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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 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
       constants:

       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
       supported 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
       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).

       __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.

CONFORMING TO
       SVr4, POSIX.1

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

Linux                             2000-07-24                           WAIT(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | NOTES | LINUX NOTES | CONFORMING TO | SEE ALSO

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