Calling into an ECL-compiled .so from C, best practices, how to pass strings, etc...
eschulte at grammatech.com
Tue Mar 9 00:01:26 UTC 2021
I’m trying to compile a common lisp system into a .so which I can then wrap a Python library around to enable easy sharing of the functionality as a Python package (wheel). I’m having trouble finding examples of other C code calling into ECL-compiled shared objects of lisp code. I hope I can solicit some answer and critical code review here, or pointers to exemplar implementations would also be much appreciated.
So far I have:
1. A Makefile  rule to build the .so from the common lisp package. This part actually seems to be working well (if slow to compile).
2. A C source file  wrapping the ECL-compiled shared object. I think this is likely where most of my problems are: specifically:
* How does one get a string back out from Common Lisp to C? Currently I’m jumping through hoops that seem like they should be unnecessary  and only work some of the time (other times I’m getting multiple copies of the string text).
* How does one get a UTF8 string from C (wchar_t*) into Common Lisp? I haven’t found a way so for now I’m limiting my input to ASCII (char*) .
* Is the best way to invoke a function really to call c_string_to_object (e.g.,  among many others)? It seems like there should be a more efficient way than converting a string into a function name for every function dispatch.
* Finally, calling init  has two problems. First, it takes a very long time every time I load the library. Is there no way to save the state of the lisp image after initialization? Second, it loads ASDF from a path relative to my local system. Can I get ASDF compiled into the shared object to remove as many external dependencies as possible?
Any help is much appreciated. Also, thanks to those who answered issues on the ECL GitLab repository to help me get this far.
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