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 The posix_spawn() function commences by calling fork(2), or possibly
73 vfork(2) (see below).
74 The PID of the new child process is placed in *pid. The posix_spawn()
76 function then returns control to the parent process.
77 Subsequently, the parent can use one of the system calls described in
79 wait(2) to check the status of the child process. If the child fails
80 in any of the housekeeping steps described below, or fails to execute
81 the desired file, it exits with a status of 127.
82 The child process is created using vfork(2) instead of fork(2) when
84 either of the following is true:
85 * the spawn-flags element of the attributes object pointed to by attrp
87 contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or
88 * file_actions is NULL and the spawn-flags element of the attributes
90 object pointed to by attrp does not contain POSIX_SPAWN_SETSIGMASK,
91 POSIX_SPAWN_SETSIGDEF, POSIX_SPAWN_SETSCHEDPARAM,
92 POSIX_SPAWN_SETSCHEDULER, POSIX_SPAWN_SETPGROUP, or
93 POSIX_SPAWN_RESETIDS.
94 In other words, vfork(2) is used if the caller requests it, or if there
96 is no cleanup expected in the child before it exec(3)s the requested
97 file.
98 pre-exec() step: housekeeping
100 In between the fork(2) and the exec(3), a child process may need to
101 perform a set of housekeeping actions. The posix_spawn() and
102 posix_spawnp() functions support a small, well-defined set of system
103 tasks that the child process can accomplish before it executes the exe‐
104 cutable file. These operations are controlled by the attributes object
105 pointed to by attrp and the file actions object pointed to by
106 file_actions. In the child, processing is done in the following
107 sequence:
108 1. Process attribute actions: signal mask, signal default handlers,
110 scheduling algorithm and parameters, process group, and effective
111 user and group IDs are changed as specified by the attributes object
112 pointed to by attrp.
113 2. File actions, as specified in the file_actions argument, are per‐
115 formed in the order that they were specified using calls to the
116 posix_spawn_file_actions_add*() functions.
117 3. File descriptors with the FD_CLOEXEC flag set are closed.
119 All process attributes in the child, other than those affected by
121 attributes specified in the object pointed to by attrp and the file
122 actions in the object pointed to by file_actions, will be affected as
123 though the child was created with fork(2) and it executed the program
124 with execve(2).
125 The process attributes actions are defined by the attributes object
127 pointed to by attrp. The spawn-flags attribute (set using posix_spaw‐
128 nattr_setflags(3)) controls the general actions that occur, and other
129 attributes in the object specify values to be used during those
130 actions.
131 The effects of the flags that may be specified in spawn-flags are as
133 follows:
134 POSIX_SPAWN_SETSIGMASK
136 Set the signal mask to the signal set specified in the spawn-
137 sigmask attribute of the object pointed to by attrp. If the
138 POSIX_SPAWN_SETSIGMASK flag is not set, then the child inherits
139 the parent's signal mask.
140 POSIX_SPAWN_SETSIGDEF
142 Reset the disposition of all signals in the set specified in
143 the spawn-sigdefault attribute of the object pointed to by
144 attrp to the default. For the treatment of the dispositions of
145 signals not specified in the spawn-sigdefault attribute, or the
146 treatment when POSIX_SPAWN_SETSIGDEF is not specified, see
147 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
157 child, 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
173 real UID and GID of the parent process. If this flag is not
174 set, then the child retains the effective UID and GID of the
175 parent. In either case, if the set-user-ID and set-group-ID
176 permission bits are enabled on the executable file, their
177 effect will override the setting of the effective UID and GID
178 (se execve(2)).
179 POSIX_SPAWN_SETPGROUP
181 Set the process group to the value specified in the spawn-
182 pgroup attribute of the object pointed to by attrp. If the
183 spawn-pgroup attribute has the value 0, the child's process
184 group ID is made the same as its process ID. If the
185 POSIX_SPAWN_SETPGROUP flag is not set, the child inherits the
186 parent's process group ID.
187 If attrp is NULL, then the default behaviors described above for each
189 flag apply.
190 The file_actions argument specifies a sequence of file operations that
192 are performed in the child process after the general processing
193 described above, and before it performs the exec(3). If file_actions
194 is NULL, then no special action is taken, and standard exec(3) seman‐
195 tics apply--file descriptors open before the exec remain open in the
196 new process, except those for which the FD_CLOEXEC flag has been set.
197 File locks remain in place.
198 If file_actions is not NULL, then it contains an ordered set of
200 requests to open(2), close(2), and dup2(2) files. These requests are
201 added to the file_actions by posix_spawn_file_actions_addopen(3),
202 posix_spawn_file_actions_addclose(3), and
203 posix_spawn_file_actions_adddup2(3). The requested operations are per‐
204 formed in the order they were added to file_actions.
205 If any of the housekeeping actions fails (due to bogus values being
207 passed or other reasons why signal handling, process scheduling,
208 process group ID functions, and file descriptor operations might fail),
209 the child process exits with exit value 127.
