1WAIT(2)                    Linux Programmer's Manual                   WAIT(2)
2
3
4

NAME

6       wait, waitpid, waitid - wait for process to change state
7

SYNOPSIS

9       #include <sys/wait.h>
10
11       pid_t wait(int *wstatus);
12       pid_t waitpid(pid_t pid, int *wstatus, int options);
13
14       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
15                       /* This is the glibc and POSIX interface; see
16                          NOTES for information on the raw system call. */
17
18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19
20       waitid():
21           Since glibc 2.26:
22               _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L
23           Glibc 2.25 and earlier:
24               _XOPEN_SOURCE
25                   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
26                   || /* Glibc <= 2.19: */ _BSD_SOURCE
27

DESCRIPTION

29       All of these system calls are used to wait for state changes in a child
30       of the calling process, and obtain information about  the  child  whose
31       state  has changed.  A state change is considered to be: the child ter‐
32       minated; the child was stopped by a signal; or the child was resumed by
33       a  signal.  In the case of a terminated child, performing a wait allows
34       the system to release the resources associated with  the  child;  if  a
35       wait  is not performed, then the terminated child remains in a "zombie"
36       state (see NOTES below).
37
38       If a child has already changed state, then these calls  return  immedi‐
39       ately.   Otherwise,  they block until either a child changes state or a
40       signal handler interrupts the call (assuming that system calls are  not
41       automatically restarted using the SA_RESTART flag of sigaction(2)).  In
42       the remainder of this page, a child whose state has changed  and  which
43       has  not  yet  been  waited upon by one of these system calls is termed
44       waitable.
45
46   wait() and waitpid()
47       The wait() system call suspends execution of the calling  thread  until
48       one  of its children terminates.  The call wait(&wstatus) is equivalent
49       to:
50
51           waitpid(-1, &wstatus, 0);
52
53       The waitpid() system call suspends execution of the calling thread  un‐
54       til  a  child specified by pid argument has changed state.  By default,
55       waitpid() waits only for terminated children, but this behavior is mod‐
56       ifiable via the options argument, as described below.
57
58       The value of pid can be:
59
60       < -1   meaning  wait  for  any  child process whose process group ID is
61              equal to the absolute value of pid.
62
63       -1     meaning wait for any child process.
64
65       0      meaning wait for any child process whose  process  group  ID  is
66              equal  to that of the calling process at the time of the call to
67              waitpid().
68
69       > 0    meaning wait for the child whose process  ID  is  equal  to  the
70              value of pid.
71
72       The  value  of  options  is an OR of zero or more of the following con‐
73       stants:
74
75       WNOHANG
76              return immediately if no child has exited.
77
78       WUNTRACED
79              also  return  if  a  child  has  stopped  (but  not  traced  via
80              ptrace(2)).   Status  for  traced children which have stopped is
81              provided even if this option is not specified.
82
83       WCONTINUED (since Linux 2.6.10)
84              also return if a stopped child has been resumed by  delivery  of
85              SIGCONT.
86
87       (For Linux-only options, see below.)
88
89       If  wstatus  is not NULL, wait() and waitpid() store status information
90       in the int to which it points.  This integer can be inspected with  the
91       following  macros  (which take the integer itself as an argument, not a
92       pointer to it, as is done in wait() and waitpid()!):
93
94       WIFEXITED(wstatus)
95              returns true if the child terminated normally, that is, by call‐
96              ing exit(3) or _exit(2), or by returning from main().
97
98       WEXITSTATUS(wstatus)
99              returns  the  exit  status  of  the child.  This consists of the
100              least significant 8 bits of the status argument that  the  child
101              specified  in  a  call to exit(3) or _exit(2) or as the argument
102              for a return statement in main().  This macro should be employed
103              only if WIFEXITED returned true.
104
105       WIFSIGNALED(wstatus)
106              returns true if the child process was terminated by a signal.
107
108       WTERMSIG(wstatus)
109              returns  the  number of the signal that caused the child process
110              to terminate.  This macro should be employed only if WIFSIGNALED
111              returned true.
