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NAME

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

SYNOPSIS

       #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)):

       waitid():
           Since glibc 2.26: _XOPEN_SOURCE >= 500 ||
               _POSIX_C_SOURCE >= 200809L
           Glibc 2.25 and earlier:
               _XOPEN_SOURCE
                   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
                   || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION

       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 to:

           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 constants:

       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()!):

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

       WEXITSTATUS(wstatus)
              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.

       WIFSIGNALED(wstatus)
              returns true if the child process was terminated by a signal.

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

       WCOREDUMP(wstatus)
              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.

       WIFSTOPPED(wstatus)
              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)).

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

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

   waitid()
       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 follows:

       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 signal.

       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  depends on the implementation.  To (portably) 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.

       POSIX.1-2008  Technical  Corrigendum  1  (2013)  adds the requirement that when WNOHANG is
       specified in options and there were no children in a waitable state, then waitid()  should
       zero  out  the  si_pid  and  si_signo  fields  of  the  structure.   On  Linux  and  other
       implementations that adhere to this requirement, it is  not  necessary  to  zero  out  the
       si_pid field before calling waitid().  However, not all implementations follow the POSIX.1
       specification on this point.

RETURN VALUE

       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 returned.

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

ERRORS

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

       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.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       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), (or by the nearest "subreaper" process
       as defined through the use of  the  prctl(2)  PR_SET_CHILD_SUBREAPER  operation);  init(1)
       automatically performs a wait to remove the zombies.

       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():

       __WCLONE
              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
              specified.

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

       __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 wait4(2).

       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.

BUGS

       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.

EXAMPLE

       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
           continued
           $ 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>

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

           cpid = fork();
           if (cpid == -1) {
               perror("fork");
               exit(EXIT_FAILURE);
           }

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

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

                   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)) {
                       printf("continued\n");
                   }
               } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
               exit(EXIT_SUCCESS);
           }
       }

SEE ALSO

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

COLOPHON

       This page is part of release 4.13 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 https://www.kernel.org/doc/man-pages/.