Export a Glu Library to C++

We can not only import C++ libraries into Glu but also export Glu libraries to C++. This also allows you to import libraries from another language, and re-export them to a second language. In this example, we will export a Glu library to C++, but the same principle can be applied to other languages.

Creating a C++ File

Create a file named main.cpp in the project directory.

touch main.cpp

Open the main.cpp file in a text editor and add the following code.

namespace libb {
__attribute__((weak)) void funcB() {}
}

int main() {
    libb::funcB();
}

This file declares a function funcB that we then call from the main function. The __attribute__((weak)) attribute tells the C++ compiler that the function is weakly linked, meaning that it can be overridden by a function with the same name in another translation unit. This is necessary because the function will be overridden by our Glu implementation. The function is empty because otherwise, the C++ compiler would not emit debug information for it, which the Glu compiler needs to generate the necessary glue code.

Implementing an external function from Glu

Create a file named libb.glu in the project directory.

touch libb.glu

Open the libb.glu file in a text editor and add the following code.

// This file implements function libb::funcB from main.cpp
@implement import main::libb::funcB;

func funcB() {
    std::print("Hello from Glu!");
}

This file defines a function funcB that we will export to C++. The @implement annotation on the import statement tells the Glu compiler to match the functions in the current Glu library with the imported declaration.

Compiling the Program

First, compile the C++ program to LLVM bitcode, which the Glu compiler can understand.

clang++ -g -emit-llvm main.cpp -o main.bc

This will create an LLVM bitcode file named main.bc from the C++ source file main.cpp. The -g flag is used to include debugging information in the generated bitcode file, which the Glu compiler will use to import the C++ code correctly.

Then, compile the Glu library to LLVM bitcode.

gluc -I . -emit-llvm libb.glu -o main.bc

Then, compile and link the C++ program.

clang++ main.cpp libb.bc -o main

This will compile the C++ program main.cpp and link it with the Glu library libb.bc to create an executable file named main. The libb.bc file contains the correct implementation of the funcB function in Glu, which will override the weakly defined function in the C++ program.

You can then run the executable with ./main on Linux or macOS, or .\main.exe on Windows.