`setvbuf`

Article
2022-12-02

Controls stream buffering and buffer size.

Syntax

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

Parameters

stream
Pointer to FILE structure.

buffer
User-allocated buffer.

mode
Mode of buffering.

size
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 isn't 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 errno, _doserrno, _sys_errlist, and _sys_nerr.

Remarks

The setvbuf function allows the program to control both buffering and buffer size for stream. stream must refer to an open file that hasn't undergone an I/O operation since it was opened. The array pointed to by buffer is used as the buffer, unless buffer 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 both 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`, this mode uses an automatically allocated buffer that's `size` bytes long.
`_IOLBF`	For some systems, this mode 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 behavior, see Global state in the CRT.

Requirements

Routine	Required header
`setvbuf`	<stdio.h>

For more compatibility information, see Compatibility.

Libraries

All versions of the C run-time libraries.

Example

// 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" );
      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();
   }
}

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

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