# Segmentation fault with Dict **URL:** \ **Category:** Mojo\ **Created:** [June 14, 2025, 6:07pm UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656 "2025-06-14T18:07:58Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![notooth](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@notooth](https://forum.modular.com/u/notooth)\ **Post date:** [June 14, 2025, 6:07pm UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/1 "2025-06-14T18:07:58Z")
Can anyone let me know why I got `Segmentation fault` with this code: ```auto struct Number(Copyable & Movable): var a: Int128 fn __init__ (out self, owned a: Int128): self.a = a fn main(): var number = Dict[String, Number]() for idx in range(5): print("Adding number", idx) number[String(idx)] = Number(Int128(idx)) ``` ```auto $ mojo life.mojo Adding number 0 Adding number 1 Please submit a bug report to https://github.com/modular/modular/issues and include the crash backtrace along with all the relevant source codes. Stack dump: 0. Program arguments: mojo life.mojo Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it): 0 mojo 0x000055f29485d6ab 1 mojo 0x000055f29485b449 2 mojo 0x000055f29485dd5a 3 libc.so.6 0x00007f1af9c67070 4 libc.so.6 0x00007f1af9cc0334 5 libc.so.6 0x00007f1af9c66fbe gsignal + 30 6 libAsyncRTMojoBindings.so 0x00007f1afbdb2ac2 7 libc.so.6 0x00007f1af9c67070 8 (error) 0x00007f1a04002595 mojo crashed! Please file a bug report. Segmentation fault (core dumped) ``` ---
**Author:** ![clattner](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/clattner/32/201_2.png) [@clattner](https://forum.modular.com/u/clattner)\ **Post date:** [June 15, 2025, 4:22am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/2 "2025-06-15T04:22:17Z")
> [@notooth](#): > > ```auto > struct Number(Copyable & Movable): > var a: Int128 > fn __init__ (out self, owned a: Int128): > self.a = a > > fn main(): > var number = Dict[String, Number]() > for idx in range(5): > print("Adding number", idx) > number[String(idx)] = Number(Int128(idx)) > > ``` this doesn’t crash for me with a recent build. I think I saw someone talking about Int128 → String having a bug and crashing, I think that recently got fixed, can you try it out with the most recent nightly? ---
**Author:** ![notooth](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@notooth](https://forum.modular.com/u/notooth)\ **Post date:** [June 15, 2025, 4:26am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/3 "2025-06-15T04:26:53Z")
I have just updated it to `Mojo 25.5.0.dev2025061405 (b81f396d)`, and it still crashes. ---
**Author:** ![ssslakter](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/ssslakter/32/435_2.png) [@ssslakter](https://forum.modular.com/u/ssslakter)\ **Post date:** [June 15, 2025, 9:39am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/4 "2025-06-15T09:39:00Z")
I’ve also experimented with this and found that the segfault occurs only when there are at least two dictionary fields in the struct. I guess original question also had dicts in the struct, or maybe it was another question on the forums, though I can’t find it now. The code below works fine; however, if you include commented lines, it will segfault. If you then also replace `Int128` with `Int64` or smaller, this would also work fine. ```mojo struct A(Writable, Copyable & Movable): var number: Int128 var dict: Dict[String, Int] # var dict2: Dict[String, Int] fn __init__ (out self, number: Int128, dict: Dict[String, Int], dict2: Dict[String, Int]): self.number = number self.dict = dict # self.dict2 = dict2 fn __copyinit__ (out self, existing: Self): print('copying') self.number = existing.number self.dict = existing.dict # self.dict2 = existing.dict2 fn __moveinit__ (out self, owned existing: Self): print('moving') self.number = existing.number self.dict = existing.dict # self.dict2 = existing.dict2 fn write_to[W: Writer](self, mut writer: W) -> None: self.write_dict(writer, self.dict) writer.write(",") # self.write_dict(writer, self.dict2) @staticmethod fn write_dict[W: Writer, K: KeyElement & Writable, V: Copyable & Movable & Writable](mut writer: W, dict: Dict[K, V]) -> None: writer.write("{") first = True for entry in dict.items(): if not first: writer.write(',') writer.write(entry.key, ":", entry.value) first = False writer.write("}") def main(): var structs = Dict[Int, A]() for i in range(2): d1 = Dict[String, Int]() d2 = Dict[String, Int]() structs[i] = A(i, d1, d2) for i in range(1): print(structs[i]) ``` UPD: This is the simplest program I found that crashes for me on the latest nightly, it won’t crash when you replace key type from `String` to `Int` or value type to something less than `Int128` ```mojo fn main(): var number = Dict[String, Int128]() for idx in range(2): number[String(idx)] = Int128(idx) ``` ---
