Runtime behavior changes depending on read-only variable access

[koljakube]

While playing around with wrapping ncurses, I ran into the following problem:

Given the following files:

main.zig:

const std = @import("std");
const nc = @import("import.zig");
const c = nc.c;

pub fn main() !void {
    _ = c.initscr();
    nc.start_color();

    // const pCLRS = &c.COLORS;

    if (nc.COLORS) |clrs| {
        std.debug.warn("COLORS: {}\n", .{clrs});
    }

    std.time.sleep(1000000000);
}

import.zig:

const std = @import("std");
pub const c = @cImport({
    @cInclude("ncurses.h");
});

pub var COLORS: ?i32 = null;

pub fn start_color() void {
    _ = c.start_color();

    COLORS = c.COLORS;
}

Compile and run with: zig build-exe --library ncurses main.zig && ./main

The output is "COLORS: 0". By commenting in line 9 of main.zig, the output changes to "COLORS: 256". Tested on macOS 10.15.4 using zig 0.5.0+1568470c4, verified on Linux.

I managed to create this problem without pointers (with a warn() instead of the pure read), but build this example first. My suspicion is that the access triggers some kind of runtime behavior, and that explicitly marking the COLORS assignment in start_color() as a runtime operation would fix this, but I can't prove it yet.

A runtime equivalent to comptime might be a good idea anyhow.

[LemonBoy]

Global variables suck.

Ncurses defines an external variable named COLORS and so does import.zig. The compiler is dumb and clumps together all the (scope-)global variables together without any kind of namespacing, this means that any variable named COLORS will alias the other with the same name.

Adding const pCLRS = &c.COLORS; generates a nice conflict that translates into the following LLVM IR:

@COLORS = external global i32, align 4
@COLORS.1 = internal unnamed_addr global %"?i32" { i32 undef, i1 false }, align 4

You were lucky enough, the extern variable won over the global one and the code behaves as intended.

[andrewrk]

To be clear the intended behavior here is that pub vars cannot clobber external symbols. The pub var COLORS is the one that is supposed to get the mangling/namespacing.

[koljakube]

Thanks for the explanation, helps a lot to understand what's going on. That lets me work around the problem for now.