What is setvbuf in C?

setvbuf is a C library function that defines how a stream is buffered according to the mode provided in the function’s argument.

Syntax

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

Parameters

stream is the pointer to a file object to which the buffer is set.
buffer is a pointer to the user allocated buffer. If set to null, the function automatically allocates the buffer of the size provided.
mode is the buffering mode to use. They are listed down below:

Mode	Description
_IOFBF	Full Buffering - Data is written to output once the buffer is full. The buffer accepts input when it is empty, and an input operation is requested.
_IOLBF	Line Buffering - Data is written on output when a newline is inserted in the stream or the buffer gets full. The buffer accepts input till the next newline when it is empty, and an input operation is requested.
_IONBF	No Buffering - No buffer is used. I/O operations are written as they arrive. `buffer` and `size` parameters are ignored.

size is the buffer size in bytes.

Return value

The function returns 0 on success and a non-zero value on failure.

Example

A file called myFile.txt is made in the program below, and a buffer of 1024 bytes is requested for the file’s stream. As shown in line 10, we have used a full buffer which means that the output to this stream is only written to the file once the buffer is full:

#include<stdio.h>
#include <string.h>
int main() {
  char buffer[1024];
  char readBuffer[1024];
  memset( buffer, '\0', sizeof( buffer )); //Initializing the buffer
  FILE *pFile;
  pFile = fopen("myFile.txt","w");
  setvbuf( pFile , buffer , _IOFBF , 1024 );
  // File operations here
  if (pFile == NULL) {
        printf("Error!");
  }
  fprintf(pFile, "%s","HelloWorld!");
  fclose (pFile);
  pFile = fopen("myFile.txt", "r");
  if (pFile == NULL) {
    printf("File does not exist.\n");
  }
  fscanf(pFile, "%s", &readBuffer);
  printf("File contains the following strings: %s", readBuffer);
  return(0);
}

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