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wait(2) - wait, waitpid, wait, waitpid - wait for process termination - man 2 wait

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



NAME
       wait, waitpid - wait for process termination

SYNOPSIS
       #include <sys/types.h>
       #include <sys/wait.h>

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

DESCRIPTION
       The  wait  function  suspends  execution of the current process until a
       child has exited, or until a signal(2,7) is delivered  whose  action  is  to
       terminate  the  current  process or to call a signal(2,7) handling function.
       If a child has already exited by the time(1,2,n)  of  the  call  (a  so-called
       "zombie"  process),  the  function  returns  immediately.   Any  system
       resources used by the child are freed.

       The waitpid function suspends execution of the current process until  a
       child as specified by the pid argument has exited, or until a signal(2,7) is
       delivered whose action is to terminate the current process or to call a
       signal(2,7)  handling  function.  If a child as requested by pid has already
       exited by the time(1,2,n) of the call  (a  so-called  "zombie"  process),  the
       function  returns  immediately.  Any system resources used by the child
       are freed.

       The value of pid can be one of:

       < -1   which means to wait for any child process whose process group ID
              is equal to the absolute value of pid.

       -1     which  means to wait for any child process; this is the same be-
              haviour which wait exhibits.

       0      which means to wait for any child process whose process group ID
              is equal to that of the calling process.

       > 0    which  means  to 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
              which means to return immediately if(3,n) no child has exited.

       WUNTRACED
              which  means  to also return for children which are stopped (but
              not traced), and whose status has not been reported.  Status for
              traced  children which are stopped is provided also without this
              option.

       (For Linux-only options, see below.)

       If status is not NULL, wait or waitpid store status information in(1,8)  the
       location pointed to by status.

       This  status  can  be evaluated with the following macros (these macros
       take the stat(1,2) buffer (an int) as an argument -- not a  pointer  to  the
       buffer!):

       WIFEXITED(status)
              returns true if(3,n) the child terminated normally, that is, by call-
              ing exit(3,n,1 builtins)() or _exit(), or by returning from main().

       WEXITSTATUS(status)
              evaluates to the least significant eight bits of the return code
              of  the  child  which terminated, which may have been set(7,n,1 builtins) as the
              argument to a call to exit(3,n,1 builtins)() or _exit() or as the argument for a
              return  statement  in(1,8)  the main program.  This macro can only be
              evaluated if(3,n) WIFEXITED returned true.

       WIFSIGNALED(status)
              returns true if(3,n) the child process terminated because of a signal(2,7)
              which was not caught.

       WTERMSIG(status)
              returns  the  number of the signal(2,7) that caused the child process
              to terminate. This macro can only be  evaluated  if(3,n)  WIFSIGNALED
              returned non-zero.

       WIFSTOPPED(status)
              returns  true  if(3,n)  the  child process which caused the return is
              currently stopped; this is only possible if(3,n) the  call  was  done
              using   WUNTRACED  or  when  the  child  is  being  traced  (see
              ptrace(2)).

       WSTOPSIG(status)
              returns the number of the signal(2,7) which caused the child to stop.
              This   macro  can  only  be  evaluated  if(3,n)  WIFSTOPPED  returned
              non-zero.

       Some versions of Unix (e.g. Linux, Solaris, but not  AIX,  SunOS)  also
       define  a  macro  WCOREDUMP(status)  to  test whether the child process
       dumped core. Only use this enclosed in(1,8) #ifdef WCOREDUMP ... #endif.

RETURN VALUE
       The process ID of the child which exited, or zero if(3,n) WNOHANG  was  used
       and  no child was available, or -1 on error(8,n) (in(1,8) which case errno is set(7,n,1 builtins)
       to an appropriate value).

ERRORS
       ECHILD (for wait)
              The calling process does not have any unwaited-for children.

       ECHILD (for waitpid)
              The process specified in(1,8) pid does not exist or is not a child of
              the  calling  process.   (This can happen for one's own child if(3,n)
              the action for SIGCHLD is set(7,n,1 builtins) to SIG_IGN.  See  also  the  LINUX
              NOTES section about threads.)

       EINTR  WNOHANG  was  not  set(7,n,1 builtins)  and an unblocked signal(2,7) or a SIGCHLD was
              caught.

       EINVAL The options argument was invalid.

NOTES
       The Single Unix Specification describes a flag SA_NOCLDWAIT  (not  sup-
       ported under Linux) such that if(3,n) either this flag is set(7,n,1 builtins), or the action
       for SIGCHLD is set(7,n,1 builtins) to SIG_IGN then children that  exit(3,n,1 builtins)  do  not  become
       zombies and a call to wait() or waitpid() will block until all children
       have exited, and then fail with errno set(7,n,1 builtins) to ECHILD.

       The original POSIX standard left the behaviour of  setting  SIGCHLD  to
       SIG_IGN  unspecified.   Later  standards,  including  SUSv2  and  POSIX
       1003.1-2001 specify the behaviour just described as  an  XSI-compliance
       option.   Linux  does  not conform to the second of the two points just
       described: if(3,n) a wait() or waitpid() call is made while SIGCHLD is being
       ignored, the call behaves just as though SIGCHLD were not being igored,
       that is, the call blocks until  the  next  child  terminates  and  then
       returns the PID 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(1,8) the same thread group.

       The following Linux-specific options are for use with children  created
       using clone(2).

       __WCLONE
              Wait  for "clone" children only.  If omitted then wait for "non-
              clone" children only.  (A "clone" child is one which delivers no
              signal(2,7), or a signal(2,7) other than SIGCHLD to its parent upon termi-
              nation.)  This option is ignored if(3,n) __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(1,8)
              the same thread group. This was the default before Linux 2.4.

CONFORMING TO
       SVr4, POSIX.1

SEE ALSO
       clone(2), ptrace(2), signal(2,7)(2), wait4(2), pthread_create(3), signal(2,7)(7)



Linux                             2000-07-24                           WAIT(2)

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