Unexpected panic when unwrapping ArenaAllocator inside struct method

Help
1

When trying to unwrap an optional ArenaAllocator inside a struct method, the program panics even though the same logic works fine outside of the method.

The issue seems related to self.arena being an optional, but the behavior is inconsistent:

  • if (self.arena) |a| {} inside a method panics
  • but if (ms1.arena) |a| {} outside works fine

Zig Version: 0.14.1

const std = @import("std");

const MyStrucut = struct {
    l: ?[]u32,
    arena: ?std.heap.ArenaAllocator,
    allocator: *const std.mem.Allocator,
    dv: ?u32, // dummy value

    const this = @This();

    pub fn Init() this {
        const ms: MyStrucut = .{
            .l = null,
            .arena = null,
            .allocator = undefined,
            .dv = 56,
        };
        return ms;
    }

    pub fn Allocate(self: *this) !void {
        self.arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
        self.allocator = &self.arena.?.allocator();
    }

    pub fn StoreValue(self: *this, v: u32) !void {
        self.l = try self.allocator.alloc(u32, 1);
        self.l.?[0] = v;
    }

    pub fn PrintValue(self: *this) void {
        std.debug.print("Stored value: {d}\n", .{self.l.?[0]});
    }

    pub fn deinit(self: *this) void {
        self.arena.?.deinit();
    }

    pub fn WhyPanic(self: *this) void {
        if (self.dv) |a| {
            std.debug.print("{any}\n", .{a});
        }
    }

    pub fn WhyPanic2(self: this) void {
        if (self.arena) |a| {
            _ = a;
        }
    }
};

pub fn main() !void {
    var ms1 = MyStrucut.Init();
    defer ms1.deinit();
    try ms1.Allocate();

    // These panic unexpectedly:
    // ms1.WhyPanic();
    // ms1.WhyPanic2();

    // But this works fine:
    if (ms1.arena) |a| {
        _ = a;
    }

    try ms1.StoreValue(21);
    ms1.PrintValue();
}

Expected Behavior

  • ms1.WhyPanic() should print 56
  • ms1.WhyPanic2() should not panic, same as the outside if (ms1.arena) |a| {} block.

Actual Behavior

  • Both ms1.WhyPanic() and ms1.WhyPanic2() panic at runtime.
  • Equivalent logic outside a struct method works as expected.

But when i remove arena from struct like this

const std = @import("std");

const MyStrucut = struct {
    l: ?[]u32,
    // arena: ?std.heap.ArenaAllocator,
    allocator: *const std.mem.Allocator,
    dv: ?u32, // dummy value

    const this = @This();

    pub fn Init() this {
        const ms: MyStrucut = .{
            .l = null,
            // .arena = null,
            .allocator = undefined,
            .dv = 56,
        };
        return ms;
    }

    pub fn Allocate(self: *this) !void {
        // self.arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
        self.allocator = &std.heap.page_allocator;
    }

    pub fn StoreValue(self: *this, v: u32) !void {
        self.l = try self.allocator.alloc(u32, 1);
        self.l.?[0] = v;
    }

    pub fn PrintValue(self: *this) void {
        std.debug.print("Stored value: {d}\n", .{self.l.?[0]});
    }

    pub fn deinit(self: *this) void {
        self.allocator.free(self.l.?);
    }

    pub fn WhyPanic(self: *this) void {
        if (self.dv) |a| {
            std.debug.print("{any}\n", .{a});
        }
    }

    pub fn WhyPanic2(self: this) void {
        if (self.l) |a| {
            _ = a;
        }
    }
};

pub fn main() !void {
    var ms1 = MyStrucut.Init();
    defer ms1.deinit();
    try ms1.Allocate();

    ms1.WhyPanic(); // now not painc
    ms1.WhyPanic2(); // now not painc

    try ms1.StoreValue(21);
    ms1.PrintValue();
}

Its work fine.

