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


   #include <sys/types.h>
   #include <sys/wait.h>

   pid_t wait(int *wstatus);

   pid_t waitpid(pid_t pid, int *wstatus, int options);

   int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
                   /* This is the glibc and POSIX interface; see
                      NOTES for information on the raw system call. */

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
           || /* Glibc versions <= 2.19: */ _BSD_SOURCE


   All of these system calls are used to wait for state changes in a child
   of the calling process, and obtain information about  the  child  whose
   state  has  changed.   A  state  change  is considered to be: the child
   terminated; the child was stopped by a signal; or the child was resumed
   by  a  signal.   In  the  case of a terminated child, performing a wait
   allows the system to release the resources associated with  the  child;
   if  a  wait  is  not  performed, then the terminated child remains in a
   "zombie" state (see NOTES below).

   If  a  child  has  already  changed  state,  then  these  calls  return
   immediately.   Otherwise, they block until either a child changes state
   or a signal handler interrupts the call (assuming that system calls are
   not automatically restarted using the SA_RESTART flag of sigaction(2)).
   In the remainder of this page, a child  whose  state  has  changed  and
   which  has  not  yet  been  waited upon by one of these system calls is
   termed waitable.

   wait() and waitpid()
   The wait() system call suspends execution of the calling process  until
   one  of its children terminates.  The call wait(&wstatus) is equivalent

       waitpid(-1, &wstatus, 0);

   The waitpid() system call suspends execution  of  the  calling  process
   until a child specified by pid argument has changed state.  By default,
   waitpid() waits only for terminated  children,  but  this  behavior  is
   modifiable via the options argument, as described below.

   The value of pid can be:

   < -1   meaning  wait  for  any  child process whose process group ID is
          equal to the absolute value of pid.

   -1     meaning wait for any child process.

   0      meaning wait for any child process whose  process  group  ID  is
          equal to that of the calling process.

   > 0    meaning  wait  for  the  child  whose process ID is equal to the
          value of pid.

   The value of options is  an  OR  of  zero  or  more  of  the  following

   WNOHANG     return immediately if no child has exited.

   WUNTRACED   also  return  if  a  child  has stopped (but not traced via
               ptrace(2)).  Status for traced children which have  stopped
               is provided even if this option is not specified.

   WCONTINUED (since Linux 2.6.10)
               also return if a stopped child has been resumed by delivery
               of SIGCONT.

   (For Linux-only options, see below.)

   If wstatus is not NULL, wait() and waitpid() store  status  information
   in  the int to which it points.  This integer can be inspected with the
   following macros (which take the integer itself as an argument,  not  a
   pointer to it, as is done in wait() and waitpid()!):

          returns  true  if  the  child  terminated  normally, that is, by
          calling exit(3) or _exit(2), or by returning from main().

          returns the exit status of the  child.   This  consists  of  the
          least  significant  8 bits of the status argument that the child
          specified in a call to exit(3) or _exit(2) or  as  the  argument
          for a return statement in main().  This macro should be employed
          only if WIFEXITED returned true.

          returns true if the child process was terminated by a signal.

          returns the number of the signal that caused the  child  process
          to terminate.  This macro should be employed only if WIFSIGNALED
          returned true.

          returns true if the child produced  a  core  dump.   This  macro
          should be employed only if WIFSIGNALED returned true.

          This macro is not specified in POSIX.1-2001 and is not available
          on some UNIX implementations  (e.g.,  AIX,  SunOS).   Therefore,
          enclose its use inside #ifdef WCOREDUMP ... #endif.

          returns  true  if the child process was stopped by delivery of a
          signal; this is  possible  only  if  the  call  was  done  using
          WUNTRACED or when the child is being traced (see ptrace(2)).

          returns the number of the signal which caused the child to stop.
          This macro should be employed only if WIFSTOPPED returned true.

          (since Linux 2.6.10) returns  true  if  the  child  process  was
          resumed by delivery of SIGCONT.

   The  waitid()  system  call (available since Linux 2.6.9) provides more
   precise control over which child state changes to wait for.

