"Undefined reference" when trying to reference Rust libraries from C.

[IoIxD]

Been scratching my head for hours wondering what I'm doing wrong. I've used this successfully with other projects, but for some reason, I can't use it with this one without getting a bunch of errors:

/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::DynamicImage(std::vector<char, std::allocator<char> >)':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:4:(.text+0x39): undefined reference to `internal__Load__DynamicImage__FromMemory(unsigned char*, unsigned long)'
/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::GetDimensions()':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:8:(.text+0x5e): undefined reference to `internal__DynamicImage__Dimensions(__DynamicImage__*)'
/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::GetPixel(unsigned int, unsigned int)':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:11:(.text+0x86): undefined reference to `internal__DynamicImage__GetPixel(__DynamicImage__*, unsigned int, unsigned int)'
/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::width()':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:14:(.text+0xa0): undefined reference to `internal__DynamicImage__GetWidth(__DynamicImage__*)'
/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::height()':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:17:(.text+0xba): undefined reference to `internal__DynamicImage__GetHeight(__DynamicImage__*)'
/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::InBounds(unsigned int, unsigned int)':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:20:(.text+0xe2): undefined reference to `internal__DynamicImage__InBounds(__DynamicImage__*, unsigned int, unsigned int)'
/usr/bin/ld: CMakeFiles/ligma-browser.dir/imageload/imageload.cpp.o: in function `DynamicImage::~DynamicImage()':
/home/gavin/Projects/ligma-browser/imageload/imageload.cpp:23:(.text+0xfc): undefined reference to `internal__DynamicImage__Free(__DynamicImage__*)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/ligma-browser.dir/build.make:109: ligma-browser] Error 1
make[1]: *** [CMakeFiles/Makefile2:351: CMakeFiles/ligma-browser.dir/all] Error 2
make: *** [Makefile:156: all] Error 2

The relevant code is:
lib.rs:

#[no_mangle]
pub extern "C" fn internal__Load__DynamicImage__FromMemory(
    data: *mut u8,
    len: usize,
) -> __DynamicImage__ {
    let slice = unsafe { std::slice::from_raw_parts(data, len) };
    let image = load_from_memory(slice).unwrap();

    __DynamicImage__ {
        image: Box::leak(Box::new(image)) as *mut image::DynamicImage as *mut c_void,
    }
}

Which is fed into cbindgen, resulting in the following __internal__imageload.hpp:

struct __DynamicImage__ internal__Load__DynamicImage__FromMemory(uint8_t *data, uintptr_t len);

Referenced by imageload.cpp:

DynamicImage::DynamicImage(std::vector<char> data) {
  this->img = internal__Load__DynamicImage__FromMemory((uint8_t*)data.data(),
                                                       data.size());
};

The function in question being what actually throws the linker error.

I'm only using the library as I have in the past.

cmake output

And if you must compile, the cmake file handles the dependencies automatically, just make sure you have glm, GLEW, and libcurl.

[jschwe]

cmake output

I don't see anything there - perhaps you copied a wrong link?

Which is fed into cbindgen, resulting in the following __internal__imageload.hpp:

Just to be sure: Is it in an extern "C" block?

If you run nm -D yourRustLib.so, does it show internal__Load__DynamicImage__FromMemory as T?
And if you look at the actual failing linker command line, is yourRustLib.so
a) on the line
b) to the right of your cpp image library?

[IoIxD]

It was that I had to make an extern "C" block, yes.

What's happening now, though, is the linker prioritizes the main function from the Rust code over the one from my C code. Putting a main function within it causes that to be seen as _start, and not putting it causes a linker error.

/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/14/../../../../lib64/crt1.o: in function `_start':
(.text+0x1b): undefined reference to `main'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/ligma-browser.dir/build.make:109: ligma-browser] Error 1
make[1]: *** [CMakeFiles/Makefile2:351: CMakeFiles/ligma-browser.dir/all] Error 2
make: *** [Makefile:156: all] Error 2

My CMakeLists.txt is here. What am I doing wrong?

[jschwe]

Why are there two main functions though? If you want to link your Rust code into a C++ executable, then the Rust code should be built as a staticlib (or cdylib if you need dynamic linking). If you want to link C++ code into a Rust executable, then the C++ code should be built as a static or shared library.

[IoIxD]

I am compiling it as a staticlib! The problem is that for some reason corrosion wants a main function anyways.

[jschwe]

Corrosion / rust doesn't require a main function for static or dynamic libraries.
Can you still reproduce the problem if your CMakeLists.txt only contains the the corrosion code for importing the library, and no other code?

[IoIxD]

For some reason, making the folder in question it's own CMakeLists.txt folder and adding it as a subdirectory worked, something I didn't have to do previously. Weird.