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waitpid

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 is delivered  whose  action  is  to
       terminate  the  current  process or to call a signal handling function.
       If a child has already exited by the time  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 is
       delivered whose action is to terminate the current process or to call a
       signal  handling  function.  If a child as requested by pid has already
       exited by the time 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
              behaviour 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 no child has exited.

       WUNTRACED
              which  means  to also return for children which are stopped, and
              whose status has not been reported.

       (For Linux-only options, see below.)

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

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

       WIFEXITED(status)
              is non-zero if the child exited normally.

       WEXITSTATUS(status)
              evaluates to the least significant eight bits of the return code
              of the child which terminated, which may have been  set  as  the
              argument  to  a  call  to exit() or as the argument for a return
              statement in the main program.  This macro can only be evaluated
              if WIFEXITED returned non-zero.

       WIFSIGNALED(status)
              returns  true  if  the  child process exited because of a signal
              which was not caught.

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

       WIFSTOPPED(status)
              returns true if the child process which  caused  the  return  is
              currently  stopped;  this  is only possible if the call was done
              using WUNTRACED.

       WSTOPSIG(status)
              returns the number of the signal which caused the child to stop.
              This   macro  can  only  be  evaluated  if  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 #ifdef WCOREDUMP ... #endif.

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

ERRORS
       ECHILD if the process specified in pid 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.)

       EINVAL if the options argument was invalid.

       EINTR  if  WNOHANG was not set and an unblocked signal or a SIGCHLD was
              caught.

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

       __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 termi-
              nation.)  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.

CONFORMING TO
       SVr4, POSIX.1

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



Linux                             2000-07-24                           WAIT(2)