I have forgotten
my Password

Or login with:

  • Facebookhttp://facebook.com/
  • Googlehttps://www.google.com/accounts/o8/id
  • Yahoohttps://me.yahoo.com


Stream buffering operations
+ View other versions (4)


#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)


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 <span class="Dv">stdin</span>). The function reference:fflush may be used to force the block out early. See also fclose.

Normally all files are block buffered. When the first <span class="Tn">I/O</span> operation occurs on a file, reference:malloc is called, and an optimally-sized buffer is obtained. If a stream refers to a terminal (as <span class="Dv">stdout</span> normally does) it is line buffered. The standard error stream <span class="Dv">stderr</span> 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)
  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> <span class="Dv">_IONBF</span> </td> <td valign="top"> unbuffered </td> </tr> <tr> <td valign="top" nowrap> <span class="Dv">_IOLBF</span> </td> <td valign="top"> line buffered </td> </tr> <tr> <td valign="top" nowrap> <span class="Dv">_IOFBF</span> </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 <span class="Dv">NULL</span>, 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 <span class="Dv">NULL</span> 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 <span class="Tn">I/O</span> 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 <span class="Dv">BUFSIZ</span>. 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 <span class="Dv">EOF</span> 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.


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.