[caladri]

For the set of features you have, with most calling conventions on most platforms you don't need to synthesize all that. You could simply have your apply function be written in assembly and handle setting up the parameters fetching them from memory.

That also isn't higher-order programming as such. Can you create new functions from doing that? There are ways to compose functions in C by hand, but the distinction is that it cannot be done elegantly. (One could always just do higher-order programming in C by constructing an abstract syntax tree, after all.)

A good first effort in that you did correctly handle calling functions without requiring them to fetch their arguments in an unusual way, which is what most people would try at first. With a varargs apply function, you can handle the case of passing values larger than a register and passing pointers without having to cast them to a list, too. Consider the case of an apply function which takes an unsigned integer as the size of the following argument. If it's zero, then the argument list is finished. Otherwise, it then fetches an argument of that size from the va_list and goes on to the next one. So then your apply function is slightly uglier (int j = 42; apply(printf, sizeof (const char ), "Hello, %s! %u\n", sizeof (const char ), "world"), sizeof j, j, 0) but perhaps more flexible, and at least avoids relying on a few nasty implicit casts.

Also, function pointers should not necessarily be cast implicitly to void *, but on only a few architectures are function pointers so strange that you'll have any trouble with your approach.

As it is, you could even avoid writing or generating any assembly by just having the apply routine be a static inline function that does a switch on the argument count and then casts the function pointer to a function pointer that takes the right number of arguments. The compiler could do constant propagation for the number of arguments and you'd have little overhead in your function calls. You couldn't handle arbitrarily-large argument lists, but it would work pretty well.

Keep at it!

[davvid]

For the set of features you have, with most calling conventions on most platforms you don't need to synthesize all that. You could simply have your apply function be written in assembly and handle setting up the parameters fetching them from memory.

Like this? https://github.com/davvid/libapply

unittest.c shows how it can be used: https://github.com/davvid/libapply/blob/master/unittest.c