指向 struct 的指针，这甚至是指向 struct 的指针吗？

Question

I have a struct and it has variables in it. Now I need to allocate memory dynamically, so initially I use

Struct struct-name *a =Null; 

Latter on , I will allocate memory to it using malloc ,then I use

a[0].variableA=allocate , a[0].variableB=allocate.

My question is , is this even a pointer to struct, because no where I give the address of struct . And moreover this technique works , how does it know the address of struct . When I'm not explicitly giving address of struct using '&'. I know this might be silly but I got confused .Thanks .

Struct st
{
 int a ;
int b;
}
Struct a *ptr=NULL;

In main

    ptr=malloc(sizeof(struct st)*2);
    //Assume array of size 2 is created 
Now I can use 
    ptr[0].a=10;

How can I use even without explicitly giving address of struct to it.

Answer 1

The thing is you are not getting how malloc works. (You mentioned I will allocate memory to it using malloc )

struct struct_name *a = NULL;
a = malloc(sizeof *a);
...

This computes the number of bytes that a struct struct_name occupy in memory, then requests that many bytes from malloc and assigns the result (ie, the starting address of the memory chunk that was just created using malloc ) to a pointer named a . That is the whole story.

Now you will access it after making it sure that it didn't return NULL . No need to apply & address of to any struct struct_name variable instance.

In your case, ptr is used to access the different struct instance allocated dynamically - where is the address? The address pointing to the first instance is contained in ptr . Then you used ptr[0] or ptr[1] to get to that structure instance. And then you accessed it.

And yes ptr is a pointer to struct.

One extra thing:-

malloc may not be able to service the request, it may return a null pointer. It is good to check for this to prevent later attempts to dereference the null pointer. If you dereference a NULL pointer then it's undefined behavior.