1wait(2)                       System Calls Manual                      wait(2)


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


9       Standard C library (libc, -lc)


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


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


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


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


259   C library/kernel differences
260       wait() is actually a library function that (in glibc) is implemented as
261       a call to wait4(2).
263       On some architectures, there is no waitpid() system call; instead, this
264       interface is implemented via a C library wrapper  function  that  calls
265       wait4(2).
267       The  raw  waitid()  system  call takes a fifth argument, of type struct
268       rusage *.  If this argument is non-NULL, then it is used to return  re‐
269       source  usage  information  about  the  child,  in  the  same manner as
270       wait4(2).  See getrusage(2) for details.


273       POSIX.1-2008.


276       SVr4, 4.3BSD, POSIX.1-2001.


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


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


347       The following program demonstrates the use of  fork(2)  and  waitpid().
348       The  program  creates  a child process.  If no command-line argument is
349       supplied to the program, then the child suspends  its  execution  using
350       pause(2),  to  allow the user to send signals to the child.  Otherwise,
351       if a command-line argument is supplied, then the  child  exits  immedi‐
352       ately,  using the integer supplied on the command line as the exit sta‐
353       tus.  The parent process executes a loop that monitors the child  using
354       waitpid(), and uses the W*() macros described above to analyze the wait
355       status value.
357       The following shell session demonstrates the use of the program:
359           $ ./a.out &
360           Child PID is 32360
361           [1] 32359
362           $ kill -STOP 32360
363           stopped by signal 19
364           $ kill -CONT 32360
365           continued
366           $ kill -TERM 32360
367           killed by signal 15
368           [1]+  Done                    ./a.out
369           $
371   Program source
373       #include <stdint.h>
374       #include <stdio.h>
375       #include <stdlib.h>
376       #include <sys/wait.h>
377       #include <unistd.h>
379       int
380       main(int argc, char *argv[])
381       {
382           int    wstatus;
383           pid_t  cpid, w;
385           cpid = fork();
386           if (cpid == -1) {
387               perror("fork");
388               exit(EXIT_FAILURE);
389           }
391           if (cpid == 0) {            /* Code executed by child */
392               printf("Child PID is %jd\n", (intmax_t) getpid());
393               if (argc == 1)
394                   pause();                    /* Wait for signals */
395               _exit(atoi(argv[1]));
397           } else {                    /* Code executed by parent */
398               do {
399                   w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
400                   if (w == -1) {
401                       perror("waitpid");
402                       exit(EXIT_FAILURE);
403                   }
405                   if (WIFEXITED(wstatus)) {
406                       printf("exited, status=%d\n", WEXITSTATUS(wstatus));
407                   } else if (WIFSIGNALED(wstatus)) {
408                       printf("killed by signal %d\n", WTERMSIG(wstatus));
409                   } else if (WIFSTOPPED(wstatus)) {
410                       printf("stopped by signal %d\n", WSTOPSIG(wstatus));
411                   } else if (WIFCONTINUED(wstatus)) {
412                       printf("continued\n");
413                   }
414               } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
415               exit(EXIT_SUCCESS);
416           }
417       }


420       _exit(2), clone(2), fork(2),  kill(2),  ptrace(2),  sigaction(2),  sig‐
421       nal(2), wait4(2), pthread_create(3), core(5), credentials(7), signal(7)
425Linux man-pages 6.05              2023-05-03                           wait(2)