NAME

       pclose - close a pipe stream to or from a process

SYNOPSIS

       #include <stdio.h>

       int pclose(FILE *stream);

DESCRIPTION

       The  pclose()  function  shall  close  a  stream  that was opened by popen(), wait for the
       command to terminate, and return the termination status of the process  that  was  running
       the  command language interpreter.  However, if a call caused the termination status to be
       unavailable to pclose(), then pclose() shall return -1  with  errno  set  to  [ECHILD]  to
       report  this  situation.  This  can  happen  if the application calls one of the following
       functions:

        * wait()

        * waitpid() with a pid argument less than or equal to 0 or equal to the process ID of the
          command line interpreter

        * Any other function not defined in this volume of IEEE Std 1003.1-2001 that could do one
          of the above

       In any case, pclose() shall not return before the child process  created  by  popen()  has
       terminated.

       If  the  command  language  interpreter  cannot  be executed, the child termination status
       returned by pclose() shall be as if the  command  language  interpreter  terminated  using
       exit(127) or _exit(127).

       The  pclose() function shall not affect the termination status of any child of the calling
       process other than the one created by popen() for the associated stream.

       If the argument stream to pclose() is not a pointer to a stream created  by  popen(),  the
       result of pclose() is undefined.

RETURN VALUE

       Upon  successful  return,  pclose()  shall  return  the  termination status of the command
       language interpreter. Otherwise, pclose() shall return -1 and set errno  to  indicate  the
       error.

ERRORS

       The pclose() function shall fail if:

       ECHILD The status of the child process could not be obtained, as described above.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       There  is a requirement that pclose() not return before the child process terminates. This
       is intended to disallow implementations that return [EINTR] if a signal is received  while
       waiting.  If  pclose() returned before the child terminated, there would be no way for the
       application to discover which child used to be associated with the stream,  and  it  could
       not do the cleanup itself.

       If  the stream pointed to by stream was not created by popen(), historical implementations
       of pclose() return -1 without setting errno.  To avoid requiring pclose() to set errno  in
       this case, IEEE Std 1003.1-2001 makes the behavior unspecified.  An application should not
       use pclose() to close any stream that was not created by popen().

       Some historical implementations of pclose() either block or  ignore  the  signals  SIGINT,
       SIGQUIT,  and SIGHUP while waiting for the child process to terminate. Since this behavior
       is not described for the pclose() function in IEEE Std 1003.1-2001,  such  implementations
       are  not  conforming.  Also, some historical implementations return [EINTR] if a signal is
       received, even though the child process has not terminated.  Such implementations are also
       considered non-conforming.

       Consider, for example, an application that uses:

              popen("command", "r")

       to start command, which is part of the same application. The parent writes a prompt to its
       standard output (presumably the terminal) and then reads from the  popen()ed  stream.  The
       child reads the response from the user, does some transformation on the response (pathname
       expansion, perhaps) and writes the result to its  standard  output.   The  parent  process
       reads  the  result  from  the pipe, does something with it, and prints another prompt. The
       cycle repeats. Assuming that both processes do appropriate buffer flushing, this would  be
       expected to work.

       To conform to IEEE Std 1003.1-2001, pclose() must use waitpid(), or some similar function,
       instead of wait().

       The code sample below illustrates how the pclose() function  might  be  implemented  on  a
       system conforming to IEEE Std 1003.1-2001.

              int pclose(FILE *stream)
              {
                  int stat;
                  pid_t pid;

                  pid = <pid for process created for stream by popen()>
                  (void) fclose(stream);
                  while (waitpid(pid, &stat, 0) == -1) {
                      if (errno != EINTR){
                          stat = -1;
                          break;
                      }
                  }
                  return(stat);
              }

FUTURE DIRECTIONS

       None.

SEE ALSO

       fork()  ,  popen()  ,  waitpid()  ,  the  Base Definitions volume of IEEE Std 1003.1-2001,
       <stdio.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2003  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .