Segfault in Allocator depending on reference creation position

[AnneKitsune]

System

Linux, zig 0.7.1 release.

Reproduce

mkdir segfault && cd segfault
zig init-exe

paste the following in src/main.zig:

const std = @import("std");
const ArgIterator = std.process.ArgIterator;

pub fn main() !void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    var alloc = arena.allocator;
    //var alloc = &arena.allocator;

    var args = ArgIterator.init();
    defer args.deinit();
    while (args.next(&alloc)) |arg| {
    //while (args.next(alloc)) |arg| {
    }
}

zig build run

Result

umaru% zig build run
Segmentation fault at address 0x0
/usr/lib/zig/std/special/c.zig:188:25: 0x234ec0 in memset (c)
        dest.?[index] = c;
                        ^
/usr/lib/zig/std/mem/Allocator.zig:186:41: 0x21df0a in std.mem.Allocator.alloc (membug)
    return self.allocAdvancedWithRetAddr(T, null, n, .exact, @returnAddress());
                                        ^
/usr/lib/zig/std/mem/Allocator.zig:469:40: 0x2325ab in std.mem.Allocator.dupeZ (membug)
    const new_buf = try allocator.alloc(T, m.len + 1);
                                       ^
/usr/lib/zig/std/process.zig:444:35: 0x232292 in std.process.ArgIterator.next (membug)
            return allocator.dupeZ(u8, self.inner.next() orelse return null);
                                  ^
/shareddata/share/prog/zig/membug/src/main.zig:12:21: 0x22b167 in main (membug)
    while (args.next(&alloc)) |arg| {
                    ^
/usr/lib/zig/std/start.zig:334:37: 0x204d5d in std.start.posixCallMainAndExit (membug)
            const result = root.main() catch |err| {
                                    ^
/usr/lib/zig/std/start.zig:162:5: 0x204a92 in std.start._start (membug)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
The following command terminated unexpectedly:
cd /shareddata/share/prog/zig/membug && /shareddata/share/prog/zig/membug/zig-cache/bin/membug
error: the following build command failed with exit code 1:
/shareddata/share/prog/zig/membug/zig-cache/o/ef07146b27f2cb074c4695700607b8a9/build /usr/bin/zig /shareddata/share/prog/zig/membug /shareddata/share/prog/zig/membug/zig-cache /home/jojolepro/.cache/zig run

If using the commented lines instead of the uncommented ones, the bug is not present.

I'm very new to zig, so I might be doing something wrong? This seems like an error that the compiler should warn about? Thanks!

[Vexu]

You're copying the allocator interface out of the arena allocator leading to bad pointer dereferences, see #591.

[AnneKitsune]

aaah that explains. thank you!