setvbuf

Controls stream buffering and buffer size.

int setvbuf( FILE *stream, char *buffer, int mode, size_t size );

For additional compatibility information, see Compatibility in the Introduction.

Libraries

Return Value

setvbuf returns 0 if successful, or a nonzero value if an illegal type or buffer size is specified.

Parameters

stream

Pointer to FILE structure

buffer

User-allocated buffer

mode

Mode of buffering

size

Buffer size in bytes. Allowable range: 2 < size < 32768. Internally, the value supplied for size is rounded down to the nearest multiple of 2.

Remarks

The setvbuf function allows the program to control both buffering and buffer size for stream. stream must refer to an open file that has not undergone an I/O operation since it was opened. The array pointed to by buffer is used as the buffer, unless it is NULL, in which case setvbuf uses an automatically allocated buffer of length size/2 * 2 bytes.

The mode must be _IOFBF, _IOLBF, or _IONBF. If mode is _IOFBF or _IOLBF, then size is used as the size of the buffer. If mode is _IONBF, the stream is unbuffered and size and buffer are ignored. Values for mode and their meanings are:

_IOFBF

Full buffering; that is, buffer is used as the buffer and size is used as the size of the buffer. If buffer is NULL, an automatically allocated buffer size bytes long is used.

_IOLBF

With MS-DOS, the same as _IOFBF.

_IONBF

No buffer is used, regardless of buffer or size.

Example

/* SETVBUF.C: This program opens two streams: stream1
 * and stream2. It then uses setvbuf to give stream1 a
 * user-defined buffer of 1024 bytes and stream2 no buffer.
 */

#include <stdio.h>

void main( void )
{
   char buf[1024];
   FILE *stream1, *stream2;

   if( ((stream1 = fopen( "data1", "a" )) != NULL) &&
       ((stream2 = fopen( "data2", "w" )) != NULL) )
   {
      if( setvbuf( stream1, buf, _IOFBF, sizeof( buf ) ) != 0 )
         printf( "Incorrect type or size of buffer for stream1\n" );
      else
         printf( "'stream1' now has a buffer of 1024 bytes\n" );
      if( setvbuf( stream2, NULL, _IONBF, 0 ) != 0 )
         printf( "Incorrect type or size of buffer for stream2\n" );
      else
         printf( "'stream2' now has no buffer\n" );
      _fcloseall();
   }
}

Output

'stream1' now has a buffer of 1024 bytes
'stream2' now has no buffer

Stream I/O Routines

See Also   fclose, fflush, fopen, setbuf