计算两个off64_t数字C ++之间的差异

Question

I would like to allocate memory in order to read (whole) very large files (15-25 Gb) in my C++ program. I have used off64_t type to get the beginning and ending positions of the file that I want to read, which are obtained from my file using function ftello64 . I would now like to get the difference between the two in a variable called size and then allocate this to a char array. However, and despite compiling successfully, my code doesn't work properly. I tried size = (off64_t)(end_of_mem-start_of_mem); where start_of_mem and end_of_mem are both off64_t numbers but when I run my program the output is

Starting position in memory (start_of_mem):152757
Ending position in memory (end_of_mem):15808475159
Size: -1371546782

What variant type should I be using to get this right? Thanks in advance for your help and suggestions.

Answer 1

You are experiencing int down-casting from 64bit to 32bit side-effects.

Take a look at following experiment:

#include <cstdint>
#include <iostream>
int main(){
  int64_t int64 = 15808475159 - 152757;
  int32_t int32 = int64;

  std::cout << "int64_t: " << int64 << "\n";
  std::cout << "int32_t: " << int32 << "\n";
}

Output:

int64_t: 15808322402
int32_t: -1371546782

Problem is the result of 15808475159 - 152757 is bigger than range of 32-bit integer. Thus due overflow it gets truncated (or mod 2^32). Then because its declared as signed 32-bit it apparently falls into the a negative range of interpretation.