Unmanaged C# calls to a static library - ios

Question

I'm using swig to generate C# wrappers for some C code base to be used from C#. When I run swig, it generates a wrapper c file that exposes all the functionality to the generated PInvoke C# file... For example:
// This is in KodLogic_wrap.c
SWIGEXPORT void SWIGSTDCALL CSharp_DMGameMode_timeLimit_set(void * jarg1, unsigned short jarg2) { ... }
// This is in KodLogicPInvoke.cs
[global::System.Runtime.InteropServices.DllImport("KodLogic", EntryPoint="CSharp_DMGameMode_timeLimit_set")]
This works great when I am building a dynamic library. However, I need to support iOS now, so I've prepared a static library, and passed in the -dllimport '__Internal' option to swig for that to work.
Unfortunately, I am getting linking errors such as:
"_DMGameMode_timeLimit_set", referenced from:
RegisterMonoModules() in RegisterMonoModules.o
(maybe you meant: _CSharp_DMGameMode_timeLimit_set)
Indeed, I did mean "CSharp_DMGameMode_timeLimit_set", but that's the point of the "entrypoint" argument?
So, since this error is thrown by the Xcode project Unity generated, I am not quite sure what's the source of the failure. Does it fail for static libraries? Is this something to be fixed on Unity side or swig side?
Update: After digging more into this, I think I have a slight idea of what's going on here..
The main issue seems to be from the AOT compiler, which tries to compile all the CS code to an ARM assembly. This seems to be required for iOS, so during Unity's AOT compilation, it generates a file RegisterMonoModules.cpp, which attempts to define access functions to the native code. RegisterMonoModules.cpp doesn't honor the entrypoint parameter, which causes undefined symbol errors to be thrown...
Still attempting to find a proper workaround.

Answer 1

The main issue seems to be from Unity, and not Swig nor Mono. As mentioned above, Unity performs AOT compilation that doesn't honor the entry point argument. This produces cpp code that calls the function name, not the entry point name..
I've confirmed this by switching the scripting backend to IL2cpp, and the entry point name was honored there.
Let's switch over to callbacks. Not exactly related to the question, but it definitely fits the context of Unity + Native plugins + iOS.
AFAIK, you can't have a managed method marshaled to native land on iOS using Mono 2x. I previously had to delete all the string callback and exception handlers from the swig generated files. Fortunately, IL2Cpp supports callbacks, after a little tweaking:
Add using AOT;
Decorate callbacks with [MonoPInvokeCallback(typeof(method_signature))]
You can use this script, just use it to process the generated swig files:
def process_csharp_callbacks(pinvoke_file):
"""Process PInvoke file by fixing the decorators for callback methods to use:
[MonoPInvokeCallback(typeof(method_signature))]
"""
# prepare requirements
with open(pinvoke_file) as f:
content = f.read()
callback_methods_regex = re.compile(r"( +)static (?:void|string) (?:SetPending|CreateString)\w*\([\s\w\,]+\)")
callback_decorator = "[MonoPInvokeCallback(typeof(ExceptionDelegate))]"
callback_arg_decorator = "[MonoPInvokeCallback(typeof(ExceptionArgumentDelegate))]"
callback_str_decorator = "[MonoPInvokeCallback(typeof(SWIGStringDelegate))]"
# add use AOT
content = content.replace("\n\n", "\nusing AOT;\n", 1)
# fix callback methods
def method_processor(match):
match_string = match.group()
indentation = match.captures(1)[0]
if match_string.find(",") != -1:
fix = callback_arg_decorator
elif match_string.find("static string") != -1:
fix = callback_str_decorator
else:
fix = callback_decorator
return indentation + fix + "\n" + match_string
content = callback_methods_regex.sub(method_processor, content)
# write it back
with open(pinvoke_file, "w+") as f:
f.write(content)
For anyone looking for help converting their generated swig CSharp PInvoke file to something mono 2x scripting backend will allow, stick this somewhere in your build process, after the CSharp files are generated:
pinvoke_template = """{extern_prefix} CSharp_{method_signature};
{normal_prefix} {method_signature} {{
{return_statement}CSharp_{method_name}({method_args});
}}"""
def process_csharp_wrapper(csharp_dir):
"""Reads the PINVOKE csharp file, and performs the following:
1. Remove EntryPoint="xxx" from the decorators
2. Make the methods match their native counterpart name
3. Add a C# method with the original name, for compatability
"""
# prepare requirements
pinvoke_file = os.path.join(csharp_dir, "KodLogicPINVOKE.cs")
with open(pinvoke_file) as f:
content = f.read()
decorator_regex = re.compile(r', EntryPoint=".*?"')
method_regex = re.compile(r"(public static extern \w+[\w:\.]+)\s(([^S]\w+)\((?:([\w:\. ]+)\,?)*\));")
# fix decorators
content = decorator_regex.sub("", content)
# fix method definitions
def method_processor(match):
extern_prefix = match.captures(1)[0]
return pinvoke_template.format(
extern_prefix=extern_prefix,
normal_prefix=extern_prefix.replace("extern ", ""),
method_signature=match.captures(2)[0],
return_statement=("return " if extern_prefix.find("void") == -1 else ""),
method_name=match.captures(3)[0],
method_args=", ".join(map(lambda s: s.strip().split()[1], match.captures(4)))
)
content = method_regex.sub(method_processor, content)
# write it back
with open(pinvoke_file, "w+") as f:
f.write(content)