初始化结构指针-c

Question

Sorry if it is a too simple question. I was trying to compile some socket invoking code, and I had to use this

SOCKADDR_IN * sin;
sin.sin_addr.s_addr    = htonl(INADDR_ANY);   

The problem is that I am told that I must initialize the pointer. (Is it necessary ?) Btw, how to initialize a structure pointer ?

Answer 1

It is necessary that you allocate memory for a SOCKADDR_IN instance. You don't have to create a pointer though; you could allocate it on the stack instead

SOCKADDR_IN sin;
sin.sin_addr.s_addr    = htonl(INADDR_ANY);

then use the address operator & if you want to pass sin to a function that expects a SOCKADDR_IN*

SOCKADDR_IN sin;
sin.sin_addr.s_addr    = htonl(INADDR_ANY);
....
function_using_socket_pointer(&sin);

Update: it seems you want to use a dynamically allocated pointer. You can do this using

SOCKADDR_IN* sin = malloc(sizeof(*sin));
if (sin == NULL) {
    /* insert oom handling here */
}
sin.sin_addr.s_addr    = htonl(INADDR_ANY);
....
free(sin);

Answer 2

You want to declare one on the stack instead of as a pointer. A pointer is just a memory address, not the actual instance of the object.

Just change your code to

SOCKADDR_IN sin;
sin.sin_addr.s_addr    = htonl(INADDR_ANY);   

And then when you need to use it, take the address of it like this: &sin

Edit for original question answer

To initialise a pointer if you really want to would be to do this:

SOCKADDR_IN* sin = (SOCKASSR_IN*)malloc(sizeof(SOCKADDR_IN));
....use sin....
free(sin);

This will allocate you memory on the heap with the size being set to the size of the SOCKADDR_IN struct. But as said before, this is unnecessary and generally should be avoided unless you need to use this structure outside of the current stack frame (function).