2GB limit on file size when using fwrite in C?

Question

I have a short C program that writes into a file until there is no more space on disk:

#include <stdio.h>

int main(void) {
  char c[] = "abcdefghij";
  size_t rez;
  FILE *f = fopen("filldisk.dat", "wb");
  while (1) {
    rez = fwrite(c, 1, sizeof(c), f);
    if (!rez) break;
  }
  fclose(f);
  return 0;
}

When I run the program (in Linux), it stops when the file reaches 2GB.

Is there an internal limitation, due to the FILE structure, or something?

Thanks.

Answer 1

On a 32 bits system (i.e. the OS is 32 bits), by default, fopen and co are limited to 32 bits size/offset/etc... You need to enable the large file support, or use the *64 bits option:

http://www.gnu.org/software/libc/manual/html_node/Opening-Streams.html#index-fopen64-931

Then your fs needs to support this, but except fat and other primitive fs, all of them support creating files > 2 gb.

Answer 2

it stops when the file reaches 2GB.

Is there an internal limitation, due to the FILE structure, or something?

This is due to the libc (the standard C library), which by default on a x86 (IA-32) Linux system is 32-bit functions provided by glibc (GNU's C Library). So by default the file stream size is based upon 32-bits -- 2^(32-1).

For using Large File Support, see the web page.

#define _FILE_OFFSET_BITS  64
/* or more commonly add -D_FILE_OFFSET_BITS=64 to CFLAGS */

#include <stdio.h>

int main(void) {
  char c[] = "abcdefghij";
  size_t rez;
  FILE *f = fopen("filldisk.dat", "wb");
  while (1) {
    rez = fwrite(c, 1, sizeof(c), f);
    if ( rez < sizeof(c) ) { break; }
  }
  fclose(f);
  return 0;
}

Note: Most systems expect fopen (and off_t) to be based on 2^31 file size limit. Replacing them with off64_t and fopen64 makes this explicit, and depending on usage might be best way to go. but is not recommended in general as they are non-standard.