void* and the Cost of Erasure
Type-erased containers: what qsort teaches, byte-wise copies, alignment, and why the compiler cannot save you.
qsort is the master class
void qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));
Four ingredients make it generic: a byte-addressed base, an element size (so the algorithm can move elements with memcpy), a count, and a comparison callback supplied by the caller. Every generic C API you will design mixes some subset of these.
What the cost is
Inside a void* container, elements are bytes: elem_at(base, i, size) is (char *)base + i * size. The compiler no longer knows element types — wrong casts, wrong sizes, and wrong comparators are all silent until runtime. Genericity in C trades compile-time checking for flexibility; document and test accordingly.
Alignment still applies
A void* handed back to the caller must be as aligned as the data it represents. If your container allocates raw bytes, align them — a double stored at an odd address is UB the moment it is loaded.