Open mjforan opened 2 months ago
NobodyXu
commented
2 months ago Sorry I don't understand how you are calling std::cout, can you provide the code please?
Just including the header file won't work, and linking with stdc++ also won't work.
C++ ABI is different from rust with its own name demangles so that it can support function overloading.
mjforan
commented
2 months ago test.cpp
#include <iostream>
extern "C" void test_cpp() {
std::cout<<"hello"<<std::endl;
}lib_x_example.rs
#[link(name = "test_cpp")]
extern { fn test_cpp(); }
pub fn main() {
unsafe {test_cpp();}
}Thanks.
And can you also show me the build.rs ?
Because you would need it to be put in the workspace, not in src or examples, but in the workspace .
Only when it's in there, it would be automatically picked up by cargo, and run by cargo to compile and link the c++ file.
mjforan
commented
2 months ago Yes, build.rs is in the workspace, as well as Cargo.toml, Cargo.lock, and the target directory. I left them out of the initial issue for simplicity. The relevant section of build.rs is listed above.
NobodyXu
commented
2 months ago Thanks.
I think you would have to define
#[link(name = "test_cpp")]
pub extern { fn test_cpp(); }in the lib.rs, when you compile and link with an external library, you usually define the API of the external library in the library itself.
mjforan
commented
2 months ago That does not work. The example still fails to compile.
NobodyXu
commented
2 months ago Can you remove
.cpp_link_stdlib("stdc++")I think using the default libstdc++ might work.
mjforan
commented
2 months ago No, that value is correct for my platform. Removing it does not help. https://docs.rs/cc/latest/cc/struct.Build.html#method.cpp_link_stdlib https://docs.rs/cc/latest/cc/#c-support
Would you mind reproducing on your end? It seems you are just taking guesses here and I have already tried these things.
Another clue: when I wrap the c++ function in my lib.rs and call that method from my example, then I am also able to use the c++ function directly in my example.
NobodyXu
commented
2 months ago Would you mind reproducing on your end? It seems you are just taking guesses here and I have already tried these things.
Sorry I don't have access to my computer for now.
Another clue: when I wrap the c++ function in my lib.rs and call that method from my example, then I am also able to use the c++ function directly in my example.
Yes, that's pretty much how most crates use cc and external crates.
Creates bindings for it in rust library crate, then call the library crate.
I think the error you experience might be due to how rust links crates.
AFAIK, cargo does a linking per crate to produce rlib, so if the external library function isn't used in the library crate, it might strip out the unused linkage to stdc++.
Since rlib isn't stablised and mostly internal to cargo, there isn't much documentation on it.
For that particular behavior, it might be faster to ask on rust's official zulip in t-cargo channel
Problem: cc is unable to find stdc++ when building an example for a library crate.
✅ works: binary crate, call c++ from example
✅ works: library crate, call c++ from library
❌ does not work: library crate, call c++ from example
build.rs:
Error output:
Environment: Raspberry Pi OS (aarch64) and Ubuntu 24.04 (amd64) with fresh install of Rust: cargo 1.81.0, cc="1.1.18"
I have tried setting various options in
build.rssuch as manually specifying the path to/lib/aarch64-linux-gnu/libstdc++.so.6