setbuf

Stream buffering operations

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Contents

Interface
Description
Return Values
See Also
Bugs
Page Comments

Interface

#include <stdio.h>void  setbuf (FILE * restrict stream, char * restrict buf)
void  setbuffer (FILE *stream, char *buf, int size)
int  setlinebuf (FILE *stream)
int  setvbuf (FILE * restrict stream, char * restrict buf, int mode, size_t size)

Description

The three types of buffering available are unbuffered, block buffered, and line buffered. When an output stream is unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered many characters are saved up and written as a block; when it is line buffered characters are saved up until a newline is output or input is read from any stream attached to a terminal device (typically stdin). The function fflush may be used to force the block out early. See also fclose.

Normally all files are block buffered. When the first I/O operation occurs on a file, malloc is called, and an optimally-sized buffer is obtained. If a stream refers to a terminal (as stdout normally does) it is line buffered. The standard error stream stderr is always unbuffered.

The code below creates a buffered stream and writes some text to it.

Example 1

#include <stdio.h>
 
int main()
{
  FILE *f = fopen("fred.txt", "wt");
 
  // make f a buffered stream
  char buffer[100];
  setbuf(f, buffer);
 
  // write some data to the buffer
  fputs("blar blar blar", f);
 
  // flush the buffer into the stream
  // (no data is written to the file until the first call to fflush)
  fflush(f);
 
  fclose(f);
  return 0;
}

The setvbuf function may be used to alter the buffering behavior of a stream. The mode argument must be one of the following three macros: <table cellspacing="0" class="refpage" style="margin-left:25px"> <tr> <td valign="top" nowrap> _IONBF </td> <td valign="top"> unbuffered </td> </tr> <tr> <td valign="top" nowrap> _IOLBF </td> <td valign="top"> line buffered </td> </tr> <tr> <td valign="top" nowrap> _IOFBF </td> <td valign="top"> fully buffered </td> </tr> </table>

The size argument may be given as zero to obtain deferred optimal-size buffer allocation as usual. If it is not zero, then except for unbuffered files, the buf argument should point to a buffer at least ;c size bytes long; this buffer will be used instead of the current buffer. If buf is not NULL, it is the caller's responsibility to free this buffer after closing the stream. (If the size argument is not zero but buf is NULL, a buffer of the given size will be allocated immediately, and released on close. This is an extension to ANSI C; portable code should use a size of 0 with any NULL buffer.)

The setvbuf function may be used at any time, but may have peculiar side effects (such as discarding input or flushing output) if the stream is 'active'. Portable applications should call it only once on any given stream, and before any I/O is performed.

The other three calls are, in effect, simply aliases for calls to setvbuf . Except for the lack of a return value, the setbuf function is exactly equivalent to the call

setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

The setbuffer function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default BUFSIZ. The setlinebuf function is exactly equivalent to the call:

setvbuf(stream, (char *)NULL, _IOLBF, 0);

Return Values

The setvbuf function returns 0 on success, or EOF if the request cannot be honored (note that the stream is still functional in this case).

The setlinebuf function returns what the equivalent setvbuf would have returned.

Bugs

The setbuffer and setlinebuf functions are not portable to versions of BSD before 4.2BSD On 4.2BSD and 4.3BSD systems, setbuf always uses a suboptimal buffer size and should be avoided.

Page Comments

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void	`setbuf` (FILE * restrict stream, char * restrict buf)
void	`setbuffer` (FILE stream, char buf, int size)
int	`setlinebuf` (FILE *stream)
int	`setvbuf` (FILE * restrict stream, char * restrict buf, int mode, size_t size)