Add std.mem.zeroes to the standard library #4092
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This zero initializes the type passed in. Can be used to zero initialize c structs.
FireFox317
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Is returning stack memory of Item undefined behavior in Zig once zero's scope returns? |
It's a by-value return, so it's fine. The memory is provided by the caller and the data is copied into it. What you have to look out for is taking the address of stack memory, e.g. with |
Yeah, I just noticed that. Thought I first saw an address return but my eyes must be fuzzy today. |
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Why is it limited to |
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@LemonBoy because I'm not sure if normal/packed structs can also be zeroed like this. Because you can get null pointers and stuff, correct me if I'm wrong |
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Indeed, I had something less safe in mind such as a member + return like the |
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I do think this function should be discouraged for non-extern, non-packed structs (even if a reflection-based version exists that writes 0 memberwise). I intentionally removed the Writing zeroes to a non-packed, non-extern struct would clobber the safety field that allows safety of |
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| var item: T = undefined; | ||
| @memset(@ptrCast([*]u8, &item), 0, @sizeOf(T)); |
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Is this defined? e.g. if T was a u24 then the size is 3 but @sizeOf returns 4.
See e.g. #4093
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The code is correct. @sizeOf gives the number of bytes the type takes up in memory. This function is valid for types which have well-defined memory layout, which includes extern structs (the main use case).
| pub fn zeroes(comptime T: type) T { | ||
| if (@sizeOf(T) == 0) return T{}; | ||
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| if (comptime meta.containerLayout(T) != .Extern) { |
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Should allow packed as well?
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not necessary for merge, made complicated by #3133
daurnimator
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Feel free to ask me to add anything. I can also allow packed structs and add a note regarding #3133 if that would be desired. |
This is a useful first implementation. Further PRs are welcome to improve it as well :) |
I'm not sure if we should only allow extern structs or specific types, but this is a start.
See: #2257 (comment)