Description

   You can use the setvbuf function after the file is opened but
   before any I/O operations are performed.

   The C RTL provides the following types of ANSI-conforming file
   buffering:

   In line-buffered I/O, characters are buffered in an area of
   memory until a new-line character is seen, at which point the
   appropriate RMS routine is called to transmit the entire buffer.
   Line buffering is more efficient than unbuffered I/O since it
   reduces the system overhead, but it delays the availability of
   the data to the user or disk on output.

   In fully buffered I/O, characters are buffered in an area of
   memory until the buffer is full, regardless of the presence of
   break characters. Full buffering is more efficient than line
   buffering or unbuffered I/O, but it delays the availability of
   output data even longer than line buffering.

   Use the values _IOLBF and _IOFBF defined in <stdio.h> for the
   type argument to specify line-buffered and fully buffered I/O,
   respectively.

   If file_ptr specifies a terminal device, the C RTL uses
   line-buffered I/O; otherwise, it uses fully buffered I/O.

   Please note that the previously documented value _IONBF is not
   supported.

   The C RTL automatically allocates a buffer to use for each
   I/O stream, so there are several buffer allocation possibilities:

   o  If buffer is not a NULL pointer and size is not smaller than
      the automatically allocated buffer, then setvbuf uses buffer
      as the file buffer.

   o  If buffer is a NULL pointer or size is smaller than the
      automatically allocated buffer, the automatically allocated
      buffer is used as the buffer area.

   o  If buffer is a NULL pointer and size is larger than the
      automatically allocated buffer, then setvbuf allocates a new
      buffer equal to the specified size and uses that as the file
      buffer.

   User programs must not depend on the contents of buffer once I/O
   has been performed on the stream. The C RTL might or might
   not use buffer for any given I/O operation.

   Generally, it is unnecessary to use setvbuf or setbuf to control
   the buffer size used by the C RTL. The automatically
   allocated buffer sizes are chosen for efficiency based on the
   kind of I/O operations performed and the device characteristics
   (such as terminal, disk, or socket).

   The setvbuf and setbuf functions are useful to introduce
   buffering for improved performance when writing a large amount of
   text to the stdout stream. This stream is unbuffered by default
   when bound to a terminal device (the normal case), and therefore
   incurs a large number of OpenVMS buffered I/O operations unless
   C RTL buffering is introduced by a call to setvbuf or
   setbuf.

   The setvbuf function is used only to control the buffering used
   by the C RTL, not the buffering used by the underlying RMS
   I/O operations. You can modify RMS default buffering behavior by
   specifying various values for the ctx, fop, rat, gbc, mbc, mbf,
   rfm, and rop RMS keywords when the file is opened by the creat,
   freopen or open functions.