(Incorrectly) calculate the free size of the directory

free_size.c

#include <stdio.h>
#include <inttypes.h>
#include <sys/statvfs.h>

typedef unsigned long long uint64;

uint64 get_free_size( const char* path )
{
	struct statvfs buffer;
	if ( statvfs( path, &buffer ) != -1 )
	{
		int64 size = (uint64)buffer.f_frsize * (uint64)buffer.f_bfree;

		if ( size > 0 )
			return size;
	}

	return 0xffffffffffffffffuLL;
}

int main( int argc, const char* argv[] )
{
	if ( argc != 2 )
	{
		printf( "Usage: %s <directory>\n", argv[0] );
		return 1;
	}

	printf( "%llu bytes free\n", get_free_size( argv[1] ) );
	return 0;
}