Linking to C library with Rust FFI

[isaac-asdf]

I'm trying to use a C library with a Rust wrapper rust-secp256k1. I downloaded the C library and built it with a target of --host=xtensa-esp32. I then added the library to my .cargo/config.toml linker flags:

rustflags = [
  "-C", "link-arg=-Tlinkall.x",
  "-C", "link-arg=-nostartfiles",
  "-C", "link-arg=-Tesp32_rom_functions.x",
  "-L", "libsecp256k1/libsecp256k1.a", "-l", "secp256k1",
]

However, when building I get an error that says

linking with 'xtensa-esp32-elf-gcc' failed: exit status: 1
... lots of stuff...
= note: ../target/xtensa-esp32-none-elf/release/deps/..... : undefined reference to `rustsecp256k1_v0_8_1_keypair_create'

Should I be linking that library differently?

Source code for project

[bjoernQ]

I think it should be "-L", "libsecp256k1/", (i.e. just the path) - in theory the linker should complain it can't find the libsecp256k1.a but it prefers to show you "undefined reference" errors

[isaac-asdf]

When switching to that, the error code is:

error: linker `xtensa-esp32-elf-gcc` not found

It seems like the crate I'm working with has a build step of the C lib which renames symbols, so I might need to skip that and include my own compiled version of the lib. I'll try what they suggested on this issue and update if that fixed it

[bjoernQ]

Maybe just having xtensa-esp32-elf-gcc on your path is enough - but precompiling the library and including it should work

[isaac-asdf]

Update: it appears the issue is from cc. This repo shows a basic C function being linked via cc and erroring out with the same undefined reference... error message. I was able to run the exact same code in a normal project without seeing the issue. @bjoernQ do you know where I should follow up on this? Would it be creating an issue in this repo, the rust fork, or elsewhere?

[bjoernQ]

Thanks for providing the example repo

I was able to get it to compile and run by defining CC and AR like this

set CC=xtensa-esp32-elf-gcc
set AR=xtensa-esp32-elf-ar

(Windows in cmd.exe syntax shown)

It looks like while the target is xtensa-esp32-none-elf the name of the executables is xtensa-esp32-elf-XXX so it fails at guessing CC/AR

Seems like this is not unexpected since their README talks about defining these env-variables here: https://github.com/rust-lang/cc-rs#external-configuration-via-environment-variables

[isaac-asdf]

That worked, thanks! I expected the compilation parameters to be passed in automatically when setting the build target, so thanks for the link and help!