Yes. Signatures of C functions often tells little about correct use of arguments, when there are pointers.
And true, without checking the docs & use example, you cannot really make it work correctly, if you will rely only on signature when designing the call to such function(s).
It is, in fact, orthogonal to FFI .. it is the way it works in C..
But what is good from FFI part, is that you can design a convenient interface to API that having such functions, in a way, that your users don't need to scratch their heads
and stumble upon the code, asking , - hey.. wtf.

That's why i said many times, that yes, it is cool to have automatic FFI wrapper generators, but they will never be as good as well thought and designed manually written interfaces
to libraries, providing features of a library without burden of strange, bizarre and inconvenient way to pass data between user and library.



On 9 November 2017 at 00:06, Nicolas Cellier <nicolas.cellier.aka.nice@gmail.com> wrote:
when I found this bizarre prototype:

^ self
����������������callUnchecked:
������������������������#(LGitReturnCodeEnum git_diff_tree_to_tree #(LGitDiff * diff , LGitRepository repo , LGitTree old_tree , LGitTree new_tree , LGitDiffOptions * opts))
����������������options: #()

While the libgit2 signature differs in indirection levels:
https://libgit2.github.com/libgit2/#v0.19.0/group/diff/git_diff_tree_to_tree

int git_diff_tree_to_tree(git_diff_list **diff, git_repository *repo, git_tree *old_tree, git_tree *new_tree, const git_diff_options *opts);

The fact that it does not differs for opts, made me think of a bug...
But no.

opts is allocated on external c heap with:

callUnchecked: #(LGitReturnCodeEnum git_diff_init_options(LGitDiffOptions * self, LGitOptionsVersionsEnum version))

int git_diff_init_options(git_diff_options *opts, unsigned int version);

What you see this time is that signatures match...
So the LGitDiffOptions handle will be an ExternalAddress that will really point on a git_diff_options
In other words, the handle is a git_diff_options *...
IOW, our LGitDiffOptions object points to an external it_diff_options so it is what it promises to be.

For the other structures not so.
We are lying on the level of indirection, so our LGitTree handle are not really a git_tree *. They are a git_tree **. (an ExternalAddress on a pointer).

As long as we lie consistently, it's OK, because it avoids too much packToArity: unpackToArity: dance.

That's the "beauty" of C pointers.
We can cast them to whatever or pretend they are handle_for_anything * in the intermediate level, as long as original producer and final consumer agree on what it really points to.

But from code quality perspective, it really stinks. Anyone like me opening a page to get the exact signature of the function will be scratching head and loose precious time. Especially when tracking vm crash down.

I'm not sure how well it's documented, I presume it's a well known conscious hack from the original developpers, but such practice really ought to be discussed here.






--
Best regards,
Igor Stasenko.