Calling C functions

[Denis]

On Friday, 26 June 2020 at 08:15:27 UTC, Jacob Carlborg wrote:
> On Friday, 26 June 2020 at 00:30:22 UTC, Denis wrote:

>>   extern(C) void cfunc(void function(int));
>>   extern(C) void dcallback(int x) {...}		<-- Why extern(C)?
>>   cfunc(&dcallback);
>>
>> Can this be rewritten, dropping the prefix from the second 
>> line? If not, it would be helpful to know why "extern(C)" is 
>> needed here too.
>
> No, it cannot be dropped. `extern(C)` is required because C and 
> D are using different calling conventions (D functions are also 
> mangled). For example, D (at least DMD and LDC) are passing the 
> arguments to the function in reverse.

OK, now this makes sense.

I tested calling the same callback function directly from D: it 
compiled and worked correctly. So at least prefixing the callback 
function with `extern(C)` doesn't prevent the rest of the D 
program from calling it too.

>> (2) Is there a way to restrict the invocation of a linked C 
>> function to one specific D function?

[...]

> For functions nested in a D language construct (class, struct, 
> function) the compiler will always use the D mangling, instead 
> of the C mangling. In theory it would be possible to workaround 
> that by forcing the mangled name using `pragma(mangle)`, but 
> for some reason the compiler doesn't allow `pragma(mangle)` 
> inside a function body, on a nested function declaration.
>
> You can wrap up everything in a struct, as follows:

I see.

Thank you very much for these explanations and code -- the 
insights are very helpful.

Denis