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The C-call syntax produces code that applies call-alien
to an alien function structure — a cache for the callout
trampoline's entry address.
(C-call "gtk_button_new" (make-alien '(* |GtkWidget|)))
==>
(call-alien '#[alien-function gtk_button_new] (make-alien ...))
The alien function contains all the information needed to load the
callout trampoline on demand (i.e. its name and library). Once the
alien function has cached the entry address, call-alien can
invoke the trampoline (via #[primitive c-call]). The
trampoline gets its arguments off the Scheme stack, converts them to C
values, calls the C function, conses a result, and returns it to
Scheme.
A function returning a pointer type is treated specially.
Its trampoline expects an extra (first) argument.
If the argument is #f, the return value is ignored.
If the argument is an alien, the function's return value clobbers the
alien's address. This makes it easy to grab pointers to toolkit
resources without dropping them, and to avoid unnecessary consing of
aliens.
A function returning a struct or union type is treated similarly.
Its trampoline expects an extra (first) argument.
If the argument is #f, the return value is ignored.
If the argument is an alien, the returned struct or union is copied to
that address.
Struct and union type parameters of a function are treated similarly. The function's trampoline expects an alien argument for each such parameter and copies the struct or union from the argument address into a local variable. Callbacks currently cannot receive struct or union type arguments, though they can receive pointer type arguments (consing an alien for each).
The alien-function structures are fasdumpable. The caching
mechanism invalidates the cache when a band is restored, or a
fasdumped object is fasloaded. The alien function will lookup the
trampoline entry point again on demand.