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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 *status);

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

       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);

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

       waitid(): _SVID_SOURCE || _XOPEN_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  per-
       formed, 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(&status) is equivalent to:

           waitpid(-1, &status, 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 status 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(status)
              returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by  returning
              from main().

       WEXITSTATUS(status)
              returns the exit status of the child.  This consists of the least significant 8 bits of the status argu-
              ment 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 only be employed if WIFEXITED returned true.

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

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

       WCOREDUMP(status)
              returns true if the child produced a core dump.  This macro  should  only  be  employed  if  WIFSIGNALED
              returned  true.  This macro is not specified in POSIX.1-2001 and is not available on some Unix implemen-
              tations (e.g., AIX, SunOS).  Only use this enclosed in #ifdef WCOREDUMP ... #endif.

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

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

       WIFCONTINUED(status)
              (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 sta-
                   tus 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
       non-zero value in this field after the call returns.

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(8), which 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 wait-
       pid() 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  treat-
       ment  of zombie process children.)  Linux 2.6 conforms to this specification.  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 cannot 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.

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  exe-
       cutes  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 status;

           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, &status, WUNTRACED | WCONTINUED);
                   if (w == -1) {
                       perror("waitpid");
                       exit(EXIT_FAILURE);
                   }

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

SEE ALSO
       _exit(2), clone(2), fork(2), kill(2), ptrace(2), sigaction(2), signal(2), wait4(2), pthread_create(3),  creden-
       tials(7), signal(7)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-04-21                           WAIT(2)