   The idtype and id arguments select  the  child(ren)  to  wait  for,  as

   idtype == P_PID
          Wait for the child whose process ID matches id.

   idtype == P_PGID
          Wait for any child whose process group ID matches id.

   idtype == P_ALL
          Wait for any child; id is ignored.

   The  child state changes to wait for are specified by ORing one or more
   of the following flags in options:

   WEXITED     Wait for children that have terminated.

   WSTOPPED    Wait for children that have been stopped by delivery  of  a

   WCONTINUED  Wait  for  (previously  stopped)  children  that  have been
               resumed by delivery of SIGCONT.

   The following flags may additionally be ORed in options:

   WNOHANG     As for waitpid().

   WNOWAIT     Leave the child in a waitable state; a later wait call  can
               be used to again retrieve the child status information.

   Upon  successful  return, waitid() fills in the following fields of the
   siginfo_t structure pointed to by infop:

   si_pid      The process ID of the child.

   si_uid      The real user ID of the child.  (This field is not  set  on
               most other implementations.)

   si_signo    Always set to SIGCHLD.

   si_status   Either  the  exit status of the child, as given to _exit(2)
               (or exit(3)), or  the  signal  that  caused  the  child  to
               terminate,  stop,  or  continue.   The si_code field can be
               used to determine how to interpret this field.

   si_code     Set  to  one  of:  CLD_EXITED  (child   called   _exit(2));
               CLD_KILLED  (child  killed  by  signal);  CLD_DUMPED (child
               killed by signal,  and  dumped  core);  CLD_STOPPED  (child
               stopped by signal); CLD_TRAPPED (traced child has trapped);
               or CLD_CONTINUED (child continued by SIGCONT).

   If WNOHANG was specified in options and there were  no  children  in  a
   waitable  state,  then  waitid() returns 0 immediately and the state of
   the siginfo_t  structure  pointed  to  by  infop  is  unspecified.   To
   distinguish  this case from that where a child was in a waitable state,
   zero out the si_pid field before the call and check for a nonzero value
   in this field after the call returns.


   wait():  on success, returns the process ID of the terminated child; on
   error, -1 is returned.

   waitpid(): on success, returns the process ID of the child whose  state
   has  changed;  if  WNOHANG  was  specified  and  one or more child(ren)
   specified by pid exist, but have not  yet  changed  state,  then  0  is
   returned.  On error, -1 is returned.

   waitid():  returns  0  on  success  or  if WNOHANG was specified and no
   child(ren) specified by id has yet  changed  state;  on  error,  -1  is

   Each  of  these calls sets errno to an appropriate value in the case of
   an error.


   ECHILD (for wait()) The calling process does not have any  unwaited-for

   ECHILD (for  waitpid()  or  waitid())  The  process  specified  by  pid
          (waitpid()) or idtype and id (waitid()) does not exist or is not
          a  child of the calling process.  (This can happen for one's own
          child if the action for SIGCHLD is set to SIG_IGN.  See also the
          Linux Notes section about threads.)

   EINTR  WNOHANG  was  not  set  and an unblocked signal or a SIGCHLD was
          caught; see signal(7).

   EINVAL The options argument was invalid.


   SVr4, 4.3BSD, POSIX.1-2001.


   A child that  terminates,  but  has  not  been  waited  for  becomes  a
   "zombie".   The kernel maintains a minimal set of information about the
   zombie process (PID, termination status, resource usage information) in
   order to allow the parent to later perform a wait to obtain information
   about the child.  As long as a zombie is not removed  from  the  system
   via  a wait, it will consume a slot in the kernel process table, and if
   this table fills, it will not be possible to create further  processes.
   If a parent process terminates, then its "zombie" children (if any) are
   adopted by init(1), which automatically performs a wait to  remove  the

   POSIX.1-2001  specifies  that  if  the disposition of SIGCHLD is set to
   SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see sigaction(2)),
   then children that terminate do not become zombies and a call to wait()
   or waitpid() will block until all children have  terminated,  and  then
   fail  with  errno set to ECHILD.  (The original POSIX standard left the
   behavior of setting SIGCHLD to SIG_IGN  unspecified.   Note  that  even
   though  the  default  disposition  of  SIGCHLD  is "ignore", explicitly
   setting the disposition to SIG_IGN results in  different  treatment  of
   zombie process children.)

