Some helpers to handle large files with pure C

large_files.c

#define _CRT_SECURE_NO_WARNINGS
#define _FILE_OFFSET_BITS 64
#include <stdint.h>
#include <stdio.h>

/* while these are standard C functions used properly, your platform may require additional macros or
 * settings to enable proper handling of files larger than 2 or 4 GB */
uint64_t large_getsize(FILE* fp)
{
  fseek(fp, 0, SEEK_END);
  uint64_t size = 0;
  enum { gigabyte = 1 << 30 };
  while (0 == fseek(fp, -1 * gigabyte, SEEK_CUR))
    size += gigabyte;
  size += ftell(fp);
  rewind(fp);
  return size;
}

void large_absseek(FILE* fp, uint64_t off)
{
  rewind(fp);
  enum { gigabyte = 1 << 30 };
  while (off > gigabyte)
  {
    fseek(fp, gigabyte, SEEK_CUR);
    off -= gigabyte;
  }
  fseek(fp, off, SEEK_CUR);
}