Apparantly, the order in which parameters are passed to a dynamically loaded C function is reversed. See the following minimal example:

----------------
%> cat dll.c
#include "stdio.h"

int dll2(const char* first, const char* second) {
    printf("dll2() - first: '%s', second: '%s'\n", first, second);
    return 0;
}

int dll3(const char* first, const char* second, const char* third) {
    printf("dll3() - first: '%s', second: '%s', third: '%s'\n",
           first,
           second,
           third);
    return 0;
}
----------------

I compiled it with the following commands:

%> gcc -c dll.c -fpic
%> gcc -shared -o libdll.dyld dll.o
%> file libdll.dyld
libdll.dyld: Mach-O 64-bit dynamically linked shared library x86_64

Now, I'm using it with a very simple D program and see very unexpected results:

----------------
%> cat main.d
import std.stdio, std.string, core.sys.posix.dlfcn;

// extern functions
alias nothrow int function(const char*, const char*) dll2_fn;
alias nothrow int function(const char*, const char*, const char*) dll3_fn;

void main() {
    string sdlLibPath = "libdll.dyld";
    auto libraryHandle = dlopen(sdlLibPath.toStringz(), RTLD_LAZY);
    scope(exit) dlclose(libraryHandle);

    dll2_fn dll2 = cast(dll2_fn)dlsym(libraryHandle, "dll2");
    dll2("one", "two");

    dll3_fn dll3 = cast(dll3_fn)dlsym(libraryHandle, "dll3");
    dll3("one", "two", "three");
}

%> rdmd main.d
dll2() - first: 'two', second: 'one'
dll3() - first: 'three', second: 'two', third: 'one'
----------------

The order in which the C functions get the parameters is exactly the reverse order in which I supply them in D.

What is happening there? I also tried to export a function that has two different types from the C library. Calling it the same way from the client D program causes a segfault - which I'd expect when the parameters are really reversed.

How can I make sure the order is correct?

On Sunday, 15 November 2015 at 17:54:27 UTC, David Nies wrote:
> How can I make sure the order is correct?

Whenever you use a C function, it must be marked as, even if it's through a function pointer as in this case. Just apply the attribute to the dll2_fn and dll3_fn declarations.

Hope this helps,
David

On Sunday, 15 November 2015 at 18:00:09 UTC, David Nadlinger wrote:
> On Sunday, 15 November 2015 at 17:54:27 UTC, David Nies wrote:
>> How can I make sure the order is correct?
>
> Whenever you use a C function, it must be marked as, even if it's through a function pointer as in this case. Just apply the attribute to the dll2_fn and dll3_fn declarations.
>
> Hope this helps,
> David

How do I mark it as such? Can you please give an example?

Thanks for the quick reply! :)

On Sunday, 15 November 2015 at 18:02:01 UTC, David Nies wrote:
> On Sunday, 15 November 2015 at 18:00:09 UTC, David Nadlinger wrote:
>> On Sunday, 15 November 2015 at 17:54:27 UTC, David Nies wrote:
>>> How can I make sure the order is correct?
>>
>> Whenever you use a C function, it must be marked as, even if it's through a function pointer as in this case. Just apply the attribute to the dll2_fn and dll3_fn declarations.
>>
>> Hope this helps,
>> David
>
> How do I mark it as such? Can you please give an example?
>
> Thanks for the quick reply! :)

I think he refers to this: http://dlang.org/interfaceToC.html

On Sunday, 15 November 2015 at 18:02:01 UTC, David Nies wrote:
> How do I mark it as such? Can you please give an example?
>
> Thanks for the quick reply! :)

Just add extern(C) to the beginning of the "alias" line.

 — David

On Sunday, 15 November 2015 at 18:12:52 UTC, David Nadlinger wrote:
> On Sunday, 15 November 2015 at 18:02:01 UTC, David Nies wrote:
>> How do I mark it as such? Can you please give an example?
>>
>> Thanks for the quick reply! :)
>
> Just add extern(C) to the beginning of the "alias" line.
>
>  — David

Great! Thanks to all of you! That was really quick and helpful!