keen plover
I am trying to copy a C array into a slice:
const spv = try alloc.alloc(u8, c.shaderc_result_get_length(result)); // []u8
const bytes = c.shaderc_result_get_bytes(result); // [*c]const u8
@memcpy(spv, bytes);
But unfortunately I get this error:
error: type '[*c]const u8' does not support field access
I read the docs of @memcpy. Source is a multi item pointer and dest is a slice, and they are both u8 types, so I don't get why this copy does not compile.
wicked summit
[*c] is not a many item pointer, it's a c pointer
fierce hollow
surely it should allow that since C pointers coerce?
keen plover
Why doesn't it automatically cast to [*]const u8? I have to manually cast it.
wicked summit
it probably should be allowed, but my guess is that it isn't checking that case
runic tapir
Isn't it a problem with nullability?
keen plover
Well, ?[*c] is a thing, but it's not the case here
fierce hollow
C pointers are nullable by default, they map as close as possible to pointers in C
fierce hollow
it probably should be allowed, but my guess is that it isn't checking that case
im guessing youre somewhat familiar with sema? https://github.com/ziglang/zig/blob/21ae64852a531c36ae3166aa2b6f1fbaaf76c6f9/src/Sema.zig#L3503 does checking for C pointers here make sense? that function is only called in zirMemcpy and seems to fix it
havent run any tests yet tho so idk if it causes any breakages
wicked summit
that is probably the right place, but as stnap mentioned, c pointers are nullable, so you can't just use them like that
fierce hollow
why not? arent C pointers generally used like anything else but assumed to not be null?
assumed in the same way as in C, youre kinda meant to check it before using it but theres no automatic safety for you
wicked summit
why not? arent C pointers generally used like anything else but assumed to not b...
no, c pointers are optional, always; you’re not supposed to be using them in the first place
fierce hollow
well i get that
i mean that theyre kinda more like allowzero pointers, youre not required to unwrap them before use. i dont understand wym by they cant be used like this
wicked summit
memcpy requires non optional pointers, because it does not check for null
fierce hollow
thats kind of my point, C pointers are always used in the same context as non optional pointers with an assumption that its not null
wicked summit
then they're being used wrong
assuming a nullable pointer is not null is how we get c-style null pointer dereferences
the short simple answer is that you should be avoiding [*c] at all costs, they should be replaced or casted to the correct zig pointer type
earnest urchin
I'm guessing that @memcpy([]u8, [*]const u8) would use the dest's length as the copy length?