C: Function pointer inside a typedef struct

Question

I am trying to create a linked list in C but trying to pack it nicely in somewhat of a C++ style class. I am having some issues however using function pointers in C.

typedef struct linkedList {
    int count;
    struct msgNode *front;
    struct msgNode *back;
    void (*addMSG)(unsigned char *, int, struct linkedList *);
} msgList;

void addMSG(unsigned char *data, int size, struct linkedList *self);

Ideally, I would like to have it such that you can make you list and then to add you can simply call a "method"(function) within the structure, simulating behavior you would see in C++.

Currently I get a segmentation fault when I call addMSG, which of-course is because addMSG is not pointing to a function. However, I don't want to have to specify a function to point to every single time I want use a linked list. Is there any nice way to have function pointers without implicitly having to point to the function, or do you have to implicitly point it to the function?

This is only the partial implementation shown here. At the end, this struct will have all the necessary functions. This is just for the sake of keeping this question short and to the point.

Answer 1

You need to assign the function to the member. i also recommend giving them different names:

typedef void (*addMSGFunc)(unsigned char *, int, struct linkedList *);

typedef struct linkedList {
    int count;
    struct msgNode *front;
    struct msgNode *back;
    addMSGFunc addMSG;
} msgList;

void addMSGImpl(unsigned char *data, int size, struct linkedList *self)
{
    ...
}

And then after creating a msgList:

msgList myList;
myList.addMSG = addMSGImpl;

Answer 2

Well you can't add a default value in the declaration of the struct but what you can do is:

• Create a function to initialize the linkedList instance - I guess you've seen that in C style libraries
• Create a default list item and use that when creating new entities.

Like:

void addMSG(unsigned char *data, int size, struct linkedList *self);

struct linkedList {
    int count;
    struct msgNode *front;
    struct msgNode *back;
    void (*addMSG)(unsigned char *, int, struct linkedList *);
} DefaultList = {0, NULL, NULL, addMSG};

Answer 3

You can have an uninitialized function pointer just fine as long as you don't actually use it. If you do want to use it to call a function, then obviously you have to assign a function to it. C is unable to guess which function you want to use.

If you have a linked list structure where sometimes you need a function, and sometimes you don't, then just assign NULL to it when you create the list, and have your list implementation only call the function when it's not NULL.

If it always points to the same function, then just do the assignment in your constructor function.