Hey everybody.

How are you supposed to pass static arrays to C functions? I'm asking because I'm getting conflicting info from how DMD works and on IRC.

The below example prints:
test1 0x7fff857c1db0
test2 0x7fff857c1db0
test3 (nil)
test4 0x7fff857c1db0


D:
void main()
{
        float[3] arr;
        test1(arr);
        test2(&arr[0]);
        test3(0, arr);
        test4(0, &arr[0]);
}

extern(C):
void test1(float[3] arr);
void test2(float *arr);
void test3(int, float[3] arr);
void test4(int, float *arr);


C:
#include <stdio.h>

void test1(float arr[3]) { printf("test1 %p\n", &arr[0]); }
void test2(float arr[3]) { printf("test2 %p\n", &arr[0]); }
void test3(int anything, float arr[3]) { printf("test3 %p\n", &arr[0]); }
void test4(int anything, float arr[3]) { printf("test4 %p\n", &arr[0]); }

Comments please.

Cheers, Jakob.

On 2012-10-24 09:54, Jakob Bornecrantz wrote:
> Hey everybody.
>
> How are you supposed to pass static arrays to C functions? I'm asking
> because I'm getting conflicting info from how DMD works and on IRC.
>
> The below example prints:
> test1 0x7fff857c1db0
> test2 0x7fff857c1db0
> test3 (nil)
> test4 0x7fff857c1db0
>
>
> D:
> void main()
> {
>          float[3] arr;
>          test1(arr);
>          test2(&arr[0]);
>          test3(0, arr);
>          test4(0, &arr[0]);
> }
>
> extern(C):
> void test1(float[3] arr);
> void test2(float *arr);
> void test3(int, float[3] arr);
> void test4(int, float *arr);
>
>
> C:
> #include <stdio.h>
>
> void test1(float arr[3]) { printf("test1 %p\n", &arr[0]); }
> void test2(float arr[3]) { printf("test2 %p\n", &arr[0]); }
> void test3(int anything, float arr[3]) { printf("test3 %p\n", &arr[0]); }
> void test4(int anything, float arr[3]) { printf("test4 %p\n", &arr[0]); }

That seems weird. Since static arrays are passed by value in D you need to send a reference:

extern (C) void test1(ref float[3] arr);

float[3] arr;
test1(arr);

BTW, do not ever use "&arr[0]", use "arr.ptr" instead to get the pointer of the array.

http://dlang.org/interfaceToC.html

Search for "Passing D Array Arguments to C Functions".

-- 
/Jacob Carlborg

On 10/24/2012 12:54 AM, Jakob Bornecrantz wrote:
> How are you supposed to pass static arrays to C functions?

C accepts static arrays as pointers, so T[n] in D would be C prototyped as T*.

Pass a pointer to the first element.

On Wednesday, 24 October 2012 at 09:03:25 UTC, Jacob Carlborg wrote:
> On 2012-10-24 09:54, Jakob Bornecrantz wrote:
>> Hey everybody.
>>
>> How are you supposed to pass static arrays to C functions? I'm asking
>> because I'm getting conflicting info from how DMD works and on IRC.
>>
>> The below example prints:
>> test1 0x7fff857c1db0
>> test2 0x7fff857c1db0
>> test3 (nil)
>> test4 0x7fff857c1db0
>>
>>
>> D:
>> void main()
>> {
>>         float[3] arr;
>>         test1(arr);
>>         test2(&arr[0]);
>>         test3(0, arr);
>>         test4(0, &arr[0]);
>> }
>>
>> extern(C):
>> void test1(float[3] arr);
>> void test2(float *arr);
>> void test3(int, float[3] arr);
>> void test4(int, float *arr);
>>
>>
>> C:
>> #include <stdio.h>
>>
>> void test1(float arr[3]) { printf("test1 %p\n", &arr[0]); }
>> void test2(float arr[3]) { printf("test2 %p\n", &arr[0]); }
>> void test3(int anything, float arr[3]) { printf("test3 %p\n", &arr[0]); }
>> void test4(int anything, float arr[3]) { printf("test4 %p\n", &arr[0]); }
>
> That seems weird. Since static arrays are passed by value in D you need to send a reference:

Yeah I'm wondering if this isn't a bug.

