Question

I am a bit confused about the behavior of c when it comes to adding large numbers. This is probably a beginner’s question but would appreciate any help.

unsigned long total = 0;
total = 1124073472 + 2835349503;
total += 2533359615;
printf("Total: %u\n", total);

The total which gets printed above is not correct. The first add result is fine but the third add throws the total off. I am thinking it is because of overflow. My question is what is a possible solution? Is there a solution without using third party libraries?

Note: I have tried various data types for total. Some of them are DWORD64, INT64, LONG64 etc.

Thanks in Advance.

Answer 1

You’re correct, it’s because of overflow. Using a 64-bit type will fix it – you will need to change your printf() format string to printa 64-bit type too, though. Making total an unsigned long long will probably get you a 64-bit type, but the safest way would be to use one of the typedefs from stdint.h. Here’s an example program – I also used inttypes.h to get PRIu64.

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

int main(int argc, char **argv)
{
  uint64_t total = 0;
  total = 1124073472 + 2835349503;
  total += 2533359615;
  printf("Total: %"PRIu64"\n", total);

  return 0;
}