
374 switch (opt) {
375 case 'c': /* -c: close standard output in child */
376 /* Create a file actions object and add a "close"
378 action to it */
379 s = posix_spawn_file_actions_init(&file_actions);
381 if (s != 0)
382 errExitEN(s, "posix_spawn_file_actions_init");
383 s = posix_spawn_file_actions_addclose(&file_actions,
385 STDOUT_FILENO);
386 if (s != 0)
387 errExitEN(s, "posix_spawn_file_actions_addclose");
388 file_actionsp = &file_actions;
390 break;
391 case 's': /* -s: block all signals in child */
393 /* Create an attributes object and add a "set signal mask"
395 action to it */
396 s = posix_spawnattr_init(&attr);
398 if (s != 0)
399 errExitEN(s, "posix_spawnattr_init");
400 s = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK);
401 if (s != 0)
402 errExitEN(s, "posix_spawnattr_setflags");
403 sigfillset(&mask);
405 s = posix_spawnattr_setsigmask(&attr, &mask);
406 if (s != 0)
407 errExitEN(s, "posix_spawnattr_setsigmask");
408 attrp = &attr;
410 break;
411 }
412 }
413 /* Spawn the child. The name of the program to execute and the
415 command-line arguments are taken from the command-line arguments
416 of this program. The environment of the program execed in the
417 child is made the same as the parent's environment. */
418 s = posix_spawnp(&child_pid, argv[optind], file_actionsp, attrp,
420 &argv[optind], environ);
421 if (s != 0)
422 errExitEN(s, "posix_spawn");
423 /* Destroy any objects that we created earlier */
425 if (attrp != NULL) {
427 s = posix_spawnattr_destroy(attrp);
428 if (s != 0)
429 errExitEN(s, "posix_spawnattr_destroy");
430 }
431 if (file_actionsp != NULL) {
433 s = posix_spawn_file_actions_destroy(file_actionsp);
434 if (s != 0)
435 errExitEN(s, "posix_spawn_file_actions_destroy");
436 }
437 printf("PID of child: %ld\n", (long) child_pid);
439 /* Monitor status of the child until it terminates */
441 do {
443 s = waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
444 if (s == -1)
445 errExit("waitpid");
446 printf("Child status: ");
448 if (WIFEXITED(status)) {
449 printf("exited, status=%d\n", WEXITSTATUS(status));
450 } else if (WIFSIGNALED(status)) {
451 printf("killed by signal %d\n", WTERMSIG(status));
452 } else if (WIFSTOPPED(status)) {
453 printf("stopped by signal %d\n", WSTOPSIG(status));
454 } else if (WIFCONTINUED(status)) {
455 printf("continued\n");
456 }
457 } while (!WIFEXITED(status) && !WIFSIGNALED(status));
458 exit(EXIT_SUCCESS);
460 }
461
463 close(2), dup2(2), execl(2), execlp(2), fork(2), open(2),
464 sched_setparam(2), sched_setscheduler(2), setpgid(2), setuid(2),
465 sigaction(2), sigprocmask(2), posix_spawn_file_actions_addclose(3),
466 posix_spawn_file_actions_adddup2(3),
467 posix_spawn_file_actions_addopen(3),
468 posix_spawn_file_actions_destroy(3), posix_spawn_file_actions_init(3),
469 posix_spawnattr_destroy(3), posix_spawnattr_getflags(3),
470 posix_spawnattr_getpgroup(3), posix_spawnattr_getschedparam(3),
471 posix_spawnattr_getschedpolicy(3), posix_spawnattr_getsigdefault(3),
472 posix_spawnattr_getsigmask(3), posix_spawnattr_init(3),
473 posix_spawnattr_setflags(3), posix_spawnattr_setpgroup(3),
474 posix_spawnattr_setschedparam(3), posix_spawnattr_setschedpolicy(3),
475 posix_spawnattr_setsigdefault(3), posix_spawnattr_setsigmask(3),
476 pthread_atfork(3), <spawn.h>, Base Definitions volume of POSIX.1-2001,
477 http://www.opengroup.org/unix/online.html
478
480 This page is part of release 5.07 of the Linux man-pages project. A
481 description of the project, information about reporting bugs, and the
482 latest version of this page, can be found at
483 https://www.kernel.org/doc/man-pages/.
484GNU 2020-04-11 POSIX_SPAWN(3)