CODEWITHSUNDEEP
rust
Frederik Deweerdt
Frederik Deweerdt

Reputation: 5271

How to export a symbol from a Rust executable?

I'm trying to export a symbol from a Rust executable:

#[allow(non_upper_case_globals)]
#[no_mangle]
pub static exported_symbol: [u8; 1] = *b"\0";

fn main() {
    println!("Hello, world!");
}

exported_symbol doesn't appear to be exported by the resulting binary:

$ cargo build
$ nm ./target/debug/test_export| grep exported_symbol

On the other hand, if I build a library with the same source, the symbol does get exported:

$ rustc --crate-type cdylib src/main.rs
$ nm libmain.so| grep exported_symbol
0000000000016c10 R exported_symbol

I'm using Rust 1.18.0 on Linux x86-64.

Upvotes: 9

Views: 4289

Answers (2)

Frank Denis
Frank Denis

Reputation: 1501

I'd recommend this in the .cargo/config file instead of the above:

[build]
rustflags = ["-C", "link-args=-rdynamic"]

-rdynamic is more portable. In particular, it works both on Linux and MacOS.

Also, in current Rust/LLVM versions, unless a symbol is actually used, the linker is very likely to remove it.

In order to avoid this, a dummy function referencing the exported functions should be called (at any point in time, such as in the main function).

An example of such a function could be:

pub fn init() {
    let funcs: &[*const extern "C" fn()] = &[
        exported_function_1 as _,
        exported_function_2 as _,
        exported_function_3 as _      
    ];
    std::mem::forget(funcs);
}

And of course, the exported functions should have the #[no_mangle] property:

#[no_mangle]
pub extern "C" fn exported_function_1() {
  // ...
}

Upvotes: 3

Evin Robertson
Evin Robertson

Reputation: 288

You can pass linker options to rustc, like:

$ rustc src/main.rs --crate-type bin -C link-args=-Wl,-export-dynamic
$ nm main|grep export
00000000000d79c4 R exported_symbol

You'll probably want to put this in your rustflags in your .cargo/config something like:

[target.x86_64-unknown-linux-gnu]
rustflags = [ "-C", "link-args=-Wl,-export-dynamic" ]

Upvotes: 3

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