C++: Getting incorrect file size

Question

I'm using Linux and C++. I have a binary file with a size of 210732 bytes, but the size reported with seekg/tellg is 210728.

I get the following information from ls-la, i.e., 210732 bytes:

-rw-rw-r-- 1 pjs pjs 210732 Feb 17 10:25 output.osr

And with the following code snippet, I get 210728:

std::ifstream handle;
handle.open("output.osr", std::ios::binary | std::ios::in);
handle.seekg(0, std::ios::end);
std::cout << "file size:" << static_cast<unsigned int>(handle.tellg()) << std::endl;

So my code is off by 4 bytes. I have confirmed that the size of the file is correct with a hex editor. So why am I not getting the correct size?

My answer: I think the problem was caused by having multiple open fstreams to the file. At least that seems to have sorted it out for me. Thanks to everyone who helped.

Answer 1

At least for me with G++ 4.1 and 4.4 on 64-bit CentOS 5, the code below works as expected, i.e. the length the program prints out is the same as that returned by the stat() call.

#include <iostream>
#include <fstream>
using namespace std;

int main () {
  int length;

  ifstream is;
  is.open ("test.txt", ios::binary | std::ios::in);

  // get length of file:
  is.seekg (0, ios::end);
  length = is.tellg();
  is.seekg (0, ios::beg);

  cout << "Length: " << length << "\nThe following should be zero: " 
       << is.tellg() << "\n";

  return 0;
}

Answer 2

Is it possible that ls -la is actually reporting the number of bytes the file takes up on the disk, instead of its actual size? That would explain why it is slightly higher.

Answer 3

When on a flavour of Unix, why do we use that, when we have the stat utlilty

long findSize( const char *filename )
{
   struct stat statbuf;
   if ( stat( filename, &statbuf ) == 0 )
   {
      return statbuf.st_size;
   }
   else
   {
      return 0;
   }
}

if not,

long findSize( const char *filename )
{
   long l,m; 
   ifstream file (filename, ios::in|ios::binary ); 
   l = file.tellg(); 
   file.seekg ( 0, ios::end ); 
   m = file.tellg(); 
   file.close(); 
   return ( m – l );
}