Why is this having trouble storing illegal the character?

Question

It's an exercise of my college. The goal is to make illegal memory store and illegal memory access and print it in char. However, using MALLOC has memory adresses doesn't store 'y', and using char x works. Why this difference? But when I use char x it shows me what I want, however, in the end show me this text "* stack smashing detected *: terminated Abortado (imagem do núcleo gravada)"

void ilegal_store(char *u)
{
    for(int i=0;i<100;i++){
        *(u+i) = 'y';
    }   
}

void ilegal_reading(char *u)
{
    for(int i=0;i<100;i++){
        printf("%d = %d\n",i,*(u+i));
    }
}

void main()
{
    //char x; WORKS
    char *x=(char *)malloc(sizeof(char)); //USING MALLOC HAS ADRESSES WHICH DOESN'T STORE THE 'y' by ilegal_store();
    if(x!=NULL){
        ilegal_store(x); //use &x when not pointer
        ilegal_reading(x); //use &x when not pointer
    }
}

---

Answer 1

The C standard does not permit you to dereference a pointer to memory that you don't own. The behaviour on attempting to do so is undefined. Undefined behaviour manifests itself in many curiosities such as "working" sometimes, not "working" sometimes, or the compiler even erasing that particular branch: it's allowed to assume that undefined behaviour does not occur.

It's possible that you will not get a crash at exactly the first undefined dereference. Because the output of printf is frequently buffered and therefore often in lag of the code generating the output, you might be able to produce a extra output for the uncommented case by flushing the buffer after every iteration using fflush(stdout). The presence of the newline character in the case you cite as working can also flush the buffer. It's probably the newline character that is producing the different behaviour.

Finally, however this is dressed up, you are circumventing contraints the language places on you. If you want to circumvent these constraints then you need to use a different language. Many C compilers support inline assembly. That's what you need to use in this instance.