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WAIT(2) Linux Programmer's Manual WAIT(2)


     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 2.26: _XOPEN_SOURCE >= 500 ||
             _POSIX_C_SOURCE >= 200809L
         Glibc 2.25 and earlier:
                 || /* 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 ter-
     minated; 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  immedi-
     ately.   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
 wait() and waitpid()
     The wait() system call suspends execution of the calling  thread  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  thread
     until a child specified by pid argument has changed state.  By default,
     waitpid() waits only for terminated children, but this behavior is mod-
     ifiable 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. 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  con-
     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 call-
            ing 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  WUN-
            TRACED 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 fol-
     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 termi-
                 nate, 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 implementa-
     tion.  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.


     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) speci-
     fied  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  (wait-
            pid())  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 "zom-
     bie".  The kernel maintains a minimal set of information about the zom-
     bie  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 set-
     ting the disposition to SIG_IGN results in different treatment of  zom-
     bie 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 con-
     struct 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 termi-
            nation.)  This option is ignored if __WALL is also specified.
     __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 immedi-
     ately, using the integer supplied on the command line as the exit  sta-
     tus.   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
     int 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),  sig-
     nal(2), wait4(2), pthread_create(3), credentials(7), signal(7)


     This  page  is  part of release 4.16 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

Linux 2018-04-30 WAIT(2)

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