Question

I have below code. In below code the value 5 gets stored in variable a(i.e abc.a). But I would like to intialize the value in c (i.e abc.c). How to do that? My requirement is to fill the data for c not for a.

typedef struct abc
{
    int a;
    int b;
    int c;
}abc;
typedef struct def{
    int *ptr;
    abc strpt;
}def;
typedef struct xyz{
int *pointer;
}xyz;
int main()
{
    int *temp, tmp;
    abc* ab;
    tmp = 5;
    temp = &tmp;
    ab = (abc*)malloc(sizeof(abc));
    xyz *x = (xyz*)malloc(sizeof(xyz));
    def *de = (def*)malloc(sizeof(def));
    x->pointer = (xyz*)temp;
    ab = (abc*)x->pointer;


    return 0;
}

Please help me.

Answer 1

I’m Affraid some serious lack of understanding pointers manifests in your code. Let me dissect it line by line to make clear where problems occur.

I numbered the statements in order to make easy references, assuming the type declarations as above.

int main()
{
    int *temp, tmp;
    abc* ab;
    /* 1 */ tmp = 5;
    /* 2 */ temp = &tmp;
    /* 3 */ ab = (abc*)malloc(sizeof(abc));
    /* 4 */ xyz *x = (xyz*)malloc(sizeof(xyz));
    /* 5 */ def *de = (def*)malloc(sizeof(def));
    /* 6 */ x = (xyz*)temp;
    /* 7 */ ab = (abc*)x;

    return 0;
}

First of all: You never clean up the memory allocated in 3, 4 and 5. Even for a simple test program, you should always take care of stuff like this.

Your first problem is line 6 where you blindly cast a pointer to an object of type int (4 bytes in memory) into a pointer to a structure of type xyz (4 or 8 bytes in memory, different type). What you do here is a complicated way to write x = (xyz*)&tmp;.

An action like that leads to following serious problems:

1. You just allocated memory in line 4. Now you overwrite this pointer with a pointer to your stack variable. Therefore you can’t free the allocated memory anymore ==> Memory leak
2. Your newly assigned pointer to the stack variable becomes invalid after exiting the scope and can cause undefined behaviour.
3. Any write attempt to elements of x after line 6 can lead to corruption of the stack (and therefore undefined behaivour) due to a possible size mismatch of elements.

By line 7 you get the same problems again and even worse. The estimated sizeof(abc) is possibly 12 bytes. Accessing any element of abc after this line can lead to stack corruption again.

I still don’t understand what you really are looking for but if you “have to” initialize a structure element through pointers there are a several ways:

If the struct ist known you can do it like this:
abc * ab = malloc(sizeof(abc));

if (ab != NULL) {
    ab->c = 5;

    // -- do other stuff

    free(ab); ab = NULL;
}

If you need a pointer to the element ‘c’ you can do it like this:

int * c_ptr = NULL;
abc * ab = malloc(sizeof(abc));

if (ab != NULL) {
    c_ptr = &(ab->c);
    *c_ptr = 5;

    //-- do other stuff
    free(ab); ab = NULL;
}