Intro to calling C libraries

[Dejan Lekic]

On Thursday, 20 February 2025 at 20:09:29 UTC, Ian wrote:
> What is the recommended documentation or introductory material 
> on how to call C libraries from D? I've seen it done in GtkD 
> (for GTK3) and I have heard of -betterC, but it's all a little 
> overwhelming. (I'm an experienced C programmer but new to D)

Ian, You do not need importC in order to interface with a C 
library. A good starting read would be: 
https://dlang.org/spec/interfaceToC.html

If you know the C code (or C library you want to interface with) 
it is relatively easy to use it directly from D. The real 
challenge sometimes is to "port" complex C macros to D. The 
following page is an essential read: 
https://dlang.org/articles/ctod.html

Here is a simple start project. Let's call liblz4's 
LZ4_versionNumber() function to get the version of the library, 
and print it out:

```d
module lz4version;

import std.stdio;

// https://github.com/lz4/lz4/blob/dev/lib/lz4.h line 142
extern(C) int LZ4_versionNumber();

int main() {
     int lz4Version = LZ4_versionNumber();

     int m = lz4Version / 10000;
     lz4Version = lz4Version % 10000;
     int n = lz4Version / 100;
     lz4Version = lz4Version % 100;
     int r = lz4Version;

     writeln("LZ4 library version: ", m, ".", n, ".",r);

     return 0;
}
```

Save this code to the lz4version.d file, and let's compile and 
link to lz4version executable with:

`gdc -o lz4version lz4version.d -llz4`

Now, let's run it:
```
shell» ./lz4version
LZ4 library version: 1.9.4
```

Indeed that is the version of the LZ4 library I have on my local 
Fedora 40 workstation: `/usr/lib64/liblz4.so.1.9.4`