fwrite

Binary stream input/output

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Contents

Interface
Description
Return Values
Page Comments

Interface

#include <stdio.h>size_t  fread (void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream)
size_t  fwrite (const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream)

Description

The function fread reads nmemb objects, each size bytes long, from the stream pointed to by stream, storing them at the location given by ptr.

The function fwrite writes nmemb objects, each size bytes long, to the stream pointed to by stream, obtaining them from the location given by ptr.

The following code attempts at reading 10 bytes at once from the text file "fred.txt", then it stores them into buffer and finally it displays this string.

Example:

Example - Binary stream input/output

Workings

#include <stdio.h>
int main()
{
  FILE *f;
  char buffer[11];
  if (f = fopen("fred.txt", "rt"))
  {
    fread(buffer, 1, 10, f);
    buffer[10] = 0;
    fclose(f);
    printf("first 10 characters of the file:\n%s\n", buffer);
  }
  return 0;
}

Return Values

The functions fread and fwrite advance the file position indicator for the stream by the number of bytes read or written. They return the number of objects read or written. If an error occurs, or the end-of-file is reached, the return value is a short object count (or zero).

The function fread does not distinguish between end-of-file and error, and callers must use reference:ferror to determine which occurred. The function fwrite returns a value less than nmemb only if a write error has occurred.

size_t	`fread` (void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream)
size_t	`fwrite` (const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream)