How to initialize the structure member with using pointer? - c

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:
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
Your newly assigned pointer to the stack variable becomes invalid after exiting the scope and can cause undefined behaviour.
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;
}

Answer 2

Are you asking for this:
abc s;
s.c = 5;
That makes c 5. Or you could do thos:
abc s = {0,0,5};
That means, the first two members are initialized to 0, and third one (which is c) is initialized to 5.
Or if you've pointer, then you can do this:
abc *p = malloc(etc);
p->c = 5;
But then avoid using pointers and malloc. Use it when you really need them. In your code, I don't see any reason why you would need it. Use automatic variables, not pointers.

Answer 3

I think you would need to add this before return in your main() to get what you need.
ab->c = *(x->pointer);