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I was using some code like this to open a 2.4GB file in my 32 bit Ubuntu installation:

#include <stdio.h>
#include <string.h>

int main ()
{
  FILE * pFile;
  pFile = fopen ("unexist.ent","r");
  if (pFile == NULL)
    printf ("Error opening file unexist.ent\\n";
  return 0;
}

But I kept getting a null pointer back.

The first step I did was try to get a more detailed error message. It turns out there’s an errno standard lib that gives you error messages about files. I implemented the changes recommended by this page:

#include <stdio.h>
#include <string.h>
#include <errno.h>

int main ()
{
  FILE * pFile;
  pFile = fopen ("unexist.ent","r");
  if (pFile == NULL)
    printf ("Error opening file unexist.ent: %s\\n",strerror(errno));
  return 0;
}

And I got this message:
Value too large for defined data type

So thanks to advice from the C irc chat room, I switched to using fopen64, and everything worked!

#include <stdio.h>
#include <string.h>
#include <errno.h>

int main ()
{
  FILE * pFile;
  pFile =fopen64("unexist.ent","r");
  if (pFile == NULL)
    printf ("Error opening file unexist.ent: %s\\n",strerror(errno));
  return 0;
}