# wait

## NAME



           wait, waitpid, waitid - wait for process to change state
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/wait.h>**
    
           **pid_t wait(int *_Nullable** _wstatus_**);**
           **pid_t waitpid(pid_t** _pid_**, int *_Nullable** _wstatus_**, int** _options_**);**
    
           **int waitid(idtype_t** _idtype_**, id_t** _id_**, siginfo_t ***_infop_**, int** _options_**);**
                           /* This is the glibc and POSIX interface; see
                              VERSIONS 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 <= 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 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
           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
                  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_ 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_ 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 failure, -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 failure, -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 failure, -1
           is returned.
    
           On failure, each of these calls sets _errno_ to indicate the error.
    


## ERRORS



           **EAGAIN** The PID file descriptor specified in _id_ is nonblocking and
                  the process that it refers to has not terminated.
    
           **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.
    
           **ESRCH**(for **wait**() or **waitpid**()) _pid_ is equal to **INT_MIN**.
    


## VERSIONS



       **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.
    


## STANDARDS



           POSIX.1-2008.
    


## 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 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 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
                  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.
    


## 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.
    


## EXAMPLES



           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 <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/types.h>
           #include <sys/wait.h>
           #include <unistd.h>
    
           int
           main(int argc, char *argv[])
           {
               int    wstatus;
               pid_t  cpid, w;
    
               cpid = fork();
               if (cpid == -1) {
                   perror("fork");
                   exit(EXIT_FAILURE);
               }
    
               if (cpid == 0) {            /* Code executed by child */
                   printf("Child PID is %jd\n", (intmax_t) 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);
               }
           }
    


* * *