Public "stdcall" symbol not exported on cdylib on i686-pc-windows-gnu

[YaLTeR]

I'm making a cdylib crate which exports a function in the following manner:

#[no_mangle]
pub extern "system" fn SomeFunction() {}

The library is then supposed to be loaded by another program at runtime and the function dlsym'd from it.

This works everywhere (32/64-bit Linux, macOS, 32/64-bit Windows MSVC, 64-bit Windows GNU) except the i686-pc-windows-gnu target where the function isn't being exported from the resulting DLL for some reason.

Minimal example is the code above in a clean crate with crate-type = ["cdylib"].

Possibly related: #50007, although for me the bug occurrs in both debug and release.

Rust: stable (1.25.0), beta, nightly.

[dpelevin]

Here is a sample project to reproduce the bug:
https://github.com/dpelevin/bug_with_dll_exports

I can reproduce it using 'x86_64-pc-windows-gnu' target, but not 'x86_64-pc-windows-msvc'.

The other interesting thing, is that in 'release' mode with

[profile.release]
lto = true

it is not reproduced. It happens only while 'debug' builds and 'release' builds with LTO turned off.

rustc 1.28.0 (9634041 2018-07-30)
gcc.exe (Rev1, Built by MSYS2 project) 8.2.0

[ancwrd1]

I can reproduce this for "system" calling convention. When using "C" calling convention the symbols are exported correctly.

Happens for i686-pc-windows-gnu target, both debug and release. The x86_64-pc-windows-gnu target works. Tried it with different MinGW versions including cross-platform build (under Linux host).

[Prior99]

Did you ever find a workaround for this? I have a similar problem with the opus-sys crate where the compiler removes all other symbols except for the opus ones.

[ancwrd1]

No solution with just cargo build. I do it with custom RUSTFLAGS and running build script for x86 target.

RUSTFLAGS="-C link-arg=exports.def"

exports.def contains symbols to be exported.

[Prior99]

@Dremon I am struggling with this for days. Thank you for your help. Do you happen to have an example project with an example .def file somewhere?

[ancwrd1]

It's pretty simple:

LIBRARY yourlibname
EXPORTS
    ExportedRustFunction1
    ExportedRustFunction2

And the Rust code is:

#[no_mangle]
pub unsafe extern "system" fn ExportedRustFunction1() {
}

[Prior99]

@Dremon With x86_64-pc-windows-gnu?

For me it still cannot find the symbols:

  = note: Cannot export MAIN: symbol not defined
          Cannot export VSTPluginMain: symbol not defined

Perhaps they vanish or are optimized out before getting exported?

[ancwrd1]

It works with both x86_64- and i686-pc-windows-gnu (I only had this issue with i686 though).

[ancwrd1]

I think it will only work for the symbols from your crate, not for symbols from any dependencies.

[Prior99]

Yeah, that was what I was trying to accomplish. Thanks for your time.

[mati865]

Triage: seems to work fine.

Tested debug and release for each i686-pc-windows-gnu and x86_64-pc-windows-gnu using examples from #50176 (comment) and #50176 (comment) but could not reproduce any issue.