Currently, I have to do something like this: @ptrCast([*c][*c]const u8, &some_cstr_array), which is very tedious to write. Is there some way to do the same with less typing?
#What's the best way to cast array to C pointer?
steel kraken
tardy pine
Why do you want to do this?
steel kraken
Because C api takes pointer
tardy pine
You can pass eg. *[16][*:0]const u8 to a function that accepts [*c][*c]const u8
It'll implicitly coerce
steel kraken
Let me try to understand this syntax...
tardy pine
Firstly what type is your array?
steel kraken
It's const char**, so in my case [16]*const u8
tardy pine
Is that the type you've written in Zig?
Because if it's an array of C strings, that's not the correct type
steel kraken
const char** is the C type I am trying to target
tardy pine
*const u8 points to a single u8, not a sequence of u8
steel kraken
I am not sure what Zig type I should use
tardy pine
`const char**` is the C type I am trying to target
Sure; I'm asking about the Zig types
steel kraken
But [16]*const u8 should point to 16 C strings, right?
tardy pine
No, it's an array of 16 pointers to singular u8
You should use [16][*:0]const u8 for 16 pointers to 0-terminated sequences of u8 (which is what a C string is)
steel kraken
It says error: expected type '[*c][*c]const u8', found '[16][*:0]const u8'
tardy pine
You need to ref it, you're currently passing the array itself rather than a pointer to it
steel kraken
Then I get error: expected type '[*c][*c]const u8', found '*[16][*:0]const u8' instead
tardy pine
hmm, that should work
steel kraken
If you want to look at a bigger snippet:
var n_exts: c_uint = undefined;
var extensions: [16][*:0]const u8 = undefined;
if (c.SDL_Vulkan_GetInstanceExtensions(window, &n_exts, &extensions) != c.SDL_TRUE) {
@panic("Failed to get required extensions");
}
Ohh ofc, the error note explains the issue
Because it's a mutable pointer type, the function could modify the array and add null pointers
So it has to be a [16]?[*:0]const u8
steel kraken
Nope: error: expected type '*[*:0]const u8', found '*[16]?[*:0]const u8'
steel kraken
src/renderer.zig:32:49: error: expected type '*[*:0]const u8', found '*[16]?[*:0]const u8'
const instance = try VkInstance(n_exts, &extensions);
Right, it's a different place
tardy pine
btw functions shouldn't start with capital letters in Zig :)
(unless they return types)
steel kraken
Hmmm, I thought Zig style guide says if it returns a type it should be TitleCase
tardy pine
Sure, but that function doesn't
steel kraken
It's a constructor?
tardy pine
It returns a vk.Instance I would assume
tardy pine
It's a constructor?
Ah, I understand the confusion
steel kraken
Wait, do you mean returning a "type" not an "instance"?
tardy pine
Yes
I mean if the return type is literally type
(which is how generics work in Zig :)
steel kraken
Are there any good explanation of pointer types in Zig?
BTW, [*c] is a bit awkward to type
Is it required? Or can it be implicitly cast in some case?
tardy pine
You almost never need to write [*c] in your own code
It's designed for use by translate-c and nothing else
tardy pine
Are there any good explanation of pointer types in Zig?
The docs have a decent reference: https://ziglang.org/documentation/master/#Pointers
For the [*:0] syntax: https://ziglang.org/documentation/master/#Sentinel-Terminated-Pointers
steel kraken
So what should this function definition be without [*c]?
fn createVkInstance(n_exts: c_uint, exts: [*c][*c]const u8) !c.VkInstance
I tried quite a few variations of arrays and pointers, but none compile
tardy pine
fn createVkInstance(n_exts: c_uint, exts: [*][*:0]const u8) !c.VkInstance (assuming it can't put null pointers into exts)
steel kraken
Ah, I got this one to work [*]?[*:0]const u8
tardy pine
But if you're writing a nice Zig wrapper, use fn createVkInstance(exts: [][*:0]const u8) !c.VkInstance
If it can put nulls in exts then []?[*:0]const u8
tardy pine
OK... `[]?[*:0]const u8` doesn't work
wdym by "doesn't work"?
tardy pine
Is there any way to remove the nullable?
only with a cast
tardy pine
But what is the error?
The reason I suggest []?[*:0]const u8 is because it gets rid of the extra n_exts arg
But it needs some tweaks inside the function
steel kraken
/opt/homebrew/Cellar/zig/HEAD-06e9b2c_1/lib/zig/std/mem.zig:461:65: error: expected type '[*c]const [*c]const u8', found '[]?[*:0]const u8'
@field(value, field.name) = @field(init, field.name);