Requirements

1. rust (with cargo)
2. gcc
3. cbindgen

Compile the Rust library

$ cd testlib
$ cargo build --release
$ cbindgen --crate testlib --lang c --output ../testlib.h
$ cp target/release/libtestlib.{so,a} ..

Compile the C executable

Using the dynamic lib

$ gcc example.c -L. -ltestlib -o example.out

Using the static lib

$ gcc example.c libtestlib.a -ldl -pthread -o example.out

Execute

$ chmod +x example.out
$ LD_LIBRARY_PATH=. ./example.out

N.B. The environment variable LD_LIBRARY_PATH is only needed for the dynamic version of the library.

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The Rust (dynamic) library could also be loaded at runtime using the dlfcn.h library.
This repository does not offer an example for this alternative.

Useful links

1. FFI - Wikipedia
2. FFI - Rustonomicon
3. A little Rust with your C
4. Exposing FFI from the Rust library
5. Creating a shared and static library with gcc
6. Minimizing Rust Binary Size
7. CBindGen Quick Start