Defining External Functions

First of all, we need to actually define a function we want to use from the extern code. Let's start with the C code for this example. Create a file named hello.h in the root directory of your project, it should contain this code:

#include <stdio.h>

void hello() {
    printf("Hello from C!\n");
}

Then you need to write the following code into your main.ft file:

extern def hello();

def main():
    hello();

As you can see, we can define extern functions simply by adding the extern keyword in front of the function definition. What you also can see is that the function does not have a body. Adding a body to extern functions is not allowed, and it will result in a compile error.

You can now try to compile the Flint file using the command

flintc --file main.ft

You should see a output like the following:

Parse Error at main.ft:1:1
└─┬┤E0000│
1 │ extern def hello();
┌─┴─┘
├─ Defined 'extern' function without the FIP running and active
└─ Check your configs in '.fip/config/' to see if there are any problems with it

This tells us that FIP is not active in the compiler yet, but how comes that? FIP is only active and activated when there exists a .fip directory containing a config directory containing a fip.toml configuration file. If the configuration file is faulty or nonexistent, FIP will not launch, so calling external functions will not work. To resolve this, you need to create a .fip directory in your source directory and in that create a config directory and put a fip.toml file in there. The fip.toml should look like this:

[fip-c]
enable = true

We also need to create a fip-c.toml configuration file to be able to use the extern defined .h header file. Create a fip-c.toml file in the config directory, it should look like this:

compiler = "gcc"
sources = ["hello.h"]
compile_flags = []

The resulting file structure should look like this:

.fip/
 └─ config/
     ├─ fip.toml
     └─ fip-c.toml
hello.h
main.ft

With these configuration files added, let's try to compile the program again:

flintc --file main.ft

And now we... still see the same error? Yes. FIP is based on the concept of Interop Modules, and we will fix our problem in the next chapter!