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       EAGAIN The  PID  file descriptor specified in id is nonblocking and the
237              process that it refers to has not terminated.
238
239       ECHILD (for wait()) The calling process does not have any  unwaited-for
240              children.
241
242       ECHILD (for  waitpid() or waitid()) The process specified by pid (wait‐
243              pid()) or idtype and id (waitid()) does not exist or  is  not  a
244              child  of  the  calling process.  (This can happen for one's own
245              child if the action for SIGCHLD is set to SIG_IGN.  See also the
246              Linux Notes section about threads.)
247
248       EINTR  WNOHANG  was  not  set  and an unblocked signal or a SIGCHLD was
249              caught; see signal(7).
250
251       EINVAL The options argument was invalid.
252
253       ESRCH  (for wait() or waitpid()) pid is equal to INT_MIN.
254

CONFORMING TO

256       SVr4, 4.3BSD, POSIX.1-2001.
257

NOTES

259       A child that terminates, but has not been waited for  becomes  a  "zom‐
260       bie".  The kernel maintains a minimal set of information about the zom‐
261       bie process (PID, termination status, resource  usage  information)  in
262       order to allow the parent to later perform a wait to obtain information
263       about the child.  As long as a zombie is not removed  from  the  system
264       via  a wait, it will consume a slot in the kernel process table, and if
265       this table fills, it will not be possible to create further  processes.
266       If a parent process terminates, then its "zombie" children (if any) are
267       adopted by init(1), (or by the nearest "subreaper" process  as  defined
268       through  the  use  of  the  prctl(2) PR_SET_CHILD_SUBREAPER operation);
269       init(1) automatically performs a wait to remove the zombies.
270
271       POSIX.1-2001 specifies that if the disposition of  SIGCHLD  is  set  to
272       SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see sigaction(2)),
273       then children that terminate do not become zombies and a call to wait()
274       or  waitpid()  will  block until all children have terminated, and then
275       fail with errno set to ECHILD.  (The original POSIX standard  left  the
276       behavior  of  setting  SIGCHLD  to SIG_IGN unspecified.  Note that even
277       though the default disposition of SIGCHLD is "ignore", explicitly  set‐
278       ting  the disposition to SIG_IGN results in different treatment of zom‐
279       bie process children.)
280
281       Linux 2.6 conforms to the POSIX requirements.  However, Linux 2.4  (and
282       earlier)  does not: if a wait() or waitpid() call is made while SIGCHLD
283       is being ignored, the call behaves just as though SIGCHLD were not  be‐
284       ing  ignored,  that is, the call blocks until the next child terminates
285       and then returns the process ID and status of that child.
286
287   Linux notes
288       In the Linux kernel, a kernel-scheduled thread is not a  distinct  con‐
289       struct  from  a process.  Instead, a thread is simply a process that is
290       created using the Linux-unique clone(2)  system  call;  other  routines
291       such  as  the  portable  pthread_create(3)  call  are implemented using
292       clone(2).  Before Linux 2.4, a thread was just  a  special  case  of  a
293       process, and as a consequence one thread could not wait on the children
294       of another thread, even when the latter  belongs  to  the  same  thread
295       group.   However,  POSIX prescribes such functionality, and since Linux
296       2.4 a thread can, and by  default  will,  wait  on  children  of  other
297       threads in the same thread group.
298
299       The  following Linux-specific options are for use with children created
300       using clone(2); they can also, since Linux 4.7, be used with waitid():
301
302       __WCLONE
303              Wait for "clone" children only.  If omitted, then wait for "non-
304              clone" children only.  (A "clone" child is one which delivers no
305              signal, or a signal other than SIGCHLD to its parent upon termi‐
306              nation.)  This option is ignored if __WALL is also specified.
307
308       __WALL (since Linux 2.4)
309              Wait  for  all  children,  regardless  of type ("clone" or "non-
310              clone").
311
312       __WNOTHREAD (since Linux 2.4)
313              Do not wait for children of other threads  in  the  same  thread
314              group.  This was the default before Linux 2.4.
315
316       Since  Linux 4.7, the __WALL flag is automatically implied if the child
317       is being ptraced.
318
319   C library/kernel differences
320       wait() is actually a library function that (in glibc) is implemented as
321       a call to wait4(2).
322
323       On some architectures, there is no waitpid() system call; instead, this
324       interface is implemented via a C library wrapper  function  that  calls
325       wait4(2).
326
327       The  raw  waitid()  system  call takes a fifth argument, of type struct
328       rusage *.  If this argument is non-NULL, then it is used to return  re‐
329       source  usage  information  about  the  child,  in  the  same manner as
330       wait4(2).  See getrusage(2) for details.
331

