> On 22 Feb 2015, at 01:19, Nicolai Hess <nicolaih...@web.de> wrote: > > 2015-02-22 0:06 GMT+01:00 Max Leske <maxle...@gmail.com > <mailto:maxle...@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 >
