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wait, waitpid, wait4, wait3 - wait for process termination

 

NAME

      wait, waitpid, wait4, wait3 - wait for process termination
 

LIBRARY

      Standard C Library (libc, -lc)
 

SYNOPSIS

      #include <sys/types.h>
      #include <sys/wait.h>
 
      pid_t
      wait(int *status);
 
      #include <sys/time.h>
      #include <sys/resource.h>
 
      pid_t
      waitpid(pid_t wpid, int *status, int options);
 
      pid_t
      wait3(int *status, int options, struct rusage *rusage);
 
      pid_t
      wait4(pid_t wpid, int *status, int options, struct rusage *rusage);
 

DESCRIPTION

      The wait() function suspends execution of its calling process until
      status information is available for a terminated child process, or a sig‐
      nal is received.  On return from a successful wait() call, the status
      area contains termination information about the process that exited as
      defined below.
 
      The wait4() system call provides a more general interface for programs
      that need to wait for certain child processes, that need resource uti‐
      lization statistics accumulated by child processes, or that require
      options.  The other wait functions are implemented using wait4().
 
      The wpid argument specifies the set of child processes for which to wait.
      If wpid is -1, the call waits for any child process.  If wpid is 0, the
      call waits for any child process in the process group of the caller.  If
      wpid is greater than zero, the call waits for the process with process id
      wpid.  If wpid is less than -1, the call waits for any process whose pro‐
      cess group id equals the absolute value of wpid.
 
      The status argument is defined below.  The options argument contains the
      bitwise OR of any of the following options.  The WCONTINUED option indi‐
      cates that children of the current process that have continued from a job
      control stop, by receiving a SIGCONT signal, should also have their sta‐
      tus reported.  The WNOHANG option is used to indicate that the call
      should not block if there are no processes that wish to report status.
      If the WUNTRACED option is set, children of the current process that are
      stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal also have
      their status reported.
 
      If rusage is non-zero, a summary of the resources used by the terminated
      process and all its children is returned (this information is currently
      not available for stopped or continued processes).
 
      When the WNOHANG option is specified and no processes wish to report sta‐
      tus, wait4() returns a process id of 0.
 
      The waitpid() function is identical to wait4() with an rusage value of
      zero.  The older wait3() call is the same as wait4() with a wpid value of
      -1.
 
      The following macros may be used to test the manner of exit of the pro‐
      cess.  One of the first three macros will evaluate to a non-zero (true)
      value:
 
      WIFCONTINUED(status)
              True if the process has not terminated, and has continued after a
              job control stop.  This macro can be true only if the wait call
              specified the WCONTINUED option).
 
      WIFEXITED(status)
              True if the process terminated normally by a call to _exit(2) or
              exit(3).
 
      WIFSIGNALED(status)
              True if the process terminated due to receipt of a signal.
 
      WIFSTOPPED(status)
              True if the process has not terminated, but has stopped and can
              be restarted.  This macro can be true only if the wait call spec‐
              ified the WUNTRACED option or if the child process is being
              traced (see ptrace(2)).
 
      Depending on the values of those macros, the following macros produce the
      remaining status information about the child process:
 
      WEXITSTATUS(status)
              If WIFEXITED(status) is true, evaluates to the low-order 8 bits
              of the argument passed to _exit(2) or exit(3) by the child.
 
      WTERMSIG(status)
              If WIFSIGNALED(status) is true, evaluates to the number of the
              signal that caused the termination of the process.
 
      WCOREDUMP(status)
              If WIFSIGNALED(status) is true, evaluates as true if the termina‐
              tion of the process was accompanied by the creation of a core
              file containing an image of the process when the signal was
              received.
 
      WSTOPSIG(status)
              If WIFSTOPPED(status) is true, evaluates to the number of the
              signal that caused the process to stop.
 

NOTES

      See sigaction(2) for a list of termination signals.  A status of 0 indi‐
      cates normal termination.
 
      If a parent process terminates without waiting for all of its child pro‐
      cesses to terminate, the remaining child processes are assigned the par‐
      ent process 1 ID (the init process ID).
 
      If a signal is caught while any of the wait() calls are pending, the call
      may be interrupted or restarted when the signal-catching routine returns,
      depending on the options in effect for the signal; see discussion of
      SA_RESTART in sigaction(2).
 
      The implementation queues one SIGCHLD signal for each child process whose
      status has changed, if wait() returns because the status of a child pro‐
      cess is available, the pending SIGCHLD signal associated with the process
      ID of the child process will be discarded.  Any other pending SIGCHLD
      signals remain pending.
 
      If SIGCHLD is blocked, wait() returns because the status of a child pro‐
      cess is available, the pending SIGCHLD signal will be cleared unless
      another status of the child process is available.
      If wait() returns due to a stopped, continued, or terminated child pro‐
      cess, the process ID of the child is returned to the calling process.
      Otherwise, a value of -1 is returned and errno is set to indicate the
      error.
 
      If wait4(), wait3(), or waitpid() returns due to a stopped, continued, or
      terminated child process, the process ID of the child is returned to the
      calling process.  If there are no children not previously awaited, -1 is
      returned with errno set to ECHILD.  Otherwise, if WNOHANG is specified
      and there are no stopped, continued or exited children, 0 is returned.
      If an error is detected or a caught signal aborts the call, a value of -1
      is returned and errno is set to indicate the error.
 

ERRORS

      The wait() function will fail and return immediately if:
 
      [ECHILD]           The calling process has no existing unwaited-for child
                         processes.
 
      [ECHILD]           No status from the terminated child process is avail‐
                         able because the calling process has asked the system
                         to discard such status by ignoring the signal SIGCHLD
                         or setting the flag SA_NOCLDWAIT for that signal.
 
      [EFAULT]           The status or rusage argument points to an illegal
                         address.  (May not be detected before exit of a child
                         process.)
 
      [EINTR]            The call was interrupted by a caught signal, or the
                         signal did not have the SA_RESTART flag set.
      _exit(2), ptrace(2), sigaction(2), exit(3), siginfo(3)
 

STANDARDS

      The wait() and waitpid() functions are defined by POSIX; wait4() and
      wait3() are not specified by POSIX.  The WCOREDUMP() macro and the abil‐
      ity to restart a pending wait() call are extensions to the POSIX inter‐
      face.
 

HISTORY

      The wait() function appeared in Version 6 AT&T UNIX.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.