Question

I have a small program where a gtk signal callback function needs 2 or 3 variables.

I don’t want to make these global variables (The entire goal of the project is to be neat and tidy) and I don’t want to make a whole struct just so I can send a widget and a compiled regex to a function.

As far as I’ve seen g_signal_connect only allows for a single data variable.

Would the most efficient way of doing this perhaps be an array of void pointers to the two objects in question? Something like this?

void * data[2];
data[0] = widget;
data[1] = compiledregex;
g_signal_connect(save,"clicked",G_CALLBACK(callbackfunction),data);

Answer 1

Of course you can use arrays of void pointers, but if you want to pass around values with different types (especially values whose type is longer than sizeof(void *)), you can’t use arrays. For these, you’ll almost certainly want to wrap them in a struct and pass the struct’s address as the data parameter.

Example:

struct my_struct *data = malloc(sizeof(*data));
data->field_one = value_one;
data->field_two = value_two; /* etc. */

g_signal_connect(save, "clicked", callback, data);

Of course, don’t forget to free(data) in the callback function (assuming it’s for single use).

Edit: as you wanted an example with void **, here you are (this is ugly, and I don’t recommend you to use this — either because allocating a one-element array for primitive types is wasting your shoot or because casting a non-pointer to void * is bad practice…):

void **data = malloc(sizeof(data[0]) * n_elements);

type1 *element1_ptr = malloc(sizeof(first_item));
*element1_ptr = first_item;
data[0] = element1_ptr;

/* etc. */

To free them:

int i;
for (i = 0; i < n_elements; i++)
    free(data[i]);

free(data);