210 exec() step
212 Once the child has successfully forked and performed all requested pre-
213 exec steps, the child runs the requested executable.
214 The child process takes its environment from the envp argument, which
216 is interpreted as if it had been passed to execve(2). The arguments to
217 the created process come from the argv argument, which is processed as
218 for execve(2).
219
221 Upon successful completion, posix_spawn() and posix_spawnp() place the
222 PID of the child process in pid, and return 0. If there is an error
223 before or during the fork(2), then no child is created, the contents of
224 *pid are unspecified, and these functions return an error number as
225 described below.
226 Even when these functions return a success status, the child process
228 may still fail for a plethora of reasons related to its pre-exec() ini‐
229 tialization. In addition, the exec(3) may fail. In all of these
230 cases, the child process will exit with the exit value of 127.
231
233 The posix_spawn() and posix_spawnp() functions fail only in the case
234 where the underlying fork(2) or vfork(2) call fails; in these cases,
235 these functions return an error number, which will be one of the errors
236 described for fork(2) or vfork(2).
237 In addition, these functions fail if:
239 ENOSYS Function not supported on this system.
241
243 The posix_spawn() and posix_spawnp() functions are available since
244 glibc 2.2.
245
247 POSIX.1-2001, POSIX.1-2008.
248
250 The housekeeping activities in the child are controlled by the objects
251 pointed to by attrp (for non-file actions) and file_actions In POSIX
252 parlance, the posix_spawnattr_t and posix_spawn_file_actions_t data
253 types are referred to as objects, and their elements are not specified
254 by name. Portable programs should initialize these objects using only
255 the POSIX-specified functions. (In other words, although these objects
256 may be implemented as structures containing fields, portable programs
257 must avoid dependence on such implementation details.)
258 According to POSIX, it unspecified whether fork handlers established
260 with pthread_atfork(3) are called when posix_spawn() is invoked. On
261 glibc, fork handlers are called only if the child is created using
262 fork(2).
263 There is no "posix_fspawn" function (i.e., a function that is to
265 posix_spawn() as fexecve(3) is to execve(2)). However, this function‐
266 ality can be obtained by specifying the path argument as one of the
267 files in the caller's /proc/self/fd directory.
268
270 POSIX.1 says that when POSIX_SPAWN_SETSCHEDULER is specified in spawn-
271 flags, then the POSIX_SPAWN_SETSCHEDPARAM (if present) is ignored.
272 However, before glibc 2.14, calls to posix_spawn() failed with an error
273 if POSIX_SPAWN_SETSCHEDULER was specified without also specifying
274 POSIX_SPAWN_SETSCHEDPARAM.
275
277 The program below demonstrates the use of various functions in the
278 POSIX spawn API. The program accepts command-line attributes that can
279 be used to create file actions and attributes objects. The remaining
280 command-line arguments are used as the executable name and command-line
281 arguments of the program that is executed in the child.
282 In the first run, the date(1) command is executed in the child, and the
284 posix_spawn() call employs no file actions or attributes objects.
285 $ ./a.out date
287 PID of child: 7634
288 Tue Feb 1 19:47:50 CEST 2011
289 Child status: exited, status=0
290 In the next run, the -c command-line option is used to create a file
292 actions object that closes standard output in the child. Consequently,
293 date(1) fails when trying to perform output and exits with a status of
294 1.
295 $ ./a.out -c date
297 PID of child: 7636
298 date: write error: Bad file descriptor
299 Child status: exited, status=1
300 In the next run, the -s command-line option is used to create an
302 attributes object that specifies that all (blockable) signals in the
303 child should be blocked. Consequently, trying to kill child with the
304 default signal sent by kill(1) (i.e., SIGTERM) fails, because that sig‐
305 nal is blocked. Therefore, to kill the child, SIGKILL is necessary
306 (SIGKILL can't be blocked).
307 $ ./a.out -s sleep 60 &
309 [1] 7637
310 $ PID of child: 7638
311 $ kill 7638
313 $ kill -KILL 7638
314 $ Child status: killed by signal 9
315 [1]+ Done ./a.out -s sleep 60
316 When we try to execute a nonexistent command in the child, the exec(3)
318 fails and the child exits with a status of 127.
