Error of memory allocation

Question

I am creating a C script to classify a triangle according to the 3 sides passed. For this I use three distinct variables, lado1, lado2 and lado3, as illustrated by the code below:

#include &ltstdio.h>
#include &ltstdlib.h>
int main(int argc, char *argv[]) {
    unsigned short int lado1, lado2, lado3;

    printf("L1: ");
    scanf("%i", &lado1);
    printf("L2: ");
    scanf("%i", &lado2);
    printf("L3: ");
    scanf("%i", &lado3);

    if (lado1 < (lado2 + lado3) && lado2 < (lado1 + lado3) && lado3 < (lado1 + lado2)) {
        if ((lado1 == lado2) && (lado2 == lado3)) {
            printf("Triângulo equilátero");
        } else if ((lado1 != lado2) && (lado2 != lado3)) {
            printf("Triângulo escaleno");
        } else {
            printf("Triângulo isóceles");
        }
    } else {
        printf("Os lados informados não constituem um triângulo");
    }

    return 0;
}

However, when I try to compile this code a memory error is shown:

stack smashing detected: <unknown> terminated

The same does not happen when I declare a string along with the other three variables lado1, lado2 and lado3. Thus, in addition to the three variables I have another variable of type string folk[90] which is not used within the code at any time, but this time I have the desired return.

Follows the code:

#include &ltstdio.h>
#include &ltstdlib.h>
int main(int argc, char *argv[]) {
    unsigned short int lado1, lado2, lado3, folk[90];

    printf("L1: ");
    scanf("%i", &lado1);
    printf("L2: ");
    scanf("%i", &lado2);
    printf("L3: ");
    scanf("%i", &lado3);

    if (lado1 < (lado2 + lado3) && lado2 < (lado1 + lado3) && lado3 < (lado1 + lado2)) {
        if ((lado1 == lado2) && (lado2 == lado3)) {
            printf("Triângulo equilátero");
        } else if ((lado1 != lado2) && (lado2 != lado3)) {
            printf("Triângulo escaleno");
        } else {
            printf("Triângulo isóceles");
        }
    } else {
        printf("Os lados informados não constituem um triângulo");
    }

    return 0;
}

What I want to know is the explanation for this, why does this occur? and how to allocate memory correctly without needing this fourth variable?

The compiler I use is the gcc

Answer 1

First it must be understood what is the stack or stack:

The stack is a contiguous portion of memory reserved to stack the necessary data while executing code blocks.

Each need for allocation is a chunk of the stack that is always used in sequence... ¹

In the case of your program 3 variables of type unsigned short which has its size set at least 16 bits, or 2 bytes, according to the wikipedia.

So we can see the stack of your show, like:

Stack Smashing is a type of attack, a buffer overflow, where the program attempts to extrapolate space (or stack), reserved for him. ⁵

And that’s what you did, reading 4 bytes in scanf("%i", &lado3);, i.e., by typing an integer in the address 0x05.

When he declared folk (NEVER DO THAT for a running problem, NEVER EVEN), the program has had slack, and may not violate the running stack, but even so, there is an invasion of memory (quite damaging as well).

In short, your program, even if it runs, doesn’t work.

To fix, align the input data type and the variable that receives its value, such as:

int lado1, lado2, lado3;

Answer 2

scanf expecting a *int to store the value of %i, you provide a *unsigned short int.

It is likely that:

sizeof(int) == 4
sizeof(unsigned short int) == 2

Thus, the CPU will write the result of size four in a region of size two and stack smashing detected: <unknown> terminated

The array folk[90] may be being allocated shortly after lado3 in memory and, coincidentally, prevents the value from being written in a region that breaks the code.

Change the type of variables:

int lado1, lado2, lado3;

Or the guy from scanf for %hu:

scanf("%hu", &lado1);

Reference: http://www.cplusplus.com/reference/cstdio/scanf/