globals fields segfault (or write fault in os.zig)

[gh0st-work]

Zig Version

0.11.0 (x86_64-linux.3.16...5.10.81-gnu.2.19)

Steps to Reproduce and Observed Behavior

const std = @import("std");

pub const User = struct { name: []const u8 };
pub const user_john = User{ .name = "John" };
pub const user_richard = User{ .name = "Richard" };
pub const users_default: [2]User = .{ user_john, user_richard };

pub const Database = struct {
    users: []User,

    pub fn default() Database {
        var users = users_default;
        return Database{
            .users = &users,
        };
    }
};

pub const Service = struct {
    users: []User,

    pub fn init(db: Database) Service {
        return Service{
            .users = db.users,
        };
    }
};

test "fault" {
    std.debug.print("\n", .{});
    var db = Database.default();
    var service = Service.init(db);
    for (service.users) |user|
        std.debug.print("name:{s}\n", .{user.name});
}

This snippet code organization / structure causes some strange behavior: segfaults, os.write faults, misreading. In this example my output is:

Test [1/1] test.fault... 
name:John
name:thread 110 panic: reached unreachable code
/home/user/.zig/lib/std/os.zig:1133:23: 0x273c8f in write (test)
            .FAULT => unreachable,
                      ^
/home/user/.zig/lib/std/fs/file.zig:1163:28: 0x26089a in write (test)
            return os.write(self.handle, bytes);
                           ^
/home/user/.zig/lib/std/io/writer.zig:17:27: 0x22e208 in write (test)
            return writeFn(self.context, bytes);
                          ^
/home/user/.zig/lib/std/io/writer.zig:23:40: 0x225a60 in writeAll (test)
                index += try self.write(bytes[index..]);
                                       ^
/home/user/.zig/lib/std/fmt.zig:1047:28: 0x22eafa in formatBuf__anon_5235 (test)
        try writer.writeAll(buf);
                           ^
/home/user/.zig/lib/std/fmt.zig:656:45: 0x225dfe in formatType__anon_3696 (test)
                            return formatBuf(value, options, writer);
                                            ^
/home/user/.zig/lib/std/fmt.zig:184:23: 0x225d2d in format__anon_3649 (test)
        try formatType(
                      ^
/home/user/.zig/lib/std/io/writer.zig:28:34: 0x224de0 in print__anon_2843 (test)
            return std.fmt.format(self, format, args);
                                 ^
/home/user/.zig/lib/std/debug.zig:90:27: 0x22468c in print__anon_1105 (test)
    nosuspend stderr.print(fmt, args) catch return;
                          ^
/home/user/projects/proj-zig/bug.zig:34:24: 0x2244e5 in test.fault (test)
        std.debug.print("name:{s}\n", .{user.name});
                       ^
/home/user/.zig/lib/test_runner.zig:176:28: 0x22dc8e in mainTerminal (test)
        } else test_fn.func();
                           ^
/home/user/.zig/lib/test_runner.zig:36:28: 0x22576a in main (test)
        return mainTerminal();
                           ^
/home/user/.zig/lib/std/start.zig:564:22: 0x224ba2 in posixCallMainAndExit (test)
            root.main();
                     ^
/home/user/.zig/lib/std/start.zig:243:5: 0x2246f1 in _start (test)
    asm volatile (switch (native_arch) {
    ^
???:?:?: 0x0 in ??? (???)

Mb I'm stupid and don't know how to work with pointers to comptime data, if so, please explain

Expected Behavior

Run correctly

[Vexu]

You are returning a pointer to a stack variable that is invalidated when the function returns.

    pub fn default() Database {
        var users = users_default;
        return Database{
            .users = &users,

Making users_defalt a var and returning a pointer to it makes the code run as expected.

[gh0st-work]

Thx for your explanations, but shouldn't it be somehow explained by the compiler? Why is it even legal saving somewhere pointer to stack? In C it is considered memory leak and C report it immediately (and LLVM/clang explains where exactly it's happened, mb with sanitizers, I don't quite remember).

[Inve1951]

afaik making it illegal is a planned improvement
see #2301, #2646, #5725