Warnings while compiling to WebAssembly

[allsey87]

Hi! I am working on compiling ROS2 nodes to WebAssembly using Emscripten so that I can simulate robot control software in the browser. For the most part, this is going smoothly so far but there are a couple warnings that I think should be addressed:

1. Conditional compilation in rcutils_get_platform_library_name

   rcutils/src/shared_library.c:289:16: warning: unused parameter 'buffer_size' [-Wunused-parameter] rcutils/src/shared_library.c:290:8: warning: unused parameter 'debug' [-Wunused-parameter]

It seems when I compile using Clang/Emscripten, none of the #ifdefs are matching my configuration in rcutils_get_platform_library_name, which is a bit unusual since I was under the impression that Emscripten defines __LINUX__ (uppercase). I will investigate this, however, there is also the possibility of checking whether __EMSCRIPTEN__ has been defined.

2. Conditional compilation in rcutils_strerror

   rcutils/src/strerror.c:32:10: warning: incompatible integer to pointer conversion initializing 'char *' with an expression of type 'int' [-Wint-conversion]

This warning seems very similar to the previous one. It seems again that conditional compilation has gone down the wrong path since the version strerror_r that comes with Emscripten's C standard library returns an int instead of a char*.

For reference, I am using the Emscripten 3.1.0 (Clang 14.0.0) with the following colcon mixin:

{
    "build": {
        "emscripten": {
            "cmake-args": [
                "-DCMAKE_TOOLCHAIN_FILE=/path/to/emsdk/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake",
                "-DCMAKE_STAGING_PREFIX=/path/to/developer/emsdk/upstream/emscripten/cache/sysroot/usr/",
                "-DBUILD_SHARED_LIBS=OFF",
                "-DBUILD_TESTING=OFF",
                "--no-warn-unused-cli"
            ]
        }
    }
}

[allsey87]

Note: Emscripten does support dynamic linking (with so or wasm as the recommended extension), however, it is still a bit strange that shared_library.c is getting compiled at all when BUILD_SHARED_LIBS=OFF...

[aprotyas]

Just for reference - since WASM is a separate virtual ISA, it might be helpful to look at what was done for other platforms (like QNX): https://github.com/ros2/ros2/issues/988.

1. For point 1, as you alluded, another condition can be added to https://github.com/ros2/rcutils/blob/67095577eaf39977f30f7db1ae92f7b0b2ec48d0/src/shared_library.c#L297

2. For point 2, rcutils_strerror does have a conditional branch that deals with the XSI-compliant strerror_r signature (second signature), but I think conditional compilation went down the wrong path here too. https://github.com/ros2/rcutils/blob/67095577eaf39977f30f7db1ae92f7b0b2ec48d0/src/strerror.c#L37-L44