C native library linking issue for Rust

help
1

Hi, I have a C native library linking issue for Rust. Let's say I have the following setup in a Linux system:

  1. A native C library (Lib A) which uses a custom build system
  2. A Rust wrapper (FFI) library (Lib B) to exports the functions of the above C library, a build.rs script is there to find the Lib A and link to Lib A statically
  3. A Rust library (Lib C) to use the Lib B and other libs for its work
  4. A binary (Bin D) which use Lib C to do its work

When I build the Bin D, I got the Undefined Reference error. The undefined references are just the functions in Lib A.

My question is, why the symbols that Bin D use are not linked to Lib B statically? I expect Lib B has all the required symbols and code, so all dependencies (Lib C and Bin D) can just use Lib B to work. What's the solution for this problem?

2

What does your build.rs script for Lib B look like?

3

That's common expectation but that's not how static libraries work on POSIX platforms. If you use library B and it depends on library A then you have to specify -lB -lA (in that order) in the command-line of the linker.

You may find approximately bazillion partial solutions on stackoverflow. Typical solution is to use pkgconfig or libtool but there are lots of other half-solutions.

But they are just the different ways to satisfy the basic requirement outlined above.

That issue is the reason C libraries often vendor in third-party code instead of using external libraries.

4

It looks like the following, but uses a special method to find path of Lib A.

println!("cargo:rustc-link-lib=static=A");
println!("cargo:rustc-link-search={Lib A path}");
5

Lib A is the only native library, Lib B and Lib C are rust libraries, so seems it's not "-lB -lA" problem.

6

Are you using an absolute or relative path for {Lib A path}? I can't reproduce the issue if I use an absolute path.

7

I'm using absolute path for Lib A. The problem seems the Lib A is not linked to Lib B. I see there is a note for linking in Using C libraries in Rust: make a sys crate (kornel.ski), seems the static linking works in a very weird way.

8

The extern crate is implied by Lib C using any symbols from Lib B, and from Bin D using any symbols from Lib C.

It's odd that it isn't linking, though. What's the full output that you get from cargo build -v following a full cargo clean, including the rustc invocations?

9

Seems the problem is the system setup. When building Bin D, the path for Lib A is inaccessible somehow. I have to embed the Lib A into the Lib B, so it can always be found. The problem solved.

This raises another question. Is it possible to let a crate (Bin D) to use the compiled form (rlib, etc.) of its dependency (Lib B), so the crate (Bin D) won't need the original native library (Lib A) during linking?

10

I can't give you a conclusive answer, because I still have no clue what your setup looks like. But you might want to look into the --extern flag, which allows you to add additional rlibs to the compilation that you can import with extern crate B;. Note that if you're using static linking, then the object files from Lib A should be copied into the rlib in Lib B.