how can I avoid buffer overflow in this case?-CodePudding

I have this exercise: "create a function in order to subtract two strings".

my attempt works just fine (with the help of an old answer here on stack overflow). The problem is that when I debug, the debugger doesn't warn me on anything, but I have a grey squiggle below the argument of malloc. visual studio says that "a sub-expression may overflow before being assigned to a wider type". This is my thought: "I need to write an if-check before malloc to make sure the sub-expression won't overflow. To do it, I've thought to do this way: int sum = length 1; if (condition), then do this (allocation)." the problem is that I don't know the condition to be checked. based on an old discussion here on stack overflow, I know that if I have a subtraction I have to do this way

int c = a - b; 
if (c >= a) {
// do allocation; 
} else {
return NULL; 
}

but it works only for subtraction, but if I have an addition this method fails. therefore, this method doesn't work.

how can I do it?

#include <stdlib.h>
#include <stdio.h>
#
char* subtract(const char* a, const char* b) {
    int a_1 = atoi(a); int b_1 = atoi(b); int result = 0; 
    result = a_1 - b_1; 
    int length = snprintf(NULL, 0, "%d", result);
    char* str = malloc(length   1); 
    if (str == NULL) {
        return NULL; 
    }
    snprintf(str, length   1, "%d", result); 
    return str; 
}



int main(void) {
    char a[] = "4567"; 
    char b[] = "568"; 
    char* c; 
    c = subtract(a, b); 

    return 0; 
}

CodePudding user response：

but I have a little warning. it's located in the argument of malloc, as I said.

malloc(length 1);

length is signed integer and it can overflow if you add 1 to it.

Solution: cast it to size_t

malloc((size_t)length   1);

BTW What is the difference between "little" and "big" warnings?

PS How to substract "4564536456645645645" and "545643563456436565436" using your solution?