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Add std.mem.zeroes to the standard library #4092

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Jan 7, 2020
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Add std.mem.zeroes to the standard library #4092

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27 changes: 27 additions & 0 deletions lib/std/mem.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -273,6 +273,33 @@ pub fn set(comptime T: type, dest: []T, value: T) void {
d.* = value;
}

/// Zero initializes the type.
/// This can be used to zero initialize a C-struct.
pub fn zeroes(comptime T: type) T {
if (@sizeOf(T) == 0) return T{};

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

@compileError("TODO: Currently this only works for extern types");
}

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).

return item;
}

test "mem.zeroes" {
const C_struct = extern struct {
x: u32,
y: u32,
};

var a = zeroes(C_struct);
a.y += 10;

testing.expect(a.x == 0);
testing.expect(a.y == 10);
}

pub fn secureZero(comptime T: type, s: []T) void {
// NOTE: We do not use a volatile slice cast here since LLVM cannot
// see that it can be replaced by a memset.
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