[Glass] FFI and C subsystems

[James Foster]

Bruno, 

My understanding is that the header parser follows all the #include directives, so will find everything that could be called from C code, including abs() in libc. But the process of calling a dynamic library at runtime involves doing a name lookup of entry points *for that library,* not for anything visible *to* the library, so libssh does not have an abs() entry point. For that you would need a reference to libc. 

The wrapper generator does not know what is actually in the given library, just what functions are defined by all included header files. So you have to do some selection on your own. One wrapper API has a filter as an argument, and so for a library like GemStone C Interface where all the functions begin with ‘gci’ we can filter to only generate wrappers for those functions (it is unlikely that libc has gci*() functions!).

So, yes, you need to have a different library object for each library so the system can do name lookups properly.

James

> On Oct 9, 2020, at 5:40 AM, Bruno Buzzi Brassesco via Glass <glass at lists.gemtalksystems.com> wrote:
> 
> Hi,
> Maybe i'm wrong in some concepts here (not a C expert) but ...
> 
> If a C library (libssh) has a subsystem (sftp) in order to create a GS wrapper i can do:
> | header wrapperClass wrapper |
> header := CHeader path: '/usr/include/libssh/sftp.h'.
> wrapperClass := header wrapperForLibraryAt:
> '/usr/lib64/libssh.so.4.4.0'.
> wrapperClass initializeFunctions.
> UserGlobals at: wrapperClass name put: wrapperClass.
> 
> In this case I got all the functions of Libssh and also SFTP, so far so good.
> 
> But what happens when there is more than one C subsystem?
> 
> How can I generate only one GS wrapper for the original C library (libssh) and all its subsystems ?
> Or I will have to create one wrapper per subsystem ?
> 
> regards,
> bruno
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