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digitalmars.D.learn - Passing string array to C

reply Andre Pany <andre s-e-a-p.de> writes:
Hi,

I have this coding. Function `sample` will later be called from C
and should provide access to a string array.

I tried to read the string values after the function call
and I can access the first string, but for the second string,
there is an access violation.

Why does it crash?

Kind regards
André

```
import std;

void main()
{
	size_t* i;
	const(wchar)* r;
	sample(&r, &i);

         // Try to read the 2 string values
	const(wchar) ** r2;
	r2 = &r;
	auto arr = r2[0..*i];
	writeln(to!string(arr[0])); // Works
	writeln(to!string(arr[1])); // Fails
}

extern(C) export void sample(const(wchar)** r, size_t** c)
{
	string[] arr = ["fooä", "bar"];
	auto z = new const(wchar)*[arr.length];
	foreach(i, ref p; z)
	{
		p = toUTF16z(arr[i]);
	}
	
         *r = z[0];
	
	*c = new size_t();
	**c = arr.length;
}
```
Sep 10 2020
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Thursday, 10 September 2020 at 14:31:41 UTC, Andre Pany wrote:

 Why does it crash?


You messed up the pointers.

A string is one star.

An array of strings is two stars.

A pointer to an array of strings is /three/ stars.

---
import std;

void main()
{
         size_t* i; // this need not be a pointer either btw
         const(wchar)** r; // array of strings
         sample(&r, &i); // pass pointer to array of strings

         // Try to read the 2 string values
         auto arr = r[0..*i]; // slice array of strings
         writeln(to!string(arr[0])); // Works
         writeln(to!string(arr[1])); // all good
}

// taking a pointer to an array of strings so 3 stars
extern(C) export void sample(const(wchar)*** r, size_t** c)
{
         string[] arr = ["foo¤", "bar"];
         auto z = new const(wchar)*[arr.length];
         foreach(i, ref p; z)
         {
                 p = toUTF16z(arr[i]);
         }

         // previously you were sending the first string
         // but not the pointer to the array
         // so then when you index above, arr[1] is bad math
         *r = &z[0];

         *c = new size_t();
         **c = arr.length;
}
---
Sep 10 2020
parent Andre Pany <andre s-e-a-p.de> writes:
On Thursday, 10 September 2020 at 15:41:17 UTC, Adam D. Ruppe 
wrote:

 On Thursday, 10 September 2020 at 14:31:41 UTC, Andre Pany 
 wrote:

 [...]


 You messed up the pointers.

 [...]


Fantastic, thank you so much Adam.

Kind regards
André
Sep 10 2020