**Author:** ![duck\_tape](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/duck_tape/32/432_2.png) [@duck\_tape](https://forum.modular.com/u/duck_tape)\ **Post date:** [June 15, 2025, 9:55am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/5 "2025-06-15T09:55:16Z")
I think it’s related to this: [UInt256 returns wrong value - #6 by sora](https://forum.modular.com/t/uint256-returns-wrong-value/1643/6) When printing numbers \> 64 bits it’s overwriting the inline array and corrupting the program. ---
**Author:** ![ssslakter](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/ssslakter/32/435_2.png) [@ssslakter](https://forum.modular.com/u/ssslakter)\ **Post date:** [June 15, 2025, 10:00am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/6 "2025-06-15T10:00:10Z")
In my latest example, I’m only populating the dictionary—there aren’t any print statements, so I’m not sure if that’s the same issue here. ---
**Author:** ![duck\_tape](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/duck_tape/32/432_2.png) [@duck\_tape](https://forum.modular.com/u/duck_tape)\ **Post date:** [June 15, 2025, 10:23am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/7 "2025-06-15T10:23:23Z")
It might be the `String(idx)`, which also goes through the int formatting methods I think. ---
**Author:** ![notooth](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@notooth](https://forum.modular.com/u/notooth)\ **Post date:** [June 16, 2025, 12:04am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/8 "2025-06-16T00:04:28Z")
@clattner Can you let me know the nightly version that you are running? ---
**Author:** ![samufi](https://avatars.discourse-cdn.com/v4/letter/s/d2c977/32.png) [@samufi](https://forum.modular.com/u/samufi)\ **Post date:** [June 16, 2025, 6:31am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/9 "2025-06-16T06:31:08Z")
To me it sounds as if this could be another version of this bug: [[BUG] UB in Variant when using SIMD · Issue #4578 · modular/modular · GitHub](https://github.com/modular/modular/issues/4578) The Dict uses Variant internally, so that the issue above causes the following issue in Dict, which resembles the one reported here: [[BUG] Segfault if using a struct based on SIMD as key in Dict · Issue #3781 · modular/modular · GitHub](https://github.com/modular/modular/issues/3781) ---
**Author:** ![notooth](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@notooth](https://forum.modular.com/u/notooth)\ **Post date:** [June 17, 2025, 12:36am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/10 "2025-06-17T00:36:52Z")
> [@samufi](#): > > [[BUG] Segfault if using a struct based on SIMD as key in Dict · Issue #3781 · modular/modular · GitHub](https://github.com/modular/modular/issues/3781) It has been a long time. Is there a fix? ---
**Author:** ![samufi](https://avatars.discourse-cdn.com/v4/letter/s/d2c977/32.png) [@samufi](https://forum.modular.com/u/samufi)\ **Post date:** [June 17, 2025, 11:48am UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/11 "2025-06-17T11:48:48Z")
Not that I am aware of. This has been blocking a project of mine for a long time now. Maybe someone from the modular staff can provide us with some insight as to when a fix may be expected. ---
**Author:** ![duck\_tape](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/duck_tape/32/432_2.png) [@duck\_tape](https://forum.modular.com/u/duck_tape)\ **Post date:** [June 17, 2025, 1:17pm UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/12 "2025-06-17T13:17:33Z")
[[stdlib] Fix int formatting to not go out of bounds, add MIN and MAX for 128 and 256 sized integers by sstadick · Pull Request #4852 · modular/modular · GitHub](https://github.com/modular/modular/pull/4852#issue-3153481339) fixes both this issue, and the original issue [UInt256 returns wrong value - #6 by sora](https://forum.modular.com/t/uint256-returns-wrong-value/1643/6). The tests show the fix for format\_int working. I also checked that: ```mojo fn main(): var number = Dict[String, Int128]() for idx in range(2): number[String(idx)] = Int128(idx) ``` failed before the fix and works after. ---
**Author:** ![duck\_tape](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/duck_tape/32/432_2.png) [@duck\_tape](https://forum.modular.com/u/duck_tape)\ **Post date:** [June 17, 2025, 2:05pm UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/13 "2025-06-17T14:05:01Z")
Well, I guess that didn’t fix it all the way. The ubuntu test runner seg-faulted with that minimal dict example in there. My changes at least fix it on mac. So @samufi’s issue does seem to also be in play. ---
**Author:** ![system](https://us1.discourse-cdn.com/flex001/uploads/modular/original/1X/2751e0fbdc595a99718b216730957e9db4448cfd.jpeg) [@system](https://forum.modular.com/u/system)\ **Post date:** [December 14, 2025, 2:05pm UTC](https://forum.modular.com/t/segmentation-fault-with-dict/1656/14 "2025-12-14T14:05:58Z")
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