Non pub functions are exported as DLL symbols

[kobe2000]

I tried this code:

#[no_mangle]
extern "C" fn foo() {}

I build a Windows DLL, I use this foo function with other static library written in C.

I think that this function should not be exported as DLL symbols, but it does

Meta

rustc --version --verbose:

rustc 1.82.0 (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: x86_64-pc-windows-msvc
release: 1.82.0
LLVM version: 19.1.1

[workingjubilee]

why did you attach no_mangle to it?

[ehuss]

This is the expected behavior of no_mangle (see docs). See #54135 and #54451 for some discussion as to why that is.

[kobe2000]

why did you attach no_mangle to it?

Without it I can not link the fooo funciton at C side(emcc) due to name mangling

extern "C" void foo();

// #[no_mangle]
extern "C" fn foo() {}

wasm-ld: error: lto.tmp: undefined symbol: foo

[kobe2000]

This is the expected behavior of no_mangle (see docs). See #54135 and #54451 for some discussion as to why that is.

I read them, but it there any way I can link rust function in static C library without export it?

flowchart
    R(Rust dynamic library) --> C(C static library)
    C -.->|call foo| R
    A(Executable application) --> R

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[ChrisDenton]

This is the expected behavior of no_mangle (see docs). See #54135 and #54451 for some discussion as to why that is.

Hm, I'm not actually sure that covers DLLs. There is a rationale and documentation for why it must be considered public for staticlibs, rlibs, etc. But this doesn't cover DLLs. E.g. the docs for no_mangle say:

Additionally, the item will be publicly exported from the produced library or object file, similar to the used attribute.

It does not mention the binary at all. And if we look to used for more clarity we see it expressly excludes the final binary:

This attribute forces the compiler to keep the variable in the output object file (.o, .rlib, etc. excluding final binaries) even if the variable is not used, or referenced, by any other item in the crate.

[ultimaweapon]

You can use .def file to export the symbols with https://learn.microsoft.com/en-us/cpp/build/reference/def-specify-module-definition-file?view=msvc-170. When specify /DEF options the MSVC linker will export only symbols in .def file.

[dtolnay]

I believe this is a duplicate of #73958.