What does `cargo:backends` mean in build.rs?

guerlando · July 22, 2022, 7:43pm

The package gtk3-rs has this line on build.rs:

gtk-rs/gtk3-rs/blob/master/gdk/sys/build.rs#L37


      
              // The pkg-config file defines a `targets` variable listing the
              // various backends that gdk was compiled for.
              // We extract that and create gdk_backend="x11" and the like
              // as configuration variables.
              // In addition we publish this as a variable which cargo will
              // provide to immediate dependents of this crate as an environment
              // variable for their `build.rs` runs called DEP_GDK_BACKENDS
              // For reference, the backend set at time of writing consists of:
              // x11 win32 quartz broadway wayland
              if let Ok(targets) = pkg_config::get_variable(PKG_CONFIG_PACKAGE, "targets") {
                  println!("cargo:backends={}", targets);
                  for target in targets.split_whitespace() {
                      println!("cargo:rustc-cfg=gdk_backend=\"{}\"", target);
                  }
              }
          }

I'm trying to find how it instructs the compiler for C dependencies, but I couldn't find what cargo:backends stands for.

How does this leads to -lX11 being added for linkage?

17cupsofcoffee · July 22, 2022, 7:50pm

I believe this is just arbitary key/value metadata that gets passed down to dependent packages, as mentioned here: https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key

Something in another crate's build script will be looking for the DEP_GDK_BACKENDS variable that gets defined as a result.

guerlando · July 22, 2022, 7:53pm

I couldn't find any other place in this lib that uses DEP_GDK_BACKENDS. My main problem is that I'm building a shared library instead of a binary, using gtk3-rs, so it tries to link with libX11.a. I need to find where in gtk3-rs it links with X11. I know that one of the backends is x11 but the .a part is still a mistery

system · October 20, 2022, 7:54pm

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What does `cargo:backends` mean in build.rs?

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