waitpid(2)



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

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 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
       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 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     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-
       stants:

       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 call-
              ing 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  WUN-
              TRACED 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  fol-
       lows:

       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 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.

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) 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
       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 (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.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

NOTES
       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():

       __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 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-
              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  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 <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),  sig-
       nal(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/.

Linux                             2017-09-15                           WAIT(2)

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