Question

I have the following form in c (gcc):

typedef struct {
  mem 1;
  mem 2;
  mem n;
} *obj;

How do I get a specific member from that type of structure, initialized with obj var;?

Answer 1

First, that declaration is invalid, you’re trying to declare members whose identifiers are numbers. I’ll assume you actually want this:

typedef struct foo {
    int mem_1;
    int mem_2;
    ...
} *obj;

You access plain struct members with a dot (.):

struct foo bar;
bar.mem_1 = 2;

This is perfectly fine, the compiler allocates stack space of the correct size automatically.
Now, since your obj var is a pointer¹, we’d access its members with an arrow (->):

obj var;
var->mem_1 = 3;

Of course, if you’re lucky, this will segfault. If not, you’re likely to have a very long and enlightening session with the debugger. Unlike declaring a struct foo itself, Declaring obj var or its equivalent struct foo *var does not create anything except an uninitialized pointer. You need to provide the memory for this yourself:

obj var = malloc(sizeof(*var));

Note that this merely provides the memory, its contents are still undefined, but you can safely assign to them.

^{1: This is the problem with typedefing away pointers like you’ve done; it’s impossible to know whether you need to use . or -> without seeing the typedef or being told off by the compiler. Don’t use typedef to hide a pointer.}