Understanding the windows-targets crate

The windows and windows-sys crates depend on the windows-targets crate for linker support. The windows-targets crate includes import libs, supports semantic versioning, and optional support for raw-dylib. It provides explicit import libraries for the following targets:

  • i686_msvc
  • x86_64_msvc
  • aarch64_msvc
  • i686_gnu
  • x86_64_gnu
  • x86_64_gnullvm
  • aarch64_gnullvm

An import lib contains information the linker uses to resolve external references to functions exported by DLLs. This allows the operating system to identify a specific DLL and function export at load time. Import libs are both toolchain- and architecture-specific. In other words, different lib files are required depending on whether you're compiling with the MSVC or GNU toolchains and whether you're compiling for the x86 or ARM64 architectures. Note that import libraries don't contain any code, as static libraries do.

While the GNU and MSVC toolchains often provide some import libs to support C++ development, those lib files are often incomplete, missing, or just plain wrong. This can lead to linker errors that are very difficult to diagnose. The windows-targets crate ensures that all functions defined by the windows and windows-sys crates can be linked without relying on implicit lib files distributed by the toolchain. This ensures that dependencies can be managed with Cargo and streamlines cross-compilation. The windows-targets crate also contains version-specific lib file names ensuring semver compatibility. Without this capability, the linker will simply pick the first matching lib file name and fail to resolve any missing or mismatched imports.

Note: Ordinarily, you don't need to think about the windows-targets crate at all. The windows and windows-sys crates depend on the windows-targets crate automatically. Only in rare cases will you need to use it directly.

Start by adding the following to your Cargo.toml file:

version = "0.52"

Use the link macro to define the external functions you wish to call:

fn main() {
windows_targets::link!("kernel32.dll" "system" fn SetLastError(code: u32));
windows_targets::link!("kernel32.dll" "system" fn GetLastError() -> u32);

Make use of any Windows APIs as needed:

fn main() {
    unsafe {
        assert_eq!(GetLastError(), 1234);

By default the link macro will cause the linker to use the bundled import libs. Compiling with the windows_raw_dylib Rust build flag will cause Cargo to skip downloading the import libs altogether and instead use raw-dylib to resolve imports automatically. The Rust compiler will then create the import entries directly. This works without having to change any of your code. Without the windows-targets crate, switching between linker and raw-dylib imports requires very intricate code changes. As of this writing, the raw-dylib feature is not yet stable.