[SOLVED] ftell always returns 0

10110111 · 12-31-2009, 02:23 PM

I need to get file size in C. So, using this example: http://www.cplusplus.com/reference/c.../cstdio/ftell/,
i try this code:

Code:

/* ftell example : getting size of a file */
#include <stdio.h>

int main ()
{
  FILE * pFile;
  long size;

  pFile = fopen ("/proc/stat","rb");
  if (pFile==NULL) perror ("Error opening file");
  else
  {
    fseek (pFile, 0, SEEK_END);
    size=ftell (pFile);
    fclose (pFile);
    printf ("Size of /proc/stat: %ld bytes.\n",size);
  }
  return 0;
}

And output of this program is this:

Quote:

Size of /proc/stat: 0 bytes.

What am i doing wrong?

bartonski · 12-31-2009, 02:33 PM

What you're doing wrong is trying to find the size of /proc/stat. It isn't a real file, rather it's an interface to the kernel which is made to act like a file... but this doesn't include having a size:

Code:

> ls -l /proc/stat
-r--r--r-- 1 root root 0 2009-12-31 15:15 /proc/stat

See? Nothing wrong with your program

10110111 · 12-31-2009, 03:57 PM

OK, but how can i determine amount of memory to allocate to read this file whole?

ta0kira · 12-31-2009, 04:47 PM

The "size" could change at any time; it doesn't exist until you read it. You probably just have to fgets until you can't read any more. If you allocate a page (malloc(sysconf(_SC_PAGESIZE))) that should be more than enough if you just want to read.
Kevin Barry

edit: You should probably read a whole page so that everything is read at once, otherwise you could be copying parts of two different versions of the file.