Linking Rust application with a dynamic library not in the runtime linker search path

12,597

Solution 1

Here's a complete solution...

I created a C library exporting a simple addition function. I also created a Cargo project to use this function.

/scratch
├── executable
│   ├── Cargo.lock
│   ├── Cargo.toml
│   ├── build.rs
│   ├── src
│   │   ├── main.rs
└── library
    ├── awesome_math.c
    └── libawesome_math.so

awesome_math.c

#include <stdint.h>

uint8_t from_the_library(uint8_t a, uint8_t b) {
  return a + b;
}

The library was compiled as gcc -g -shared awesome_math.c -o libawesome_math.so.

src.rs

extern crate libc;

extern {
    fn from_the_library(a: libc::uint8_t, b: libc::uint8_t) -> libc::uint8_t;
}

fn main() {
    unsafe {
        println!("Adding: {}", from_the_library(1, 2));
    }
}

build.rs

fn main() {
    println!("cargo:rustc-link-lib=dylib=awesome_math");
    println!("cargo:rustc-link-search=native=/scratch/library");
}

Cargo.toml

[package]
name = "executable"
version = "0.1.0"
authors = ["An Devloper <[email protected]>"]
build = "build.rs"

[dependencies]
libc = "*"

[profile.dev]
rpath = true

Doing all of this exhibits the same problem that you experienced. This is called a Minimal, Complete, and Verifiable Example and you should provide one when asking a question. If this were provided, this answer might have been created 12 hours earlier.

Investigating further, I asked the Rust compiler to print out the linker args it was going to use:

cargo rustc -- -Z print-link-args

This printed out a bunch of stuff, but the two important lines were

"-Wl,-rpath,$ORIGIN/../../../../root/.multirust/toolchains/stable-2016-11-08-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib"
"-Wl,-rpath,/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib"

These are directives to the linker to add specific values to the rpath of the finished binary. Missing is any reference to the dynamic library that we are linking to. In retrospect, this probably makes sense, as how would the compiler know that we want to include it in the rpath?

A workaround is to add another directive to the linker. There are interesting options (like $ORIGIN), but for simplicity, we will just use an absolute path:

cargo rustc -- -C link-args="-Wl,-rpath,/scratch/library/"

And the resulting binary prints the right thing for ldd and runs without setting LD_LIBRARY_PATH:

$ ldd target/debug/executable | grep awesome
    libawesome_math.so => /scratch/library/libawesome_math.so (0x00007fe859085000)
$ ./target/debug/executable
Adding: 3

Turning to making it relative, we can use $ORIGIN:

cargo rustc -- -C link-args='-Wl,-rpath,$ORIGIN/../../../library/'

Be careful to escape $ORIGIN properly for your shell, and remember that the path is relative to the executable, not the current working directory.

Solution 2

Adding to what Shepmaster said (apparently I don't have enough reputation to comment): I'm not sure when this feature was added, but as of at least Rust 1.20, you can achieve the same effect by setting the environment variable RUSTFLAGS:

$ RUSTFLAGS="-C link-args=-Wl,-rpath,/the/lib/path" cargo build

This can be more convenient than the cargo rustc option if, for instance, you're using build scripts that just invoke cargo build.

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Ameo
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Ameo

https://cprimozic.net

Updated on June 07, 2022

Comments

  • Ameo
    Ameo over 1 year

    I have a shared library that I'd like to dynamically link into several separate binary Cargo applications. I include its location in the linker using the -- -L /path/to/dir format and the application compiles correctly with the significant decrease in binary size I expect. However, when checking the generated binary using ldd, I get a message saying that the library couldn't be found:

    casey@Gilthar-II:~/bot4/backtester/target/release$ ldd backtester 
        linux-vdso.so.1 =>  (0x00007ffc642f7000)
        libalgobot_util.so => not found
    

    If I add the library to the /lib/x86_64-linux-gnu directory, the application runs without issue.

    Is there a way to get Rust to look for .so files in the same directory as the binary or in a directory like lib in the binary's directory to be loaded at runtime? If that's not possible, is there a way to at least get Rust to insert the absolute path of the library it was linked with?

    I've tried setting rpath = true with no effect.

    • BurntSushi5
      BurntSushi5 almost 7 years
      Does adding the path to /etc/ld.so.conf and running ldconfig not work for you? (Or failing that, setting the LD_LIBRARY_PATH environment variable?)
    • Shepmaster
      Shepmaster almost 7 years
      Is there a way to get Rust to look for .so files — this isn't something about Rust once the binary is created; it's up to the OS and executable loaders. Setting the rpath is something that Rust can control (more accurately that Rust instructs the linker to deal with).
  • Ameo
    Ameo almost 7 years
    Thanks for this incredibly informative response! You simplified and clarified what took me several hours of trawling the internet into a single well-written format. Just one question: Is $ORIGIN relative to the location of the binary or the location of the source code being compiled?
  • Shepmaster
    Shepmaster almost 7 years
    @Ameo $ORIGIN is relative to the executable.
  • Parisa.H.R
    Parisa.H.R over 1 year
    @Shepmaster Each time I want to run the program I should run those commands? Is there any way that the program remembers the library and If I changed the value or ask it from the user just build and run it?
  • Shepmaster
    Shepmaster over 1 year
    @Parisa.H.R Each time I want to run the program I should run those commands — I don't understand your question; please expand and clarify it. The commands I show (cargo rustc ...) are about building the executable. Running the executable (./target/debug/executable) does remember the library; that's how dynamic linking and rpath work.
  • Parisa.H.R
    Parisa.H.R over 1 year
    means that if I changed numbers from_the_library(1, 2) to 3 and 4 and build it again I should link to the library again? Sorry I am new to rust
  • Shepmaster
    Shepmaster over 1 year
    @Parisa.H.R see How can I specify linker flags/arguments in a build script?; TL;DR you can also usecargo:rustc-link-arg in your build script.