man_2_waitpid.txt

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 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 thread until one of its  chil‐
       dren terminates.  The call wait(&wstatus) is equivalent to:

           waitpid(-1, &wstatus, 0);

       The waitpid() system call suspends execution of the calling thread until a child speci‐
       fied by pid argument has changed state.  By default, waitpid() waits  only  for  termi‐
       nated  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  abso‐
              lute 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 at the time of the call to waitpid().

       > 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  wait‐
       pid()!):

       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 (see core(5)).  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_PIDFD (since Linux 5.4)
              Wait for the child referred to by the PID file descriptor specified in id.  (See
              pidfd_open(2) for further information on PID file descriptors.)

       idtype == P_PGID
              Wait for any child whose process group ID matches id.  Since Linux 5.4, if id is
              zero, then wait for any child that is in the same process group as the  caller's
              process group at the time of the call.

       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  struc‐
       ture 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 imple‐
                   mentations.)

       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 re‐
       turned.

       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 sig‐
              nal(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 sta‐
       tus, 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  ta‐
       ble  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 be‐
       haves 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 pre‐
       scribes 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 cre‐
       ates 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 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), core(5), credentials(7), signal(7)

COLOPHON
       This page is part of release 5.05 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                                     2019-11-19                                   WAIT(2)