112
113       WCOREDUMP(wstatus)
114              returns  true  if  the child produced a core dump (see core(5)).
115              This macro should be employed only if WIFSIGNALED returned true.
116
117              This macro is not specified in POSIX.1-2001 and is not available
118              on some UNIX implementations (e.g., AIX, SunOS).  Therefore, en‐
119              close its use inside #ifdef WCOREDUMP ... #endif.
120
121       WIFSTOPPED(wstatus)
122              returns true if the child process was stopped by delivery  of  a
123              signal;  this  is  possible only if the call was done using WUN‐
124              TRACED or when the child is being traced (see ptrace(2)).
125
126       WSTOPSIG(wstatus)
127              returns the number of the signal which caused the child to stop.
128              This macro should be employed only if WIFSTOPPED returned true.
129
130       WIFCONTINUED(wstatus)
131              (since  Linux  2.6.10) returns true if the child process was re‐
132              sumed by delivery of SIGCONT.
133
134   waitid()
135       The waitid() system call (available since Linux  2.6.9)  provides  more
136       precise control over which child state changes to wait for.
137
138       The  idtype and id arguments select the child(ren) to wait for, as fol‐
139       lows:
140
141       idtype == P_PID
142              Wait for the child whose process ID matches id.
143
144       idtype == P_PIDFD (since Linux 5.4)
145              Wait for the child referred to by the PID file descriptor speci‐
146              fied  in  id.  (See pidfd_open(2) for further information on PID
147              file descriptors.)
148
149       idtype == P_PGID
150              Wait for any child whose process group  ID  matches  id.   Since
151              Linux 5.4, if id is zero, then wait for any child that is in the
152              same process group as the caller's process group at the time  of
153              the call.
154
155       idtype == P_ALL
156              Wait for any child; id is ignored.
157
158       The  child state changes to wait for are specified by ORing one or more
159       of the following flags in options:
160
161       WEXITED
162              Wait for children that have terminated.
163
164       WSTOPPED
165              Wait for children that have been stopped by delivery of  a  sig‐
166              nal.
167
168       WCONTINUED
169              Wait for (previously stopped) children that have been resumed by
170              delivery of SIGCONT.
171
172       The following flags may additionally be ORed in options:
173
174       WNOHANG
175              As for waitpid().
176
177       WNOWAIT
178              Leave the child in a waitable state; a later wait  call  can  be
179              used to again retrieve the child status information.
180
181       Upon  successful  return, waitid() fills in the following fields of the
182       siginfo_t structure pointed to by infop:
183
184       si_pid The process ID of the child.
185
186       si_uid The real user ID of the child.  (This field is not set  on  most
187              other implementations.)
188
189       si_signo
190              Always set to SIGCHLD.
191
192       si_status
193              Either  the  exit  status of the child, as given to _exit(2) (or
194              exit(3)), or the signal that  caused  the  child  to  terminate,
195              stop,  or  continue.  The si_code field can be used to determine
196              how to interpret this field.
197
198       si_code
199              Set to one of: CLD_EXITED (child  called  _exit(2));  CLD_KILLED
200              (child  killed  by  signal); CLD_DUMPED (child killed by signal,
201              and  dumped  core);  CLD_STOPPED  (child  stopped  by   signal);
202              CLD_TRAPPED  (traced child has trapped); or CLD_CONTINUED (child
203              continued by SIGCONT).
204
205       If WNOHANG was specified in options and there were  no  children  in  a
206       waitable  state,  then  waitid() returns 0 immediately and the state of
207       the siginfo_t structure pointed to by infop depends on the  implementa‐
208       tion.   To (portably) distinguish this case from that where a child was
209       in a waitable state, zero out the si_pid  field  before  the  call  and
210       check for a nonzero value in this field after the call returns.
211
212       POSIX.1-2008  Technical  Corrigendum 1 (2013) adds the requirement that
213       when WNOHANG is specified in options and there were no  children  in  a
214       waitable  state,  then waitid() should zero out the si_pid and si_signo
215       fields of the structure.  On Linux and other implementations  that  ad‐
216       here  to  this  requirement, it is not necessary to zero out the si_pid
217       field before calling waitid().  However, not all implementations follow
218       the POSIX.1 specification on this point.
219

