C.h to D conversion (structs)

[Chris via Digitalmars-d-learn]

On Tuesday, 15 March 2016 at 18:47:22 UTC, Adam D. Ruppe wrote:

> Like you would another D library.

Now I get it! Yes, that works as expected.

>
> The problem isn't the struct itself, but the D initializer. 
> Structs in C don't have initalizers but do in D:
>
> struct Foo {
>    int a = 10; /* illegal in C, legal in D */
> };
>
>
> Note that extern(C) doesn't actually do anything on a struct, 
> it really only applies to functions and global variables.

That's where I got it wrong!

> D realizes this by creating a variable called 
> _DStructName__init which has the initial value of the struct. 
> When you make a variable of that type, it copies the value of 
> this hidden init thing over to your new variable to initialize 
> it. C does nothing of the sort, so the C library will not 
> provide this - you need to link in the one from D.
>
>
> You might be thinking "but I don't use = anything in this 
> struct", but there's a few places where D will put one in 
> automatically:
>
> float and double = NaN by default
> char, wchar, and dchar = -1 (well, 0xff) by default
>
> And everything else =0 (or null) by default.
>
>
> If *everything* in a struct is initialized to 0, the __init 
> variable is left out and memset(&struct, struct.sizeof, 0) is 
> called instead.
>
> But if *anything* in there is non-zero, the __init variable is 
> created and referenced when you declare the variable (unless 
> you =void it at the usage point).
>
>
> Which causes the undefined reference problem you're seeing - 
> the usage code is trying to initialize, but the C library 
> doesn't provide that (since it isn't a C feature) and the D 
> library isn't linked in.

Thanks a million! Now I understand why it didn't work with my 
implementation.

[snip]