How to use a compiled C .so file in rust

Question

has updated because of some suggestions

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System：macOS 10.14.6

The question I want to ask here is how do I use rust to call a compiled .so file, sorry, I am new to this part.

I have a very simple c file:

#include "add.h"

int add(int a, int b) {
    return a + b;
}

Then I used gcc-fPIC -shared -o libadd.so add.c to compile it into a .so file and put it in the lib directory

Then I wrote this in rust's build.rs file:

use std::env;
use std::path::{Path};

fn main() {
    let pwd_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
    let path = Path::new(&*pwd_dir).join("lib");
    println!("cargo:rustc-link-search=native={}", path.to_str().unwrap());
    println!("cargo:rustc-link-lib=dylib=add");
    // println!("cargo:rustc-link-lib=static=add");
    // println!("cargo:rerun-if-changed=src/hello.c");
}

I expect I can get and use this function, main.rs is:

extern { fn add(a: i32, b: i32) -> i32; }

fn main() {
    let c = unsafe { let d = add(3, 5); d };
    println!("c: {:?}", c);
}

cargo build is ok, but cargo run with error:

   Compiling hello-from-generated-code-3 v0.1.0 (/Users/niexiaotao/work/rust-server/rust-ffi/hello-from-generated-code-3)
    Finished dev [unoptimized + debuginfo] target(s) in 0.34s
     Running `target/debug/hello-from-generated-code-3`
dyld: Library not loaded: libadd.so
  Referenced from: /Users/niexiaotao/work/rust-server/rust-ffi/hello-from-generated-code-3/target/debug/hello-from-generated-code-3
  Reason: image not found
[1]    81811 abort      cargo run

other thing: I change the .so to .a and cargo run is ok.

Sample code here

Thank you for your help!

Answer 1

I ran into the same problem. It seems that Cargo can properly build your library because you're explicitly telling it where to look for the file (using rustc-link-search). However, when you go to run it, Linux doesn't know where it is.

If you were to run ldd on your compiled Rust binary, you'd get something like this:

$ ldd target/debug/hello-from-generated-code-3
        linux-vdso.so.1 (0xabcd)
        libadd.so => not found      <----- ldd can't find your library
        ...

This is because your .so file isn't found in LD_LIBRARY_PATH. You can fix this either by copying your library into a relevant folder or just setting it when you run your program. For example, you should be able to say:

LD_LIBRARY_PATH=. cargo run

(Or any other path where your .so file is - not necessarily .)

Answer 2

I didn`t use env variables at all. For instance you have /your-crate/lib/libmystuff.so shared library. In order to link it, specify the following:

println!("cargo:rustc-link-search=native=./lib");
println!("cargo:rustc-link-lib=dylib=mystuff");

Pay attention, library without prefix lib, and path is relative. And just in case. You can also define include folder with your header files:

let headers = Path::new("/headers");
cc::Build::new()
   .file("src/foo.c")
   .include(headers)
   .compile("foo");

Tested on Linux. Best regards.