`setbuf`

Article
12/02/2022

Controls stream buffering. This function is deprecated; use setvbuf instead.

Syntax

void setbuf(
   FILE *stream,
   char *buffer
);

Parameters

stream
Pointer to FILE structure.

buffer
User-allocated buffer.

Remarks

The setbuf function controls buffering for stream. The stream argument must refer to an open file that hasn't been read or written. If the buffer argument is NULL, the stream is unbuffered. If not, the buffer must point to a character array of length BUFSIZ, where BUFSIZ is the buffer size as defined in STDIO.H. The user-specified buffer, instead of the default system-allocated buffer for the given stream, is used for I/O buffering. The stderr stream is unbuffered by default, but you can use setbuf to assign buffers to stderr.

setbuf has been replaced by setvbuf, which is the preferred routine for new code. Unlike setvbuf, setbuf has no way of reporting errors. setvbuf also lets you control both the buffering mode and the buffer size. setbuf exists for compatibility with existing code.

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
`setbuf`	<stdio.h>

For more compatibility information, see Compatibility.

Example

// crt_setbuf.c
// compile with: /W3
// This program first opens files named DATA1 and
// DATA2. Then it uses setbuf to give DATA1 a user-assigned
// buffer and to change DATA2 so that it has no buffer.

#include <stdio.h>

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

   fopen_s( &stream1, "data1", "a" );
   fopen_s( &stream2, "data2", "w" );

   if( (stream1 != NULL) && (stream2 != NULL) )
   {
      // "stream1" uses user-assigned buffer:
      setbuf( stream1, buf ); // C4996
      // Note: setbuf is deprecated; consider using setvbuf instead
      printf( "stream1 set to user-defined buffer at: %Fp\n", buf );

      // "stream2" is unbuffered
      setbuf( stream2, NULL ); // C4996
      printf( "stream2 buffering disabled\n" );
      _fcloseall();
   }
}

stream1 set to user-defined buffer at: 0012FCDC
stream2 buffering disabled