zig_strange_behaviour.zig

const std = @import("std");
const c = @cImport({
    @cInclude("X11/Xlib.h");
    @cInclude("time.h");
});

const X11Error = error{
    NoDisplay,
    InternalError,
    SyncFailed,
};

const X11 = struct {
    handle: ?*c.Display,

    pub fn new() X11Error!X11 {
        const handle = c.XOpenDisplay(null);
        if (handle == null) {
            return X11Error.NoDisplay;
        }

        return X11 {
            .handle = handle
        };
    }

    pub fn setName(self: X11, name: [:0]const u8) X11Error!void {
        std.debug.warn("{}\n", .{name});
        if (c.XStoreName(self.handle, c.XDefaultRootWindow(self.handle),
                         name) == 0) {
            return X11Error.InternalError;
        }

        if (c.XSync(self.handle, c.True) == 0) {
            return X11Error.SyncFailed;
        }
    }
};

fn currentTime() [:0]const u8 {
    var buffer: [64]u8 = undefined;
    const clock: c.time_t = c.time(null);
    const time: ?*c.tm = c.localtime(&clock);
    const size = c.strftime(&buffer, buffer.len, "%Y-%m-%d %H:%M:%S", time);
    return buffer[0..size :0];
}

pub fn main() anyerror!void {
    const desktop = try X11.new();

    const time = currentTime();
    std.debug.warn("{}\n", .{time});
    try desktop.setName(time);
}

keegan@zzz:~/code/dwmstatus % rm -rf zig-cache/
keegan@zzz:~/code/dwmstatus % zig build run
2020-01-17 16:46:38
N 17 16:46:38

Now with release safe:

keegan@zzz:~/code/dwmstatus % rm -rf zig-cache/
keegan@zzz:~/code/dwmstatus % zig build -Drelease-safe=true run
2020-01-17 16:47:18
2020-01-17 16:47:18

buffer Is stack-allocated and becomes invalid as soon as currentTime returns.

My assumption was that it would be copied. Will I need to allocate it to return the value?