2

Please provide the panic message, stack trace and zig version

3

Sure!

panic message of first code

when i call ms1.WhyPanic()

~/flix/test_files/zig main* ⇡ ❯ zig run arena.zig
56
Segmentation fault at address 0x3800000038
/snap/zig/14333/lib/std/mem/Allocator.zig:129:26: 0x10de30b in allocBytesWithAlignment__anon_23841 (arena)
    return a.vtable.alloc(a.ptr, len, alignment, ret_addr);
                         ^
/snap/zig/14333/lib/std/mem/Allocator.zig:263:40: 0x10e0142 in allocWithSizeAndAlignment__anon_24212 (arena)
    return self.allocBytesWithAlignment(alignment, byte_count, return_address);
                                       ^
/snap/zig/14333/lib/std/mem/Allocator.zig:257:75: 0x10dedd1 in alloc__anon_24056 (arena)
    const ptr: [*]align(a) T = @ptrCast(try self.allocWithSizeAndAlignment(@sizeOf(T), a, n, return_address));
                                                                          ^
/home/parthdegama/flix/test_files/zig/arena.zig:27:42: 0x10decad in StoreValue (arena)
        self.l = try self.allocator.alloc(u32, 1);
                                         ^
/home/parthdegama/flix/test_files/zig/arena.zig:66:23: 0x10def90 in main (arena)
    try ms1.StoreValue(21);
                      ^
/snap/zig/14333/lib/std/start.zig:660:37: 0x10deaba in posixCallMainAndExit (arena)
            const result = root.main() catch |err| {
                                    ^
/snap/zig/14333/lib/std/start.zig:271:5: 0x10de66d in _start (arena)
    asm volatile (switch (native_arch) {
    ^
???:?:?: 0x0 in ??? (???)
[1]    404972 IOT instruction (core dumped)  zig run arena.zig

and, when i call ms1.WhyPanic2()

~/flix/test_files/zig main* ⇡ ❯ zig run arena.zig
General protection exception (no address available)
/snap/zig/14333/lib/std/mem/Allocator.zig:129:26: 0x10de11b in allocBytesWithAlignment__anon_23841 (arena)
    return a.vtable.alloc(a.ptr, len, alignment, ret_addr);
                         ^
/snap/zig/14333/lib/std/mem/Allocator.zig:263:40: 0x10dfdb2 in allocWithSizeAndAlignment__anon_24208 (arena)
    return self.allocBytesWithAlignment(alignment, byte_count, return_address);
                                       ^
/snap/zig/14333/lib/std/mem/Allocator.zig:257:75: 0x10debe1 in alloc__anon_24056 (arena)
    const ptr: [*]align(a) T = @ptrCast(try self.allocWithSizeAndAlignment(@sizeOf(T), a, n, return_address));
                                                                          ^
/home/parthdegama/flix/test_files/zig/arena.zig:27:42: 0x10deabd in StoreValue (arena)
        self.l = try self.allocator.alloc(u32, 1);
                                         ^
/home/parthdegama/flix/test_files/zig/arena.zig:66:23: 0x10deda0 in main (arena)
    try ms1.StoreValue(21);
                      ^
/snap/zig/14333/lib/std/start.zig:660:37: 0x10de8ca in posixCallMainAndExit (arena)
            const result = root.main() catch |err| {
                                    ^
/snap/zig/14333/lib/std/start.zig:271:5: 0x10de47d in _start (arena)
    asm volatile (switch (native_arch) {
    ^
???:?:?: 0x0 in ??? (???)
[1]    405267 IOT instruction (core dumped)  zig run arena.zig

System Info:
Os: Ubuntu 25.04
Kernel: Linux 6.14.0-27-generic

4

self.allocator is referencing temporary stack memory in Allocate(), firstly don’t store a pointer to an Allocator instead store it by value, it is just a struct with pointers. That should make it work, if not, replace .? with if(self.arena) |*a| a.allocator(). IDK off the top of my head if .? copies or not.

Other issues:

  • don’t store the arena in your struct, instead take it as a parameter to init and get your allocator there.
  • preferably take an Allocator instead of an arena, so as not to restrict the allocation strategy, even if you rely on the arena strategy, FixedBufferAllocator is also an arena, just one that cant grow the backing memory.
  • while an optional slice doesn’t have aditional cost since it’s a pointer, you can use an empty slice in place of null, it’s just easier to work with while not loosing information.
  • don’t use undefined unless you immediately overwrite it, returning it is worse as it’s invisible unless you look at the function source.
5 Likes