Functions Missing from Cross-Compiled DLL for Windows

[sbuercklin]

I am compiling a shared library (.so, .dylib, .dll) that I need to call using C FFI from another language. The problem I'm running into is that this works for so and dylib files on Linux + Mac respectively, but the DLL seems to be missing functions marked extern "C".

This might be a mis-use of FFI in the language I'm using (Julia), but it works on both Linux + Mac. When I open up the DLL in DLL Export Viewer, I don't see the names of the exported functions either (e.g. test_fn below)

For an MWE, I have the following lib.rs and Cargo.toml:

#[no_mangle]
pub extern "C" fn test_fn(tag_id_c: i32) -> i32 {
    return tag_id_c;
}

[package]
name = "wincompile"
version = "0.1.0"
edition = "2021"

[dependencies]

[lib]
crate-type = ["cdylib"]

And I compile it using cargo-zigbuild for Windows and Linux (x86_64-unknown-linux-gnu and x86_64-pc-windows-gnu).

I'm compiling the code inside a slightly modified form of the standard cargo-zigbuild image via

cargo zigbuild --release --target x86_64-pc-windows-gnu

For completeness, the Julia code I'm trying to verify this with

# Called from Linux, prints a pointer indicating it found the function
lib = Libdl.dlopen("/path/to/libwincompile.so")
Libdl.dlsym(lib, :test_fn)
# Ptr{...}

# Call on Windows, throws error
lib = Libdl.dlopen("/path/to/wincompile.dll")
Libdl.dlsym(lib, :test_fn)
# ERROR: could not load symbol "test_fn"

[Coding-Badly]

Are any exports making it into the DLL?

[sbuercklin]

There are functions showing up in the DLL, just not anything I defined. Everything is either _Unwind_... or unw_..., or _GCC_specific_handler.

[Coding-Badly]

The export table is assembled by the linker. Were I in your shoes I'd try building the DLL using off-the-shelf Rust tools (e.g. no zigbuild nor gnu; use Microsoft's linker). If that works then the linker you're using is probably the culprit.

[sbuercklin]

Building directly on my machine did work, I opened an issue on cargo-zigbuild to look into what's going wrong with the linker there. Thanks!