static arrays in C functions

Lutger lutger.blijdestijn at gmail.com
Tue Dec 8 15:54:45 PST 2009


Steven Schveighoffer wrote:

> On Tue, 08 Dec 2009 11:53:12 -0500, Bill Baxter <wbaxter at gmail.com> wrote:
> 
>> On Tue, Dec 8, 2009 at 2:08 AM, Lutger <lutger.blijdestijn at gmail.com>
>> wrote:
>>> Since a while some extern(C) functions which take arrays seem to be
>>> broken.
>>> Can anybody clarify /confirm how they should be declared?
>>>
>>> For example I stumbled upon this:
>>>
>>> import core.sys.posix.unistd, std.stdio;
>>>
>>> void main()
>>> {
>>>    int[2] fd;
>>>    writeln( pipe(fd) ); // failes with errno == EFAULT
>>> }
>>>
>>> In core.sys.posix.unistd, pipe is declared as: int pipe(int[2]);
>>>
>>>
>>> This works though:
>>>
>>> extern (C) { int pipe(int*); }
>>>
>>> void main()
>>> {
>>>    int[2] fd;
>>>    writeln( pipe(fd.ptr) );
>>> }
>>
>> (Assuming you're talking about D2 here...)
>> A few releases ago fixed-size arrays changed to be pass-by-value.
>> But I guess there's still some logic in there to interpret int[2] as
>> int* when inside an extern(C) block.
> 
> No it compiles *because* that logic is not there.  It now thinks int[2] is
> a pass-by-value entity.  It links because you are using C linkage which
> does not do name-mangling.  I could define pipe as:
> 
> extern (C) int pipe(char c, int x, float y);
> 
> and it will still link :)

Interesting, I didn't realize that but it makes sense!

> 
>> It does seem like there's a bug there, though.  I think pipe(fd) in
>> the first case should fail to compile because it's attempting to pass
>> by value where a pointer is expected.
> 
> The error is either:
> 
> a) you now need to declare C functions that were declared in C taking an
> array to taking a pointer, so core.sys.posix.unistd (and likely others)
> needs to be fixed.
> b) as you suggested, inside a C block, int[2] should be interpreted as int
> *.
> 
> I'd prefer a, because I don't care much about direct translation of C
> headers :)  If b is chosen as a solution, I'd also like to have the
> compiler automatically pass the pointer when calling a C function.
> 
> -Steve

Thanks for the explanation, looks like there is some work to do in the 
binding department. This looks like a case where a piece of C code silently  
does something different in D.


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