Zig 0.17.0-dev (Split by item)

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Header

Needed because libc memory allocators don't provide old alignment and size which are required by Zig memory allocators.

malloc.Header

const Header = packed struct(u64)

File

lib/c/malloc.zig:51

Code

const Header = packed struct(u64) {
    alignment: Alignment,
    /// Does not include the extra alignment bytes added.
    size: Size,
    canary: Canary = magic,

    comptime {
        assert(@sizeOf(Header) <= alignment_bytes);
    }

    const safety = switch (builtin.mode) {
        .debug, .safe => true,
        .fast, .small => false,
    };
    const max_addr_bits = switch (safety) {
        true => 48, // Ensures space for Canary bits.
        false => 64,
    };
    const Size = @Int(.unsigned, @min(max_addr_bits, 64 - @bitSizeOf(Alignment), @bitSizeOf(usize)));
    const Canary = @Int(.unsigned, 64 - @bitSizeOf(Alignment) - @bitSizeOf(Size));
    const magic: Canary = switch (safety) {
        true => @truncate(@as(u64, 0x76fa65bebb3d7a39)), // statically chosen entropy
        false => 0,
    };

    fn get(base: [*]align(alignment_bytes) u8) Header {
        const header: *Header = @ptrCast(base - @sizeOf(Header));
        assert(header.canary == magic);
        return header.*;
    }

    fn set(base: [*]align(alignment_bytes) u8, a: Alignment, size: Size) [*]align(alignment_bytes) u8 {
        const header: *Header = @ptrCast(base - @sizeOf(Header));
        header.* = .{ .alignment = a, .size = size };
        return base;
    }
}