319 $ ./a.out xxxxx
321 PID of child: 10190
322 Child status: exited, status=127
323 Program source
325 #include <spawn.h>
327 #include <stdio.h>
328 #include <unistd.h>
329 #include <stdlib.h>
330 #include <string.h>
331 #include <wait.h>
332 #include <errno.h>
333 #define errExit(msg) do { perror(msg); \
335 exit(EXIT_FAILURE); } while (0)
336 #define errExitEN(en, msg) \
338 do { errno = en; perror(msg); \
339 exit(EXIT_FAILURE); } while (0)
340 char **environ;
342 int
344 main(int argc, char *argv[])
345 {
346 pid_t child_pid;
347 int s, opt, status;
348 sigset_t mask;
349 posix_spawnattr_t attr;
350 posix_spawnattr_t *attrp;
351 posix_spawn_file_actions_t file_actions;
352 posix_spawn_file_actions_t *file_actionsp;
353 /* Parse command-line options, which can be used to specify an
355 attributes object and file actions object for the child. */
356 attrp = NULL;
358 file_actionsp = NULL;
359 while ((opt = getopt(argc, argv, "sc")) != -1) {
361 switch (opt) {
362 case 'c': /* -c: close standard output in child */
363 /* Create a file actions object and add a "close"
365 action to it */
366 s = posix_spawn_file_actions_init(&file_actions);
368 if (s != 0)
369 errExitEN(s, "posix_spawn_file_actions_init");
370 s = posix_spawn_file_actions_addclose(&file_actions,
372 STDOUT_FILENO);
373 if (s != 0)
374 errExitEN(s, "posix_spawn_file_actions_addclose");
375 file_actionsp = &file_actions;
377 break;
378 case 's': /* -s: block all signals in child */
380 /* Create an attributes object and add a "set signal mask"
382 action to it */
383 s = posix_spawnattr_init(&attr);
385 if (s != 0)
386 errExitEN(s, "posix_spawnattr_init");
387 s = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK);
388 if (s != 0)
389 errExitEN(s, "posix_spawnattr_setflags");
390 sigfillset(&mask);
392 s = posix_spawnattr_setsigmask(&attr, &mask);
393 if (s != 0)
394 errExitEN(s, "posix_spawnattr_setsigmask");
395 attrp = &attr;
397 break;
398 }
399 }
400 /* Spawn the child. The name of the program to execute and the
402 command-line arguments are taken from the command-line arguments
403 of this program. The environment of the program execed in the
404 child is made the same as the parent's environment. */
405 s = posix_spawnp(&child_pid, argv[optind], file_actionsp, attrp,
407 &argv[optind], environ);
408 if (s != 0)
409 errExitEN(s, "posix_spawn");
410 /* Destroy any objects that we created earlier */
412 if (attrp != NULL) {
414 s = posix_spawnattr_destroy(attrp);
415 if (s != 0)
416 errExitEN(s, "posix_spawnattr_destroy");
417 }
418 if (file_actionsp != NULL) {
420 s = posix_spawn_file_actions_destroy(file_actionsp);
421 if (s != 0)
422 errExitEN(s, "posix_spawn_file_actions_destroy");
423 }
424 printf("PID of child: %ld\n", (long) child_pid);
426 /* Monitor status of the child until it terminates */
428 do {
430 s = waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
431 if (s == -1)
432 errExit("waitpid");
433 printf("Child status: ");
435 if (WIFEXITED(status)) {
436 printf("exited, status=%d\n", WEXITSTATUS(status));
437 } else if (WIFSIGNALED(status)) {
438 printf("killed by signal %d\n", WTERMSIG(status));
439 } else if (WIFSTOPPED(status)) {
440 printf("stopped by signal %d\n", WSTOPSIG(status));
441 } else if (WIFCONTINUED(status)) {
442 printf("continued\n");
443 }
444 } while (!WIFEXITED(status) && !WIFSIGNALED(status));
445 exit(EXIT_SUCCESS);
447 }
448
450 close(2), dup2(2), execl(2), execlp(2), fork(2), open(2),
451 sched_setparam(2), sched_setscheduler(2), setpgid(2), setuid(2),
452 sigaction(2), sigprocmask(2), posix_spawn_file_actions_addclose(3),
453 posix_spawn_file_actions_adddup2(3),
454 posix_spawn_file_actions_addopen(3),
455 posix_spawn_file_actions_destroy(3), posix_spawn_file_actions_init(3),
456 posix_spawnattr_destroy(3), posix_spawnattr_getflags(3),
457 posix_spawnattr_getpgroup(3), posix_spawnattr_getschedparam(3),
458 posix_spawnattr_getschedpolicy(3), posix_spawnattr_getsigdefault(3),
459 posix_spawnattr_getsigmask(3), posix_spawnattr_init(3),
460 posix_spawnattr_setflags(3), posix_spawnattr_setpgroup(3),
461 posix_spawnattr_setschedparam(3), posix_spawnattr_setschedpolicy(3),
462 posix_spawnattr_setsigdefault(3), posix_spawnattr_setsigmask(3),
463 pthread_atfork(3), <spawn.h>, Base Definitions volume of POSIX.1-2001,
464 http://www.opengroup.org/unix/online.html
465
467 This page is part of release 5.04 of the Linux man-pages project. A
468 description of the project, information about reporting bugs, and the
469 latest version of this page, can be found at
470 https://www.kernel.org/doc/man-pages/.
471GNU 2019-03-06 POSIX_SPAWN(3)