BUGS

333       According to POSIX.1-2008, an application calling waitid() must  ensure
334       that infop points to a siginfo_t structure (i.e., that it is a non-null
335       pointer).  On Linux, if infop is NULL, waitid() succeeds,  and  returns
336       the  process ID of the waited-for child.  Applications should avoid re‐
337       lying on this inconsistent, nonstandard, and unnecessary feature.
338

EXAMPLES

340       The following program demonstrates the use of  fork(2)  and  waitpid().
341       The  program  creates  a child process.  If no command-line argument is
342       supplied to the program, then the child suspends  its  execution  using
343       pause(2),  to  allow the user to send signals to the child.  Otherwise,
344       if a command-line argument is supplied, then the  child  exits  immedi‐
345       ately,  using the integer supplied on the command line as the exit sta‐
346       tus.  The parent process executes a loop that monitors the child  using
347       waitpid(), and uses the W*() macros described above to analyze the wait
348       status value.
349
350       The following shell session demonstrates the use of the program:
351
352           $ ./a.out &
353           Child PID is 32360
354           [1] 32359
355           $ kill -STOP 32360
356           stopped by signal 19
357           $ kill -CONT 32360
358           continued
359           $ kill -TERM 32360
360           killed by signal 15
361           [1]+  Done                    ./a.out
362           $
363
364   Program source
365
366       #include <sys/wait.h>
367       #include <stdint.h>
368       #include <stdlib.h>
369       #include <unistd.h>
370       #include <stdio.h>
371
372       int
373       main(int argc, char *argv[])
374       {
375           pid_t cpid, w;
376           int wstatus;
377
378           cpid = fork();
379           if (cpid == -1) {
380               perror("fork");
381               exit(EXIT_FAILURE);
382           }
383
384           if (cpid == 0) {            /* Code executed by child */
385               printf("Child PID is %jd\n", (intmax_t) getpid());
386               if (argc == 1)
387                   pause();                    /* Wait for signals */
388               _exit(atoi(argv[1]));
389
390           } else {                    /* Code executed by parent */
391               do {
392                   w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
393                   if (w == -1) {
394                       perror("waitpid");
395                       exit(EXIT_FAILURE);
396                   }
397
398                   if (WIFEXITED(wstatus)) {
399                       printf("exited, status=%d\n", WEXITSTATUS(wstatus));
400                   } else if (WIFSIGNALED(wstatus)) {
401                       printf("killed by signal %d\n", WTERMSIG(wstatus));
402                   } else if (WIFSTOPPED(wstatus)) {
403                       printf("stopped by signal %d\n", WSTOPSIG(wstatus));
404                   } else if (WIFCONTINUED(wstatus)) {
405                       printf("continued\n");
406                   }
407               } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
408               exit(EXIT_SUCCESS);
409           }
410       }
411

SEE ALSO

413       _exit(2), clone(2), fork(2),  kill(2),  ptrace(2),  sigaction(2),  sig‐
414       nal(2), wait4(2), pthread_create(3), core(5), credentials(7), signal(7)
415

COLOPHON

417       This  page  is  part of release 5.13 of the Linux man-pages project.  A
418       description of the project, information about reporting bugs,  and  the
419       latest     version     of     this    page,    can    be    found    at
420       https://www.kernel.org/doc/man-pages/.
421
422
423
424Linux                             2021-08-27                           WAIT(2)
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