1WAIT(2) Linux Programmer's Manual WAIT(2)
2
6 wait, waitpid, waitid - wait for process to change state
7
9 #include <sys/types.h>
10 #include <sys/wait.h>
11 pid_t wait(int *status);
13 pid_t waitpid(pid_t pid, int *status, int options);
15 int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19 waitid(): _SVID_SOURCE || _XOPEN_SOURCE
21
23 All of these system calls are used to wait for state changes in a child
24 of the calling process, and obtain information about the child whose
25 state has changed. A state change is considered to be: the child ter‐
26 minated; the child was stopped by a signal; or the child was resumed by
27 a signal. In the case of a terminated child, performing a wait allows
28 the system to release the resources associated with the child; if a
29 wait is not performed, then the terminated child remains in a "zombie"
30 state (see NOTES below).
31 If a child has already changed state, then these calls return immedi‐
33 ately. Otherwise they block until either a child changes state or a
34 signal handler interrupts the call (assuming that system calls are not
35 automatically restarted using the SA_RESTART flag of sigaction(2)). In
36 the remainder of this page, a child whose state has changed and which
37 has not yet been waited upon by one of these system calls is termed
38 waitable.
39 wait() and waitpid()
41 The wait() system call suspends execution of the calling process until
42 one of its children terminates. The call wait(&status) is equivalent
43 to:
44 waitpid(-1, &status, 0);
46 The waitpid() system call suspends execution of the calling process
48 until a child specified by pid argument has changed state. By default,
49 waitpid() waits only for terminated children, but this behavior is mod‐
50 ifiable via the options argument, as described below.
51 The value of pid can be:
53 < -1 meaning wait for any child process whose process group ID is
55 equal to the absolute value of pid.
56 -1 meaning wait for any child process.
58 0 meaning wait for any child process whose process group ID is
60 equal to that of the calling process.
61 > 0 meaning wait for the child whose process ID is equal to the
63 value of pid.
64 The value of options is an OR of zero or more of the following con‐
66 stants:
67 WNOHANG return immediately if no child has exited.
69 WUNTRACED also return if a child has stopped (but not traced via
71 ptrace(2)). Status for traced children which have stopped
72 is provided even if this option is not specified.
73 WCONTINUED (since Linux 2.6.10)
75 also return if a stopped child has been resumed by delivery
76 of SIGCONT.
77 (For Linux-only options, see below.)
79 If status is not NULL, wait() and waitpid() store status information in
81 the int to which it points. This integer can be inspected with the
82 following macros (which take the integer itself as an argument, not a
83 pointer to it, as is done in wait() and waitpid()!):
84 WIFEXITED(status)
86 returns true if the child terminated normally, that is, by call‐
87 ing exit(3) or _exit(2), or by returning from main().
88 WEXITSTATUS(status)
90 returns the exit status of the child. This consists of the
91 least significant 8 bits of the status argument that the child
92 specified in a call to exit(3) or _exit(2) or as the argument
93 for a return statement in main(). This macro should only be
94 employed if WIFEXITED returned true.
95 WIFSIGNALED(status)
97 returns true if the child process was terminated by a signal.
98 WTERMSIG(status)
100 returns the number of the signal that caused the child process
101 to terminate. This macro should only be employed if WIFSIGNALED
102 returned true.
103 WCOREDUMP(status)
105 returns true if the child produced a core dump. This macro
106 should only be employed if WIFSIGNALED returned true. This
107 macro is not specified in POSIX.1-2001 and is not available on
108 some Unix implementations (e.g., AIX, SunOS). Only use this
109 enclosed in #ifdef WCOREDUMP ... #endif.
110 WIFSTOPPED(status)
112 returns true if the child process was stopped by delivery of a
113 signal; this is only possible if the call was done using WUN‐
114 TRACED or when the child is being traced (see ptrace(2)).
115 WSTOPSIG(status)
117 returns the number of the signal which caused the child to stop.
118 This macro should only be employed if WIFSTOPPED returned true.
119 WIFCONTINUED(status)
121 (since Linux 2.6.10) returns true if the child process was
122 resumed by delivery of SIGCONT.
123 waitid()
125 The waitid() system call (available since Linux 2.6.9) provides more
126 precise control over which child state changes to wait for.
