Description

   The popen function creates a pipe between the calling program
   and a shell command awaiting execution. It returns a pointer to a
   FILE structure for the stream.

   The popen function uses the value of the DECC$PIPE_BUFFER_SIZE
   feature logical to set the buffer size of the mailbox it creates
   for the pipe. You can specify a DECC$PIPE_BUFFER_SIZE value of
   512 to 65024 bytes. If DECC$PIPE_BUFFER_SIZE is not specified,
   the default buffer size of 512 is used.

                                 NOTES

      o  When you use the popen function to invoke an output
         filter, beware of possible deadlock caused by output
         data remaining in the program buffer. You can avoid this
         by either using the setvbuf function to ensure that the
         output stream is unbuffered, or the fflush function to
         ensure that all buffered data is flushed before calling
         the pclose function.

      o  For added UNIX portability, you can use the following
         feature logicals to control the behavior of the C RTL
         pipe implementation:

         -  Define the DECC$STREAM_PIPE feature logical name to
            ENABLE to direct the pipe function to use stream I/O
            instead of record I/O.

         -  Define the DECC$POPEN_NO_CRLF_REC_ATTR feature logical
            to ENABLE to prevent CR/LF carriage control from being
            added to pipe records for pipes opened with the popen
            function. Be aware that enabling this feature might
            result in undesired behavior from other functions such
            as gets that rely on the carriage-return character.

   See also fflush, pclose, and setvbuf.