How does a Swift function differ from a C one?

[Dante-Broggi]

In this thread about function equality in Swift @Joe_Groff said ("[]" inserted for clarification):

@Joe_Groff

The ABI for [Swift] function values is intentionally general—the only guarantees are that the invocation function is a valid function pointer, and that for escaping closures the context word can be passed to swift_retain and swift_release . Any other details of what the context points to are a private contract between the invocation function and the context object. It is not necessarily even a valid pointer, because swift_retain on some platforms recognizes certain input values as obviously invalid and ignores them, and when it is, it isn't necessarily a pointer to something that's specifically a closure context, since a closure capturing a single refcounted value will directly adopt that one pointer as its context value.

However C function pointers, IIRC, are just raw pointers to the function code.