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digitalmars.D.learn - static init c struct with array filed

reply test <test123 gmail.com> writes:
```c
struct Test {
     int32_t a;
}
struct Test2 {
     int32_t a;
     Test arr[];
}
```

I need static const init Test2, then pass it to c library 
late(third library, can not change the type def).


I am not sure how to do it in D.
Mar 16 2022
next sibling parent reply user1234 <user1234 12.de> writes:
On Wednesday, 16 March 2022 at 07:27:06 UTC, test wrote:

 ```c
 struct Test {
     int32_t a;
 }
 struct Test2 {
     int32_t a;
     Test arr[];
 }
 ```

 I need static const init Test2, then pass it to c library 
 late(third library, can not change the type def).


 I am not sure how to do it in D.


```d
extern(C) struct Test {
     int a;
}

extern(C) struct Test2 {
      int a;
      Test* arr;
}


static const Test2 t2 = Test2(2, null);

void main(){
    to_c_library(&t2);
    to_c_library(new Test2(2, null));
    Test2 local;
    to_c_library(&local);
}
```

assuming the c library takes by reference
Mar 16 2022
parent reply Era Scarecrow <rtcvb32 yahoo.com> writes:
On Wednesday, 16 March 2022 at 11:27:20 UTC, user1234 wrote:

 assuming the c library takes by reference


  My experience all arrays are effectively just pointers, and the 
brackets/length is only really applicable to stack allocated 
fixed length allocations. In my own C projects i always used 
pointers to pass arrays around.

  There's also static construction, though i don't see how that 
improves anything

https://dlang.org/spec/module.html#staticorder
Mar 16 2022
parent reply user1234 <user1234 12.de> writes:
On Wednesday, 16 March 2022 at 14:42:02 UTC, Era Scarecrow wrote:

 On Wednesday, 16 March 2022 at 11:27:20 UTC, user1234 wrote:

 assuming the c library takes by reference


  My experience all arrays are effectively just pointers


I meant the function that takes the Test2 as parameter, but to be 
honest I dont get exactly what does OP want actually... that's 
true that passing a Test2 is somewhat strange as there's no info 
for the array length.
Mar 16 2022
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 3/16/22 10:03, user1234 wrote:

 On Wednesday, 16 March 2022 at 14:42:02 UTC, Era Scarecrow wrote:

 On Wednesday, 16 March 2022 at 11:27:20 UTC, user1234 wrote:

 assuming the c library takes by reference


  My experience all arrays are effectively just pointers


 I meant the function that takes the Test2 as parameter, but to be honest
 I dont get exactly what does OP want actually...


They want to pass Test2 to C and be able to use it. D ABI defines arrays 
(slices) as a pair of size_t and a pointer:

   https://dlang.org/spec/abi.html#arrays

I haven't passed such a struct to C but I believe the equivalent C 
struct can assume there are the following members in place of the array:

struct Test2 {
     int32_t a;
     size_t length;
     Test * ptr;
}

However, the lifetime of ptr must be ensured on the D side because 
utless there is a reference to it on the D side, the GC can free the 
memory at any time and C side may access invalid memory.

That is explained here:

   https://dlang.org/spec/interfaceToC.html#storage_allocation

Ali
Mar 16 2022
prev sibling parent reply Mike Parker <aldacron gmail.com> writes:
On Wednesday, 16 March 2022 at 07:27:06 UTC, test wrote:

 ```c
 struct Test {
     int32_t a;
 }
 struct Test2 {
     int32_t a;
     Test arr[];
 }
 ```

 I need static const init Test2, then pass it to c library 
 late(third library, can not change the type def).


Any time you see a '[]' in C, the equivalent declaration in D has 
to be a pointer. Like Ali said, D's arrays are a pointer and 
length pair, so declaring the declaration of the `arr` member 
will not match the declaration on the C side.

Since you say you can't change the declaration on the C side, 
user1234's answer is the direction you want to go (though you 
don't need the `extern(C)` on the structs).
Mar 16 2022
parent Adam Ruppe <destructionator gmail.com> writes:
On Thursday, 17 March 2022 at 00:16:39 UTC, Mike Parker wrote:

 On Wednesday, 16 March 2022 at 07:27:06 UTC, test wrote:

 ```c
 struct Test {
     int32_t a;
 }
 struct Test2 {
     int32_t a;
     Test arr[];
 }
 ```

 I need static const init Test2, then pass it to c library 
 late(third library, can not change the type def).


 Any time you see a '[]' in C, the equivalent declaration in D 
 has to be a pointer.


Not always with structs....... if it has a fixed size or an 
initializer in C, then you need to match the size in D.

Otherwise yeah use the pointer.
Mar 16 2022