Is there a way to pass a function pointer with generic arguments? - c

Question

I am implementing a generic singly linked list where list nodes store a pointer to their data.
typedef struct sll_node
{
void *data;
struct sll_node *next;
} sll_node;
To implement a generic find subroutine that works with any kind of data, I wrote it so that it takes as an argument a function pointer to the comparison function as follows:
/* eq() must take 2 arguments. ex: strcmp(char *, char *) */
sll_node *sll_find(void *data, int (*eq)(), sll_node *root);
You can pass the appropriate function pointer that works with the data type at hand.. So if you store strings in the list nodes, you can pass strcmp as the eq() function, and so on. It works but I'm still not satisfied..
Is there a way to explicitly specify the number of comparison function parameters without giving up its generality?
I tried this at first:
sll_node *sll_find(void *data, int (*eq)(void *, void *), sll_node *root);
I expected it to work. But no (edit: it compiles with a warning but I have -Werror on!), I had to write a wrapper function around strcmp to make it conform to the eq prototype.
I then tried:
sll_node *sll_find(void *data, int (*eq)(a, b), sll_node *root);
or:
typedef int (*equality_fn)(a, b);
sll_node *sll_find(void *data, equality_fn eq, sll_node *root);
which both wouldn't compile since: "a parameter list without types is only allowed in a function definition"

Answer 1

To use strcmp without a wrapper or a cast, the declaration needs to be
sll_node *findNode(void *data, int (*eq)(const char *, const char *), sll_node *root);
On the other hand, if you declare the args as const void *, then you can avoid the wrapper by casting strcmp to the appropriate type.
Method 1: direct cast, messy but effective
result = findNode( "hello", (int(*)(const void *, const void *))strcmp, root );
Method 2: typedef the comparison function, and then use it to cast
typedef int (*cmpfunc)(const void *, const void *);
result = findNode( "world", (cmpfunc)strcmp, root );
Edit: After reading this post that #WilburVandrsmith linked, I've decided to leave this answer as is. I leave it up to the reader to decide whether the proposed cast violates the following paragraph from the specification:
If a converted pointer is used to call a function whose type is not
compatible with the pointed-to type, the behavior is undefined.
Compatible or not compatible, that is the question, you decide.

Answer 2

Your last attempted solution is the closest to being correct. The parameters in your defined-type function pointer need to be declared with their data types, just like you would with a regular function declaration, like so:
typedef int (*equality_fn)(char *a, char *b);
sll_node *sll_find(void *data, equality_fn eq, sll_node *root);
UPDATE
To make it more generic use void pointers, and then type cast the passed void pointers to the needed data type in the matching function definition for equality_fn:
typedef int (*equality_fn)(void *a, void *b);
sll_node *sll_find(void *data, equality_fn eq, sll_node *root);
Something else important to remember is that a pointer is a pointer is a pointer, regardless of what it's pointing at or how it was originally defined. So, you can have some function pointer, or a void pointer, or a pointer to a byte, a char, an int--anything--as long as you handle it properly in your code and cast it back to a valid type before attempting to use it.
Something else that most coders don't take much advantage of in C is that function names themselves are really just addresses that are called at run-time, and so they are also pointers. ;)

Answer 3

My solution to this conundrum would be (avoiding pointer typedefs, incidentally):
typedef int equality_fn(const void *a, const void *b);
sll_node *sll_find(void *data, equality_fn *eq, sll_node *root);
Then make all your comparators be of type equality_fn. If you need to actually have a function then so be it:
equality_fn eq_strcmp; // a prototype
// ...
int eq_strcmp(const void *a, const void *b) { return strcmp(a, b); }
Gain lots of type safety in exchange for a potential picosocopic runtime penalty - which end of this trade you want to be on depends on your application.