<stdio.h> provides two functions for writing and reading chunks of bytes to and from open streams.

#include <stdio.h>
size_t fread(void *ptr,
             size_t chsz, size_t nch,
             FILE *str);
size_t fwrite(const void *ptr,
              size_t chsz, size_t nch,
              FILE *str);

fread reads nch chunks of chsz bytes each from the stream str, and writes them to the array of at least nch×chsz bytes starting at ptr. Only whole chunks are read, and the number of whole chunks read is returned.

fwrite reads nch chunks of chsz bytes each from the array of at least nch×chsz bytes starting at ptr, and writes them to the stream str. Only whole chunks are written, and the number of whole chunks written is returned.

These functions stop on error or end-of-file, and so feof and ferror should be called to test for these conditions if the return value is less than nch.