On 22 Feb 2015, at 01:19, Nicolai Hess <nicolaihess@web.de> wrote:

2015-02-22 0:06 GMT+01:00 Max Leske <maxleske@gmail.com>:
Hi.

I have a situation with libgit2 where the function I need to call is a field in a struct (the field gets populated with different functions with the same signature depending on certain conditions).

I��m facing two problems now:
1. how do I define a function as the field type of an NBExternalStructure?

Athens creates a structure with callbacks as a surface plugin for the bitblter (SQSurfaceDispatch).
But this is a structur with functions (generated with NB) that are called "from the outside"

2. how can I call the function?

I don't know of any example in the image, where we are calling a function defined in a struct.
Maybe you can extend NBFFICallout to generate a call from a function spec + function address

NBFFICallout>>#generateCall:module: does this, it generates a call from a function spec and and
address loockuped in a module, maybe you can do something similiar, but use the function address from
the structure isntead of calling nbGetSymbolAddress:module:

I��ll give that a try, thanks.

I��ve tried using callbacks as field types. That works as far as I can access the functions by pointer. But how would I then call that function?
I��ve looked at NBNativeFunction and NBNativeFunctionGen but I can��t see how I should use either of those to make a plain call.

Here��s a more precise description of my problem:

writepack := LG2GitOdbWritepackS new.
�� initialize the struct externally��

��now call the function ��commit�� and pass the arguments (this is how I imagine it could work��)"
writepack commit
value: writepack
value: anotherStruct.

Needed struct fields:

backend: git_odb_backend *
append: int (*)(git_odb_writepack *, const void *, size_t, git_transfer_progress *)
commit: int (*)(git_odb_writepack *, git_transfer_progress *)
free: void (*)(git_odb_writepack *)

For what is this needed? Do you want to write your own git backend?

No. I need to implement packing of loose objects to save space. libgit2 only does this for network transport. For this I need to work with the pack backend directly and the backend provides the function I need to call for finishing the write in the ��commit�� field of the struct. Of course, I know which function that will be since I��m working with one particular backend. But if I wanted to support multiple backends in the future it would be better to call the provided function instead of explicitly naming it.

I find this strange. I only know of two use cases for function pointers in a structs.
1. to define the callbacks (like athens does it for the SQSurfaceDispatch). But we only define the functions and don't call them.
2. a library defines a public api and delegates the implementation to different backends, but structure and backend functions
are hidden. Users of the library don't call them.

Exactly. I��m dipping deeper here than one usually needs to.

Thanks Nicolai.

My current (ad hoc) struct fields:

fieldsDesc
^#(
LG2GitOdbO backend;
LG2WritePackAppendCb append;
LG2WritePackCommitCb commit;
LG2WritePackFreeCb free;
)

I��d even be happy if I have to implement a method on LG2GitOdbWritepackS for every function call (e.g. ��writepack commit: arg1 and: arg2��) but how would I make the call?

Any ideas?
Max