cocoa-rs | C bindings for the Rust programming language | iOS library

This is a non-ABI-specific answer.

Yes, formally the caller can (and, in general case, will) treat functions with variadic arguments in a special way. This is actually the reason why from the beginning of standardized times C language required all variadic functions to be declared with prototype before the point of the call. Note that even though it was possible to safely call undeclared functions in C89/90, the permission to do so did not extend to variadic functions: those always had to be declared in advance. Otherwise, the behavior was undefined.

In a slightly different form the rule still stands in modern C. Even though post-C99 C no longer allows calling undeclared functions, it still does not require prototype declarations. Yet, variadic functions have to be declared with prototype before the point of the call. The rationale is the same: the caller has to know that it is calling a variadic function and, possibly, handle the call differently.

And historically, there were implementations that used completely differrent calling conventions when calling variadic functions.