Controls stream buffering and buffer size.


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


Pointer to FILE structure.

User-allocated buffer.

Mode of buffering.

Buffer size in bytes. Allowable range: 2 <= size <= INT_MAX (2147483647). Internally, the value supplied for size is rounded down to the nearest multiple of 2.

Return Value

Returns 0 if successful.

If stream is NULL, or if mode or size is not within a valid change, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, this function returns -1 and sets errno to EINVAL.

For information on these and other error codes, see _doserrno, errno, _sys_errlist, and _sys_nerr.


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:

mode value Meaning
_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 For some systems, this provides line buffering. However, for Win32, the behavior is the same as _IOFBF - Full Buffering.
_IONBF No buffer is used, regardless of buffer or size.

By default, this function's global state is scoped to the application. To change this, see Global state in the CRT.


Routine Required header
setvbuf <stdio.h>

For additional compatibility information, see Compatibility.


All versions of the C run-time libraries.


// crt_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>

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

   if( fopen_s( &stream1, "data1", "a" ) == 0 &&
       fopen_s( &stream2, "data2", "w" ) == 0 )
      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

See also

Stream I/O
fclose, _fcloseall
fopen, _wfopen