Creating your first DLL in Rust

As a systems programming language with similar linkage support to that of C and C++, it is quite straightforward to build a DLL in Rust. Rust does however have it's own notion of libraries that are quite different to that of C and C++, so it's just a matter of finding the right configuration to produce the desired output.

As with most Rust projects, you can start with Cargo and get started with a basic template but it's so simple we'll just build it by hand here to see what's involved. Let's create a directory structure as follows:

> hello_world
  Cargo.toml
  > src
    lib.rs

Just two directories and two files. There's the hello_world directory that contains the project as a whole. In that directory we have a Cargo.toml file that contains metadata for the project or package, information needed to compile the package:

[package]
name = "hello_world"
edition = "2021"

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

At a minimum, the [package] section includes the name and Rust edition your package is compiled with.

Rust-only libraries don't generally include a [lib] section. This is necessary when you need to specifically control how the project will be used and linked. In this case, we're using 'cdylib' that represents a dynamic system library and maps to a DLL on Windows.

The src sub directory contains the lib.rs Rust source file where we can add any functions that we'd like to export from the DLL. Here's a simple example:

#[no_mangle]
extern "system" fn HelloWorld() -> i32 {
    123
}

The [no_mangle] attribute just tells the compiler to disable any name mangling and use the function name verbatim as the exported identifier. The extern "system" function qualifier indicates the ABI or calling convention expected for the function. The "system" string represents the system-specific calling convention which generally maps to "stdcall" on Windows.

And that's it! You can now build the package and it will produce a DLL:

> cargo build -p hello_world

Cargo will drop the resulting binaries in the target directory where you can then use them from any other programming language:

> dir /b target\debug\hello_world.*
hello_world.d
hello_world.dll
hello_world.dll.exp
hello_world.dll.lib
hello_world.pdb

Here's a simple example in C++:

#include <stdint.h>
#include <stdio.h>

extern "C" {
    int32_t __stdcall HelloWorld();
}

int main() {
    printf("%d\n", HelloWorld());
}

You can build it with MSVC as follows:

cl hello_world.cpp hello_world.dll.lib

The dumpbin tool can be used to further inspect imports and exports.

> dumpbin /nologo /exports hello_world.dll

Dump of file hello_world.dll

File Type: DLL

  Section contains the following exports for hello_world.dll

    00000000 characteristics
    FFFFFFFF time date stamp
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 00001000 HelloWorld = HelloWorld

> dumpbin /nologo /imports hello_world.exe

Dump of file hello_world.exe

File Type: EXECUTABLE IMAGE

  Section contains the following imports:

    hello_world.dll
             140017258 Import Address Table
             140021200 Import Name Table
                     0 time date stamp
                     0 Index of first forwarder reference

                           0 HelloWorld

Kenny Kerr

Creating your first DLL in Rust