On windows _fseeki64 does not seek to SEEK_END correctly for large files.

Question

I have reduced the problem to the following basic function which should simply print the number of bytes in the file.

When I execute it for a file of 83886080 bytes (80 MB) it prints the correct number. However for a file of 4815060992 bytes (4.48 GB) it prints 520093696 which is way to low.

It seems to have something to do with the SEEK_END option because if I set the pointer to 4815060992 bytes manually (e.g. _fseeki64(fp, (__int64)4815060992, SEEK_SET) _ftelli64 does return the correct position. So a workaround would be to get the proper file size without using SEEK_END, how is this done?

The code is compiled on a 32 bit Windows system (hence __int64, _iseeki64 and _ftelli64) with MinGW.

In short: what am I doing wrong here?

void printbytes(char* filename)
{
  FILE *fp;
  __int64 n;
  int result;

  /* Open file */
  fp = fopen(filename, "rb");
  if (fp == NULL)
  {
    perror("Error: could not open file!\n");
    return -1;
  }

  /* Find end of file */
  result = _fseeki64(fp, (__int64)0, SEEK_END);
  if (result)
  {
    perror("Error: fseek failed!\n");
    return result;
  }

  /* Get number of bytes */
  n = _ftelli64(fp);

  printf("%I64d\n", n);

  /* Close file */
  fclose(fp);
}

Answer 1

In Windows, you should be able to "go native" and just use GetFileSizeEx().

I would also advise you to read the generated code to see if it maybe is some 64-bit confusion that prevents your stdio-based code from working.