   Linux  2.6 conforms to the POSIX requirements.  However, Linux 2.4 (and
   earlier) does not: if a wait() or waitpid() call is made while  SIGCHLD
   is  being  ignored,  the  call  behaves just as though SIGCHLD were not
   being ignored, that is, the call blocks until the next child terminates
   and then returns the process ID and status of that child.

   Linux notes
   In  the  Linux  kernel,  a  kernel-scheduled  thread  is not a distinct
   construct from a process.  Instead, a thread is simply a  process  that
   is  created using the Linux-unique clone(2) system call; other routines
   such as the  portable  pthread_create(3)  call  are  implemented  using
   clone(2).   Before  Linux  2.4,  a  thread was just a special case of a
   process, and as a consequence one thread could not wait on the children
   of  another  thread,  even  when  the latter belongs to the same thread
   group.  However, POSIX prescribes such functionality, and  since  Linux
   2.4  a  thread  can,  and  by  default  will, wait on children of other
   threads in the same thread group.

   The following Linux-specific options are for use with children  created
   using clone(2); they can also, since Linux 4.7, be used with waitid():

          Wait for "clone" children only.  If omitted, then wait for "non-
          clone" children only.  (A "clone" child is one which delivers no
          signal,  or  a  signal  other  than  SIGCHLD  to its parent upon
          termination.)   This  option  is  ignored  if  __WALL  is   also

   __WALL (since Linux 2.4)
          Wait  for  all  children,  regardless  of type ("clone" or "non-

   __WNOTHREAD (since Linux 2.4)
          Do not wait for children of other threads  in  the  same  thread
          group.  This was the default before Linux 2.4.

   Since  Linux 4.7, the __WALL flag is automatically implied if the child
   is being ptraced.

   C library/kernel differences
   wait() is actually a library function that (in glibc) is implemented as
   a call to wait4(2).

   On some architectures, there is no waitpid() system call; instead, this
   interface is implemented via a C library wrapper  function  that  calls

   The  raw  waitid()  system  call takes a fifth argument, of type struct
   rusage *.  If this argument is non-NULL, then  it  is  used  to  return
   resource  usage  information  about  the  child,  in the same manner as
   wait4(2).  See getrusage(2) for details.


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


   The following program demonstrates the use of  fork(2)  and  waitpid().
   The  program  creates  a child process.  If no command-line argument is
   supplied to the program, then the child suspends  its  execution  using
   pause(2),  to  allow the user to send signals to the child.  Otherwise,
   if  a  command-line  argument  is  supplied,  then  the   child   exits
   immediately, using the integer supplied on the command line as the exit
   status.  The parent process executes a loop  that  monitors  the  child
   using  waitpid(),  and  uses the W*() macros described above to analyze
   the wait status value.

   The following shell session demonstrates the use of the program:

       $ ./a.out &
       Child PID is 32360
       [1] 32359
       $ kill -STOP 32360
       stopped by signal 19
       $ kill -CONT 32360
       $ kill -TERM 32360
       killed by signal 15
       [1]+  Done                    ./a.out

   Program source

   #include <sys/wait.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <stdio.h>

   main(int argc, char *argv[])
       pid_t cpid, w;
       int wstatus;

       cpid = fork();
       if (cpid == -1) {

       if (cpid == 0) {            /* Code executed by child */
           printf("Child PID is %ld\n", (long) getpid());
           if (argc == 1)
               pause();                    /* Wait for signals */

       } else {                    /* Code executed by parent */
           do {
               w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
               if (w == -1) {

               if (WIFEXITED(wstatus)) {
                   printf("exited, status=%d\n", WEXITSTATUS(wstatus));
               } else if (WIFSIGNALED(wstatus)) {
                   printf("killed by signal %d\n", WTERMSIG(wstatus));
               } else if (WIFSTOPPED(wstatus)) {
                   printf("stopped by signal %d\n", WSTOPSIG(wstatus));
               } else if (WIFCONTINUED(wstatus)) {
           } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));


   _exit(2),  clone(2),   fork(2),   kill(2),   ptrace(2),   sigaction(2),
   signal(2), wait4(2), pthread_create(3), credentials(7), signal(7)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at

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