Why the linker cannot find my local C shared library for use with FFI in Rust?

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06-10-2024

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The Great Shared Library Hunt: Why Your Rust FFI Can't Find Your C Code

You've built your beautiful C library, ready to be used in your Rust project. You've even compiled it into a shared object (.so on Linux, .dylib on macOS). Yet, when you try to use it with Rust's Foreign Function Interface (FFI), you're met with a cryptic error message: "undefined symbol" or "cannot find -l...". What gives?

This frustrating situation arises from the linker's inability to locate your lovingly crafted C library. It's a common issue that stems from the delicate dance between Rust, your C library, and the linker.

Let's break down the problem:

• Rust's world: Rust uses Cargo for dependency management. Cargo, in turn, relies on the linker to build your Rust project and link it with any necessary libraries.
• Your C library: You've compiled your C library into a shared object (.so or .dylib). This shared object contains the compiled code, ready to be used by other programs.
• The linker: This crucial piece of software, responsible for combining compiled code and libraries, struggles to find your shared object and link it with your Rust project.

Scenario: A Code Example

Let's assume you have a simple C library (mylib.c) with a function add:

// mylib.c
int add(int a, int b) {
  return a + b;
}

You compile it to a shared object:

gcc -shared -fPIC -o libmylib.so mylib.c

Now, your Rust code (main.rs) attempts to call add:

// main.rs
#[link(name = "mylib")]
extern "C" {
    fn add(a: i32, b: i32) -> i32;
}

fn main() {
    println!("2 + 3 = {}", unsafe { add(2, 3) });
}

When you run cargo build, you get the infamous error:

error: linking with `cc` failed: exit status: 1
...
undefined symbol: add

Why the linker is lost:

The linker, in this case, is looking for a library named mylib but can't find it in its usual search paths. This is because the linker, while looking for libmylib.so, doesn't know where to find it.

Solutions to the Library Hunt:

1. Explicitly tell the linker where your library is:

   • RUSTFLAGS: Add the library path to your environment variable:

     RUSTFLAGS="-L /path/to/your/lib" cargo build
     

   • cargo build: Use the --target-dir flag to specify the build directory where the linker should look:

     cargo build --target-dir=target/debug
     

2. Install your library using a package manager:

   • If you're using Linux, consider using tools like pkg-config or creating a .deb or .rpm package. This simplifies sharing your library with others.

3. Link directly using cargo build:

   • cargo build: Pass the library path directly to the linker:

     cargo build --target-dir=target/debug --target-lib=libmylib.so
     

Additional Tips:

• Make sure your library is in a standard location: Some systems (like macOS) have specific locations for shared libraries (like /usr/local/lib).
• Use the --verbose flag: Run cargo build --verbose to see the linker commands and identify the exact paths it's searching.
• Check for naming inconsistencies: Ensure your library name (e.g., mylib in the #[link(name = "mylib")] annotation) matches the actual file name (e.g., libmylib.so).
• Clean and rebuild: Sometimes, a simple cargo clean followed by cargo build can resolve lingering issues.

Remember: The linker is a meticulous detective, and sometimes it needs a little guidance to find your valuable C library. By understanding the linker's search patterns and using these solutions, you can ensure your Rust project finds its way to your C code.