How to use a compiled C .so file in rust - c

Question

has updated because of some suggestions
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.