Interfaces Built from Pointers

Function pointers name behavior

typedef int (*cmp_fn)(const void *a, const void *b);
void sort(void *base, size_t n, size_t w, cmp_fn cmp);

qsort is exactly this shape: the algorithm is generic, the comparison policy is a runtime pointer. Calling through the pointer is an indirect call — the one thing preventing inlining unless the compiler can see the target.

Opaque handles hide the struct

Publish a header with typedef struct cache cache_t; and functions taking cache_t *; define the struct only in the .c file. Clients cannot depend on layout (you can change it freely), and every access goes through your validation. This is the standard C module boundary.

The context-pointer pattern

Callbacks that need state take void *ctx:

void foreach(list_t *l, void (*fn)(void *item, void *ctx), void *ctx);

The alternative — global variables — breaks reentrancy and threads. Every serious C API you will meet (event loops, hash iterators, thread pools) uses the context pointer.