127 The idtype and id arguments select the child(ren) to wait for, as fol‐
129 lows:
130 idtype == P_PID
132 Wait for the child whose process ID matches id.
133 idtype == P_PGID
135 Wait for any child whose process group ID matches id.
136 idtype == P_ALL
138 Wait for any child; id is ignored.
139 The child state changes to wait for are specified by ORing one or more
141 of the following flags in options:
142 WEXITED Wait for children that have terminated.
144 WSTOPPED Wait for children that have been stopped by delivery of a
146 signal.
147 WCONTINUED Wait for (previously stopped) children that have been
149 resumed by delivery of SIGCONT.
150 The following flags may additionally be ORed in options:
152 WNOHANG As for waitpid().
154 WNOWAIT Leave the child in a waitable state; a later wait call can
156 be used to again retrieve the child status information.
157 Upon successful return, waitid() fills in the following fields of the
159 siginfo_t structure pointed to by infop:
160 si_pid The process ID of the child.
162 si_uid The real user ID of the child. (This field is not set on
164 most other implementations.)
165 si_signo Always set to SIGCHLD.
167 si_status Either the exit status of the child, as given to _exit(2)
169 (or exit(3)), or the signal that caused the child to termi‐
170 nate, stop, or continue. The si_code field can be used to
171 determine how to interpret this field.
172 si_code Set to one of: CLD_EXITED (child called _exit(2));
174 CLD_KILLED (child killed by signal); CLD_DUMPED (child
175 killed by signal, and dumped core); CLD_STOPPED (child
176 stopped by signal); CLD_TRAPPED (traced child has trapped);
177 or CLD_CONTINUED (child continued by SIGCONT).
178 If WNOHANG was specified in options and there were no children in a
180 waitable state, then waitid() returns 0 immediately and the state of
181 the siginfo_t structure pointed to by infop is unspecified. To distin‐
182 guish this case from that where a child was in a waitable state, zero
183 out the si_pid field before the call and check for a non-zero value in
184 this field after the call returns.
185
187 wait(): on success, returns the process ID of the terminated child; on
188 error, -1 is returned.
189 waitpid(): on success, returns the process ID of the child whose state
191 has changed; if WNOHANG was specified and one or more child(ren) speci‐
192 fied by pid exist, but have not yet changed state, then 0 is returned.
193 On error, -1 is returned.
194 waitid(): returns 0 on success or if WNOHANG was specified and no
196 child(ren) specified by id has yet changed state; on error, -1 is
197 returned. Each of these calls sets errno to an appropriate value in
198 the case of an error.
199
201 ECHILD (for wait()) The calling process does not have any unwaited-for
202 children.
203 ECHILD (for waitpid() or waitid()) The process specified by pid (wait‐
205 pid()) or idtype and id (waitid()) does not exist or is not a
206 child of the calling process. (This can happen for one's own
207 child if the action for SIGCHLD is set to SIG_IGN. See also the
208 Linux Notes section about threads.)
209 EINTR WNOHANG was not set and an unblocked signal or a SIGCHLD was
211 caught; see signal(7).
212 EINVAL The options argument was invalid.
214
216 SVr4, 4.3BSD, POSIX.1-2001.
217
219 A child that terminates, but has not been waited for becomes a "zom‐
220 bie". The kernel maintains a minimal set of information about the zom‐
221 bie process (PID, termination status, resource usage information) in
222 order to allow the parent to later perform a wait to obtain information
223 about the child. As long as a zombie is not removed from the system
224 via a wait, it will consume a slot in the kernel process table, and if
225 this table fills, it will not be possible to create further processes.
226 If a parent process terminates, then its "zombie" children (if any) are
227 adopted by init(8), which automatically performs a wait to remove the
228 zombies.
229 POSIX.1-2001 specifies that if the disposition of SIGCHLD is set to
231 SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see sigaction(2)),
232 then children that terminate do not become zombies and a call to wait()
233 or waitpid() will block until all children have terminated, and then
234 fail with errno set to ECHILD. (The original POSIX standard left the
235 behavior of setting SIGCHLD to SIG_IGN unspecified. Note that even
236 though the default disposition of SIGCHLD is "ignore", explicitly set‐
237 ting the disposition to SIG_IGN results in different treatment of zom‐
238 bie process children.) Linux 2.6 conforms to this specification. How‐
239 ever, Linux 2.4 (and earlier) does not: if a wait() or waitpid() call
240 is made while SIGCHLD is being ignored, the call behaves just as though
241 SIGCHLD were not being ignored, that is, the call blocks until the next
242 child terminates and then returns the process ID and status of that
243 child.
