Visual Studio 6.0

Controls stream buffering and buffer size.

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

Routine Required Header Compatibility
setvbuf <stdio.h> ANSI, Win 95, Win NT

For additional compatibility information, see Compatibility in the Introduction.


LIBC.LIB Single thread static library, retail version
LIBCMT.LIB Multithread static library, retail version
MSVCRT.LIB Import library for MSVCRT.DLL, retail version

Return Value

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



Pointer to FILE structure


User-allocated buffer


Mode of buffering


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


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:


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.


With MS-DOS, the same as _IOFBF.


No buffer is used, regardless of buffer or size.


/* 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" );
         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" );
         printf( "'stream2' now has no buffer\n" );


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

Stream I/O Routines

See Also   fclose, fflush, fopen, setbuf