Checked Add

Arithmetic checks can reject a result before assigning it to program state.

checked_add.c

#include <stdio.h>

int checkedAdd(int left, int right, int limit, int *outSum) {
    if (right > limit - left) {
        return 0;
    }
    *outSum = left + right;
    return 1;
}

int main(void) {
    int right = ;
    int left = 40;
    int limit = 100;
    int sum = 0;
    int ok = checkedAdd(left, right, limit, &sum);

    printf("left=%d right=%d ok=%d sum=%d\n", left, right, ok, sum);
    return 0;
}

#include <stdio.h>

int checkedAdd(int left, int right, int limit, int *outSum) {
    if (right > limit - left) {
        return 0;
    }
    *outSum = left + right;
    return 1;
}

int main(void) {
    int right = ;
    int left = 40;
    int limit = 100;
    int sum = 0;
    int ok = checkedAdd(left, right, limit, &sum);

    printf("left=%d right=%d ok=%d sum=%d\n", left, right, ok, sum);
    return 0;
}

#include <stdio.h>

int checkedAdd(int left, int right, int limit, int *outSum) {
    if (right > limit - left) {
        return 0;
    }
    *outSum = left + right;
    return 1;
}

int main(void) {
    int right = ;
    int left = 40;
    int limit = 100;
    int sum = 0;
    int ok = checkedAdd(left, right, limit, &sum);

    printf("left=%d right=%d ok=%d sum=%d\n", left, right, ok, sum);
    return 0;
}

pre-check Checking against `limit - left` avoids computing a value that should be rejected.

output parameter The function writes the sum only when the status says it is valid.