244 Linux Notes
246 In the Linux kernel, a kernel-scheduled thread is not a distinct con‐
247 struct from a process. Instead, a thread is simply a process that is
248 created using the Linux-unique clone(2) system call; other routines
249 such as the portable pthread_create(3) call are implemented using
250 clone(2). Before Linux 2.4, a thread was just a special case of a
251 process, and as a consequence one thread could not wait on the children
252 of another thread, even when the latter belongs to the same thread
253 group. However, POSIX prescribes such functionality, and since Linux
254 2.4 a thread can, and by default will, wait on children of other
255 threads in the same thread group.
256 The following Linux-specific options are for use with children created
258 using clone(2); they cannot be used with waitid():
259 __WCLONE
261 Wait for "clone" children only. If omitted then wait for "non-
262 clone" children only. (A "clone" child is one which delivers no
263 signal, or a signal other than SIGCHLD to its parent upon termi‐
264 nation.) This option is ignored if __WALL is also specified.
265 __WALL (since Linux 2.4)
267 Wait for all children, regardless of type ("clone" or "non-
268 clone").
269 __WNOTHREAD (since Linux 2.4)
271 Do not wait for children of other threads in the same thread
272 group. This was the default before Linux 2.4.
273
275 The following program demonstrates the use of fork(2) and waitpid().
276 The program creates a child process. If no command-line argument is
277 supplied to the program, then the child suspends its execution using
278 pause(2), to allow the user to send signals to the child. Otherwise,
279 if a command-line argument is supplied, then the child exits immedi‐
280 ately, using the integer supplied on the command line as the exit sta‐
281 tus. The parent process executes a loop that monitors the child using
282 waitpid(), and uses the W*() macros described above to analyze the wait
283 status value.
284 The following shell session demonstrates the use of the program:
286 $ ./a.out &
288 Child PID is 32360
289 [1] 32359
290 $ kill -STOP 32360
291 stopped by signal 19
292 $ kill -CONT 32360
293 continued
294 $ kill -TERM 32360
295 killed by signal 15
296 [1]+ Done ./a.out
297 $
298 Program source
300 #include <sys/wait.h>
302 #include <stdlib.h>
303 #include <unistd.h>
304 #include <stdio.h>
305 int
307 main(int argc, char *argv[])
308 {
309 pid_t cpid, w;
310 int status;
311 cpid = fork();
313 if (cpid == -1) {
314 perror("fork");
315 exit(EXIT_FAILURE);
316 }
317 if (cpid == 0) { /* Code executed by child */
319 printf("Child PID is %ld\n", (long) getpid());
320 if (argc == 1)
321 pause(); /* Wait for signals */
322 _exit(atoi(argv[1]));
323 } else { /* Code executed by parent */
325 do {
326 w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
327 if (w == -1) {
328 perror("waitpid");
329 exit(EXIT_FAILURE);
330 }
331 if (WIFEXITED(status)) {
333 printf("exited, status=%d\n", WEXITSTATUS(status));
334 } else if (WIFSIGNALED(status)) {
335 printf("killed by signal %d\n", WTERMSIG(status));
336 } else if (WIFSTOPPED(status)) {
337 printf("stopped by signal %d\n", WSTOPSIG(status));
338 } else if (WIFCONTINUED(status)) {
339 printf("continued\n");
340 }
341 } while (!WIFEXITED(status) && !WIFSIGNALED(status));
342 exit(EXIT_SUCCESS);
343 }
344 }
345
347 _exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2), sig‐
348 nal(2), wait4(2), pthread_create(3), credentials(7), signal(7)
349
351 This page is part of release 3.22 of the Linux man-pages project. A
352 description of the project, and information about reporting bugs, can
353 be found at http://www.kernel.org/doc/man-pages/.
354Linux 2009-04-21 WAIT(2)