>
> extern (C) void test1(ref float[3] arr);
>
> float[3] arr;
> test1(arr);
>
> BTW, do not ever use "&arr[0]", use "arr.ptr" instead to get the pointer of the array.

Right you are, thats what you get when mixing C and D code.

>
> http://dlang.org/interfaceToC.html
>
> Search for "Passing D Array Arguments to C Functions".

Thanks for the info.

Cheers, Jakob.

On Wednesday, 24 October 2012 at 09:16:30 UTC, Walter Bright wrote:
> C accepts static arrays as pointers, so T[n] in D would be C prototyped as T*.
>
> Pass a pointer to the first element.

 In GCC (or any c compiler) isn't it possible in optimizing to copy the static array to the stack (say 8 bytes or less as a guess)? Course this assumes the static size is set as part of the function signature too...

 It makes sense for it to work both ways (in their own way); But then more bugs can lie in wait as well.

On 24-10-2012 12:00, Era Scarecrow wrote:
> On Wednesday, 24 October 2012 at 09:16:30 UTC, Walter Bright wrote:
>> C accepts static arrays as pointers, so T[n] in D would be C
>> prototyped as T*.
>>
>> Pass a pointer to the first element.
>
>   In GCC (or any c compiler) isn't it possible in optimizing to copy the
> static array to the stack (say 8 bytes or less as a guess)? Course this
> assumes the static size is set as part of the function signature too...
>
>   It makes sense for it to work both ways (in their own way); But then
> more bugs can lie in wait as well.

Passing static arrays by reference to the first element is part of the calling convention/ABI. If the C compiler generated code that doesn't work this way, the C compiler has a bug.

-- 
Alex Rønne Petersen
alex@lycus.org
http://lycus.org

On 10/24/2012 2:55 AM, Jakob Bornecrantz wrote:
> On Wednesday, 24 October 2012 at 09:03:25 UTC, Jacob Carlborg wrote:
>> On 2012-10-24 09:54, Jakob Bornecrantz wrote:
>>> Hey everybody.
>>>
>>> How are you supposed to pass static arrays to C functions? I'm asking
>>> because I'm getting conflicting info from how DMD works and on IRC.
>>>
>>> The below example prints:
>>> test1 0x7fff857c1db0
>>> test2 0x7fff857c1db0
>>> test3 (nil)
>>> test4 0x7fff857c1db0
>>>
>>>
>>> D:
>>> void main()
>>> {
>>>         float[3] arr;
>>>         test1(arr);
>>>         test2(&arr[0]);
>>>         test3(0, arr);
>>>         test4(0, &arr[0]);
>>> }
>>>
>>> extern(C):
>>> void test1(float[3] arr);
>>> void test2(float *arr);
>>> void test3(int, float[3] arr);
>>> void test4(int, float *arr);
>>>
>>>
>>> C:
>>> #include <stdio.h>
>>>
>>> void test1(float arr[3]) { printf("test1 %p\n", &arr[0]); }
>>> void test2(float arr[3]) { printf("test2 %p\n", &arr[0]); }
>>> void test3(int anything, float arr[3]) { printf("test3 %p\n", &arr[0]); }
>>> void test4(int anything, float arr[3]) { printf("test4 %p\n", &arr[0]); }
>>
>> That seems weird. Since static arrays are passed by value in D you need to
>> send a reference:
>
> Yeah I'm wondering if this isn't a bug.

In D, static arrays are passed by value, in C by pointer. Hence, you have a mismatch between the C and D prototypes of test1, and you'll get garbage results.

Walter Bright:

> In D, static arrays are passed by value, in C by pointer. Hence, you have a mismatch between the C and D prototypes of test1, and you'll get garbage results.

Isn't it possible to help the programmer avoid some similar mistakes with some warnings or errors?

I think the extern(C) annotation, plus the function signatures, give the D compiler all the info it needs.

Bye,
bearophile

On 10/24/12, bearophile <bearophileHUGS@lycos.com> wrote:
> Isn't it possible to help the programmer avoid some similar mistakes with some warnings or errors?
>
> I think the extern(C) annotation, plus the function signatures, give the D compiler all the info it needs.

Agreed. File to bugzilla unless Walter disagrees?

On 10/24/12, Andrej Mitrovic <andrej.mitrovich@gmail.com> wrote:
> Agreed. File to bugzilla unless Walter disagrees?

Also, what should the error message look like? I've got a pull in works fwiw.