man_2_wait.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 re‐
       sources 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 children 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 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 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 in‐
       spected 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 (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).  There‐
              fore, 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 re‐
              turned 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 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  ter‐
                   minate, 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_CON‐
                   TINUED (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().  How‐
       ever, 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 de‐
       fined 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  chil‐
       dren 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  disposi‐
       tion 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 de‐
              livers 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 func‐
       tion 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.  Ap‐
       plications 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), 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)