Question

This is my first post, so please be gentle.

I’ve been playing around with C from time to time in the past. Now I’ve gotten to the point where I’ve started a real project (a 2D graphics engine using SDL, but that’s irrelevant for the question), to be able to say that I have some real C experience. Yesterday, while working on the event system, I ran into a problem which I couldn’t solve.

There’s this typedef,

//the void parameter is really an SDL_Event*.  
//but that  is irrelevant for this question.  
typedef void (*event_callback)(void);  

which specifies the signature of a function to be called on engine events.

I want to be able to support multiple event_callbacks, so an array of these callbacks would be an idea, but do not want to limit the amount of callbacks, so I need some sort of dynamic allocation. This is where the problem arose. My first attempt went like this:

//initial size of callback vector  
static const int initial_vecsize = 32;  
//our event callback vector  
static event_callback* vec = 0;  
//size  
static unsigned int vecsize = 0;  

void register_event_callback(event_callback func) {  
    if (!vec)  
        __engine_allocate_vec(vec);  
    vec[vecsize++] = func; //error here!  
}  

static void __engine_allocate_vec(engine_callback* vec) {  
    vec = (engine_callback*) malloc(sizeof(engine_callback*) * initial_vecsize);  
}  

First of all, I have omitted some error checking as well as the code that reallocates the callback vector when the number of callbacks exceed the vector size.

However, when I run this code, the program crashes as described in the code. I’m guessing segmentation fault but I can’t be sure since no output is given. I’m also guessing that the error comes from a somewhat flawed understanding on how to declare and allocate an array of pointers to function pointers.

Please Stack Overflow, guide me.

EDIT: I can’t seem to grasp how to indent the code blocks. That’s almost a tad embarassing…

EDIT: Nevermind that. Checked the page source of some other posts =).

Answer 1

At the line:

vec[vecsize++] = func; //error here!  

What happens if vecsize is >= initial_vecsize ?

Also __engine_allocate_ve doesn’t work since it only modifies the local copy of vec, you have to change the signature to a ** and pass the argument with &.

static void __engine_allocate_vec(engine_callback** vec)

__engine_allocate_vec(&vec);