C/C++: Hack to use an array of 4 characters as one unsigned 32 bit integer and increment their value by one


In this scenario, we had 4 unsigned char variables.
The first variable was counting ‘something’.
The second variable was counting how many times the first variable reached its maximum value and then overflowed.
The third variable was counting how many times the second variable reached its maximum value and then overflowed.
Finally, the fourth variable was counting how many times the third variable reached its maximum value and then overflowed.

What we wanted to achieve was to perform this process without using several if statements that check when each variable overflowed and then perform the corrective actions.
The trick that we did to achieve the expected result works on the principle that an array in C/C++ always holds consecutive memory spaces.

What we did was cast the array pointer as a pointer of an unsigned int variable, this allowed us to operate on all 4 bytes at the same time as if they were one.
We can do the same for an array composed of two unsigned short integers.
This trick can of course work on 64bit variables and arrays that contain 8 unsigned char elements or 4 unsigned short elements or 2 unsigned int elements.

++(*((unsigned int *)&t));

Full Example

array-of-char-as-integer.c (compressed) (2 downloads)

#include <stdio.h>
#include <time.h>
#include <stdlib.h>

static inline void increment(unsigned char t[4]) {
  if (t[0] == 0xFF) {
    if (t[1] == 0xFF) {
      if (t[2] == 0xFF) {
        ++t[3];
      }
      ++t[2];
    }
    ++t[1];
  }
  ++t[0];
}

int main() {
  const unsigned int final_limit = (unsigned int) -1; //or 0xFFFFFFFF
  const unsigned int intermediate_limit = 0xABCDEF00;

  {
    unsigned char t[4] = {0, 0, 0, 0};
    printf("%02X %02X %02X %02X - Initial Values\n", t[3], t[2], t[1], t[0]);
    const clock_t start = clock();

    unsigned int i;
    for (i = 0; i < intermediate_limit; ++i) {
      ++(*((unsigned int *)&t));
    }

    printf("%02X %02X %02X %02X - Intermediate Values\n", t[3], t[2], t[1], t[0]);

    for (; i < final_limit; ++i) {
      ++(*((unsigned int *)&t));
    }

    const clock_t end = clock();
    const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
    printf("%02X %02X %02X %02X - Final Values\n", t[3], t[2], t[1], t[0]);
    printf("Seconds elapsed %f\t by casting array of unsigned chars to an unsigned int\n", seconds);
  }

  {
    unsigned char t[4] = {0, 0, 0, 0};
    printf("%02X %02X %02X %02X - Initial Values\n", t[3], t[2], t[1], t[0]);
    const clock_t start = clock();

    unsigned int i;
    for (i = 0; i < intermediate_limit; ++i) {
      increment(t);
    }

    printf("%02X %02X %02X %02X - Intermediate Values\n", t[3], t[2], t[1], t[0]);

    for (; i < final_limit; ++i) {
      increment(t);
    }

    const clock_t end = clock();
    const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
    printf("%02X %02X %02X %02X - Final Values\n", t[3], t[2], t[1], t[0]);
    printf("Seconds elapsed %f\t by using various if statements to control overflow\n", seconds);
  }

  return 0;
}

array-of-char-as-integer.c (compressed) (2 downloads)

Output

00 00 00 00 - Initial Values
AB CD EF 00 - Intermediate Values
FF FF FF FF - Final Values
Seconds elapsed 7.938144 by casting array of unsigned chars to an unsigned int
00 00 00 00 - Initial Values
AB CD EF 00 - Intermediate Values
FF FF FF FF - Final Values
Seconds elapsed 12.820061 by using various if statements to control overflow

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