Function pointers and comparator contracts

The idea

A function pointer lets an algorithm call behavior supplied by its caller. Its type must match the function’s parameter and return contract; casting an incompatible function pointer does not make the actual call safe. State whether callbacks may retain pointers, mutate arguments, or access caller-owned context, because the type alone cannot express every lifetime rule.

The standard qsort callback receives pointers to elements and returns a negative, zero, or positive result according to their order. Subtracting two signed int values is a tempting comparison but can overflow. Comparing with relational operators and subtracting their boolean results avoids that overflow. qsort does not promise stable ordering of equal elements, so include a tie-breaker or choose a stable algorithm when that ordering matters.

#include <stdio.h>
#include <stdlib.h>
int compare_int(const void *left, const void *right) {
  int a = *(const int *)left, b = *(const int *)right;
  return (a > b) - (a < b);
}
int main(void) {
  int values[] = {9, -2, 4};
  qsort(values, 3, sizeof values[0], compare_int);
  printf("%d %d %d\n", values[0], values[1], values[2]);
}

-2 4 9

The comparator returns -1, 0, or 1 using safe relational results.