RETURN VALUE

221       wait():  on success, returns the process ID of the terminated child; on
222       failure, -1 is returned.
223
224       waitpid(): on success, returns the process ID of the child whose  state
225       has changed; if WNOHANG was specified and one or more child(ren) speci‐
226       fied by pid exist, but have not yet changed state, then 0 is  returned.
227       On failure, -1 is returned.
228
229       waitid():  returns  0  on  success  or  if WNOHANG was specified and no
230       child(ren) specified by id has yet changed state; on failure, -1 is re‐
231       turned.
232
233       On failure, each of these calls sets errno to indicate the error.
234

ERRORS

236       ECHILD (for  wait()) The calling process does not have any unwaited-for
237              children.
238
239       ECHILD (for waitpid() or waitid()) The process specified by pid  (wait‐
240              pid())  or  idtype  and id (waitid()) does not exist or is not a
241              child of the calling process.  (This can happen  for  one's  own
242              child if the action for SIGCHLD is set to SIG_IGN.  See also the
243              Linux Notes section about threads.)
244
245       EINTR  WNOHANG was not set and an unblocked signal  or  a  SIGCHLD  was
246              caught; see signal(7).
247
248       EINVAL The options argument was invalid.
249

CONFORMING TO

251       SVr4, 4.3BSD, POSIX.1-2001.
252

NOTES

254       A  child  that  terminates, but has not been waited for becomes a "zom‐
255       bie".  The kernel maintains a minimal set of information about the zom‐
256       bie  process  (PID,  termination status, resource usage information) in
257       order to allow the parent to later perform a wait to obtain information
258       about  the  child.   As long as a zombie is not removed from the system
259       via a wait, it will consume a slot in the kernel process table, and  if
260       this  table fills, it will not be possible to create further processes.
261       If a parent process terminates, then its "zombie" children (if any) are
262       adopted  by  init(1), (or by the nearest "subreaper" process as defined
263       through the use  of  the  prctl(2)  PR_SET_CHILD_SUBREAPER  operation);
264       init(1) automatically performs a wait to remove the zombies.
265
266       POSIX.1-2001  specifies  that  if  the disposition of SIGCHLD is set to
267       SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see sigaction(2)),
268       then children that terminate do not become zombies and a call to wait()
269       or waitpid() will block until all children have  terminated,  and  then
270       fail  with  errno set to ECHILD.  (The original POSIX standard left the
271       behavior of setting SIGCHLD to SIG_IGN  unspecified.   Note  that  even
272       though  the default disposition of SIGCHLD is "ignore", explicitly set‐
273       ting the disposition to SIG_IGN results in different treatment of  zom‐
274       bie process children.)
275
276       Linux  2.6 conforms to the POSIX requirements.  However, Linux 2.4 (and
277       earlier) does not: if a wait() or waitpid() call is made while  SIGCHLD
278       is  being ignored, the call behaves just as though SIGCHLD were not be‐
279       ing ignored, that is, the call blocks until the next  child  terminates
280       and then returns the process ID and status of that child.
281
282   Linux notes
283       In  the  Linux kernel, a kernel-scheduled thread is not a distinct con‐
284       struct from a process.  Instead, a thread is simply a process  that  is
285       created  using  the  Linux-unique  clone(2) system call; other routines
286       such as the  portable  pthread_create(3)  call  are  implemented  using
287       clone(2).   Before  Linux  2.4,  a  thread was just a special case of a
288       process, and as a consequence one thread could not wait on the children
289       of  another  thread,  even  when  the latter belongs to the same thread
290       group.  However, POSIX prescribes such functionality, and  since  Linux
291       2.4  a  thread  can,  and  by  default  will, wait on children of other
292       threads in the same thread group.
293
294       The following Linux-specific options are for use with children  created
295       using clone(2); they can also, since Linux 4.7, be used with waitid():
296
297       __WCLONE
298              Wait for "clone" children only.  If omitted, then wait for "non-
299              clone" children only.  (A "clone" child is one which delivers no
300              signal, or a signal other than SIGCHLD to its parent upon termi‐
301              nation.)  This option is ignored if __WALL is also specified.
302
303       __WALL (since Linux 2.4)
304              Wait for all children, regardless  of  type  ("clone"  or  "non-
305              clone").
306
307       __WNOTHREAD (since Linux 2.4)
308              Do  not  wait  for  children of other threads in the same thread
309              group.  This was the default before Linux 2.4.
310
311       Since Linux 4.7, the __WALL flag is automatically implied if the  child
312       is being ptraced.
313
314   C library/kernel differences
315       wait() is actually a library function that (in glibc) is implemented as
316       a call to wait4(2).
317
318       On some architectures, there is no waitpid() system call; instead, this
319       interface  is  implemented  via a C library wrapper function that calls
320       wait4(2).
321
322       The raw waitid() system call takes a fifth  argument,  of  type  struct
323       rusage *.   If this argument is non-NULL, then it is used to return re‐
324       source usage information  about  the  child,  in  the  same  manner  as
325       wait4(2).  See getrusage(2) for details.
326

