How to combine two 32-bit integers into one 64-bit integer?

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It might be advantageous to use unsigned integers with explicit sizes in this case:

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

int main(void) {
  uint32_t leastSignificantWord = 0;
  uint32_t mostSignificantWord = 1;
  uint64_t i = (uint64_t) mostSignificantWord << 32 | leastSignificantWord;
  printf("%" PRIu64 "\n", i);

  return 0;
}

Output

4294967296

Break down of (uint64_t) mostSignificantWord << 32 | leastSignificantWord

  • (typename) does typecasting in C. It changes value data type to typename.

    (uint64_t) 0x00000001 -> 0x0000000000000001

  • << does left shift. In C left shift on unsigned integers performs logical shift.

    0x0000000000000001 << 32 -> 0x0000000100000000

left logical shift

  • | does ‘bitwise or’ (logical OR on bits of the operands).

    0b0101 | 0b1001 -> 0b1101

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