Dynamic linking between crates

Question

The state of dynamic linking in rust is very confusing to me currently. I can only find old sources that say rust doesn't support dynamic linking, yet I know multiple libraries that do dynamic linking (ex bevy_dylib).
Is it currently possible to create a bin crate and a lib crate, and link the bin to the lib at runtime using dylib or rlib?
Something like

// lib.rs
fn hi() {
   println!("hi");
}


// main.rs
fn main() {
   // whatever to link to lib.rs/lib.so
   link("./lib.so");
   hi();
}

Not sure what you're asking, the reference page on linkage seems pretty clear. — cafce25
– cafce25, Commented Mar 31, 2023 at 23:11
I've looked through that but it doesn't really say if and how the above is possible, it just lists what rust can be compiled too and stuff about linking to C. — salmon
– salmon, Commented Mar 31, 2023 at 23:17
"--crate-type=dylib, #![crate_type = "dylib"] - A dynamic Rust library will be produced. This is different from the lib output type in that this forces dynamic library generation. The resulting dynamic library can be used as a dependency for other libraries and/or executables. This output type will create *.so files on Linux, *.dylib files on macOS, and *.dll files on Windows" clearly says it's possible, you use the type dylib — cafce25
– cafce25, Commented Mar 31, 2023 at 23:18
I know how to make a dylib, but how do I actually link to it from the main binary? — salmon
– salmon, Commented Mar 31, 2023 at 23:22
"use [it] as a dependency" in Cargo.toml [dependencies] yourcrate = {path = "whatever", version = "*" } the same as for a static library — cafce25
– cafce25, Commented Mar 31, 2023 at 23:39

cafce25 · Accepted Answer · 2023-03-31 23:56:03Z

To link dynamically instead of statically the only thing you have to change is the crate type of your library from the default to dylib:

Generate a library:

cargo new --lib dy-lib

Add the type in dy-lib/Cargo.toml everything else can stay unchanged:

[lib]
crate-type = ["dylib"]

Generate a binary and add library as dependency:

cargo new dy-bin
cd dy-bin
cargo add dy-lib --path ../dy-lib

Replace the contents of dy-bin/src/main.rs with something that uses the library:

fn main() {
    println!("{}", dy_lib::add(1, 2));
}

build the binary

cargo build # or cargo run, ...

The binary is now dynamically linked to our dynamic library as can be seen by for example readelf on linux:

$ readelf -d target/debug/dy-bin

Dynamic section at offset 0x2d70 contains 30 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libdy_lib.so]
...

And if you remove it there will be a runtime error:

$ rm target/debug/libdy_lib.so
$ target/debug/dy-bin
target/debug/dy-bin: error while loading shared libraries: libdy_lib.so: cannot open shared object file: No such file or directory

Note that Rust ABI is not stable, so one would better use cdylib instead of dylib and make all necessary functions extern "C".

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