std.rand.DefaultPrng causes memory corruption

[samhattangady]

I am storing a randomiser variable of type std.rand.Random in a struct. Whenever I use that randomiser to get a random number, it corrupts memory after it in the struct.

const std = @import("std");

const RNG = struct {
    const Self = @This();
    num1: u32 = 42,
    randomiser: std.rand.Random,
    num2: u32 = 42,

    pub fn init() Self {
        return Self{
            .randomiser = std.rand.DefaultPrng.init(0).random,
        };
    }
};

pub fn main() anyerror!void {
    var rng = RNG.init();
    std.debug.print("{d}, {d}\n", .{rng.num1, rng.num2});
    _ = rng.randomiser.int(u32);
    std.debug.print("{d}, {d}\n", .{rng.num1, rng.num2});
}

Output:

42, 42
42, 17465386

I am able to get around this by changing the type stored in the struct to std.rand.DefaultPrng, so I am not sure whether I am making some mistake by storing a std.rand.Random, or whether this is actually a bug. I also tried this with std.rand.DefaultCsprng, and while there was no memory corruption in that case, it did lead to a segmentation fault may mean that there is a bug there as well.

I am using zig version zig-windows-x86_64-0.9.0-dev.71+138afd5cb

Thanks,

[Vexu]

You are taking the interface out of the implementation and causing invalid pointer dereferences, this is a known footgun and will be fixed by #7769.

[samhattangady]

Good to note. Thank you!