What if the function is written in assembler and takes some pointer to some opaque memory region? void (*)(char *) and void (*)(void *) and even void\()(struct

What if the function is written in assembler and takes some pointer to some opaque memory region? void (*)(char *) and void (*)(void *) and even void\()(struct SomeStruct*) where SomeStruct is declared but never defined could all be correct and reasonable ways to declare that function in a c-header.