C Standard Library

C <stdio.h> - setvbuf() Function

The C <stdio.h> setvbuf() function is used to change the buffering mode of the given file stream stream. In addition, the function allows to specify the mode and size of the buffer (in bytes). If buffer is a null pointer, the function automatically resizes the internal buffer to size. Otherwise, it instructs the stream to use the user-provided buffer of size size beginning at buffer.

This function should be called after the stream pointed to by stream is associated with an open file but before any input or output operation is performed on the stream.

Syntax

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

Parameters

stream Specify a pointer to a FILE object that identifies an open stream.

buffer Specify pointer to a buffer for the stream to use or null pointer to change size and mode only.

mode

Specify a mode for file buffering. It can be one of the following macro constants:

Macros	Description
_IOFBF	`Full buffering` - On output, data is written once the buffer is full (or flushed). On Input, the buffer is filled when an input operation is requested and the buffer is empty.
_IOLBF	`Line buffering` - On output, data is written when a newline character is inserted into the stream or when the buffer is full (or flushed), whatever happens first. On Input, the buffer is filled up to the next newline character when an input operation is requested and the buffer is empty.
_IONBF	`No buffering` - No buffer is used. Each I/O operation is written as soon as possible. In this case, the buffer and size parameters are ignored.

size Specify size of the buffer.

Return Value

Returns 0 on success. Otherwise returns a non-zero value.

Example:

The example below describes the usage of setvbuf() function.

#include <stdio.h>
#include <stdlib.h>

int main() {
  char buf[50];

  if(setvbuf(stdout, buf, _IOFBF, sizeof buf)) {
    perror("Failed to change the buffer of stdout");
    return EXIT_FAILURE;
  }
  
  printf("Programming"); //The buffer contains "Programming" 
  //but nothing is written to stdout yet
  fflush(stdout);  //Now "Programming" is written to stdout

  if(setvbuf(stdout, NULL, _IONBF, 0)) {
    perror("Failed to change the buffer of stdout");
    return EXIT_FAILURE;
  }

  printf(" is"); //" is" is written to stdout
  //there is no buffering

  if(setvbuf(stdout, buf, _IOLBF, sizeof buf)) {
    perror("Failed to change the buffer of stdout");
    return EXIT_FAILURE;
  }
  printf(" fun."); //The buffer now contains " fun."
  //but nothing is written to stdout yet
  putchar('\n'); //stdout is line buffered 
  //everything in the buffer is now written to 
  //stdout along with the newline

  return EXIT_SUCCESS;
}

The output of the above code will be:

Programming is fun.

❮ C <stdio.h> Library