BUGS

328       According  to POSIX.1-2008, an application calling waitid() must ensure
329       that infop points to a siginfo_t structure (i.e., that it is a non-null
330       pointer).   On  Linux, if infop is NULL, waitid() succeeds, and returns
331       the process ID of the waited-for child.  Applications should avoid  re‐
332       lying on this inconsistent, nonstandard, and unnecessary feature.
333

EXAMPLES

335       The  following  program  demonstrates the use of fork(2) and waitpid().
336       The program creates a child process.  If no  command-line  argument  is
337       supplied  to  the  program, then the child suspends its execution using
338       pause(2), to allow the user to send signals to the  child.   Otherwise,
339       if  a  command-line  argument is supplied, then the child exits immedi‐
340       ately, using the integer supplied on the command line as the exit  sta‐
341       tus.   The parent process executes a loop that monitors the child using
342       waitpid(), and uses the W*() macros described above to analyze the wait
343       status value.
344
345       The following shell session demonstrates the use of the program:
346
347           $ ./a.out &
348           Child PID is 32360
349           [1] 32359
350           $ kill -STOP 32360
351           stopped by signal 19
352           $ kill -CONT 32360
353           continued
354           $ kill -TERM 32360
355           killed by signal 15
356           [1]+  Done                    ./a.out
357           $
358
359   Program source
360
361       #include <sys/wait.h>
362       #include <stdint.h>
363       #include <stdlib.h>
364       #include <unistd.h>
365       #include <stdio.h>
366
367       int
368       main(int argc, char *argv[])
369       {
370           pid_t cpid, w;
371           int wstatus;
372
373           cpid = fork();
374           if (cpid == -1) {
375               perror("fork");
376               exit(EXIT_FAILURE);
377           }
378
379           if (cpid == 0) {            /* Code executed by child */
380               printf("Child PID is %jd\n", (intmax_t) getpid());
381               if (argc == 1)
382                   pause();                    /* Wait for signals */
383               _exit(atoi(argv[1]));
384
385           } else {                    /* Code executed by parent */
386               do {
387                   w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
388                   if (w == -1) {
389                       perror("waitpid");
390                       exit(EXIT_FAILURE);
391                   }
392
393                   if (WIFEXITED(wstatus)) {
394                       printf("exited, status=%d\n", WEXITSTATUS(wstatus));
395                   } else if (WIFSIGNALED(wstatus)) {
396                       printf("killed by signal %d\n", WTERMSIG(wstatus));
397                   } else if (WIFSTOPPED(wstatus)) {
398                       printf("stopped by signal %d\n", WSTOPSIG(wstatus));
399                   } else if (WIFCONTINUED(wstatus)) {
400                       printf("continued\n");
401                   }
402               } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
403               exit(EXIT_SUCCESS);
404           }
405       }
406

SEE ALSO

408       _exit(2),  clone(2),  fork(2),  kill(2),  ptrace(2), sigaction(2), sig‐
409       nal(2), wait4(2), pthread_create(3), core(5), credentials(7), signal(7)
410

COLOPHON

412       This page is part of release 5.12 of the Linux  man-pages  project.   A
413       description  of  the project, information about reporting bugs, and the
414       latest    version    of    this    page,    can     be     found     at
415       https://www.kernel.org/doc/man-pages/.
416
417
418
419Linux                             2021-03-22                           WAIT(2)
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