getmousestate function Open BGI library

Question

I am currently using the out of date open BGI library for C as that is what my uni wants us to do but I can't figure out how to use this function.

extern void getmousestate(g_mousestate *state);

Definition:

void getmousestate(g_mousestate * state)
{
    CHECK_GRAPHCS_INITED
    state->x = sharedStruct->mouseX;
    state->y = sharedStruct->mouseY;
    state->buttons = sharedStruct->mouseButton;
}

g_mousestate Definition:

typedef struct mousestate {
    int x, y;
    int buttons;
}g_mousestate;

sharedStruct definition:

static SHARED_STRUCT * sharedStruct;

SHARED_STRUCT Definition:

typedef struct
{
  int mouseX, mouseY;
  int mouseButton;
  int keyCode;
  int keyLetter;
  int visualPage;
} SHARED_STRUCT;

The sort of thing I was trying to do to call:

g_mousestate *a;
getmousestate(a->x);

But I don't know what to initialize a to...

I assumed this function could tell me what position the mouse is in and what buttons are being pressed etc, but I can't figure out how to call the function properly. Very much a beginner here, any help would be appreciated.

Answer 1

OK, beginner, let's start. You want to do:

g_mousestate *a;
getmousestate(a->x);

to get the x-position of the mouse. First note that you have declared a as a pointer, but no memory has been allocated for it (it points to nothing yet). So first you must have memory for the g_mousestate object:

g_mousestate a;
getmousestate(&a);

Now a is no longer a pointer but an object and you pass a pointer to the getmousestate function by taking the address of a with the & operator ("address-of").

You need to do no more since, if you look at the definition of the function, you see that all members are filled in by it.