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Clarify compute-sanitizer warnings #244

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dunnoconnor merged 1 commit into from
Apr 21, 2026
+11 −1
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Clarify compute-sanitizer warnings #244

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12 changes: 11 additions & 1 deletion book/src/puzzle_10/memcheck.md
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Original file line number Diff line number Diff line change
Expand Up @@ -168,6 +168,14 @@ MODULAR_DEVICE_CONTEXT_MEMORY_MANAGER_SIZE_PERCENT=0 pixi run compute-sanitizer

```txt
========= COMPUTE-SANITIZER
[...]:WARNING close_multiple.cc:66] close: Bad file descriptor (9)
[...]:WARNING close_multiple.cc:66] close: Bad file descriptor (9)
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: .../.pixi/envs/default/bin/mojo problems/p10/p10.mojo --memory-bug
#0 0x... (/.../.pixi/envs/default/bin/mojo+0x...)
...
[...] intermediate process terminated by signal 11 (Segmentation fault) (core dumped)
out shape: 2 x 2
Running memory bug example (bounds checking issue)...
out: HostBuffer([10.0, 11.0, 12.0, 13.0])
Expand All @@ -178,6 +186,8 @@ expected: HostBuffer([10.0, 11.0, 12.0, 13.0])

**✅ SUCCESS:** No memory violations detected!

> **Note on the segfault**: The crash lines above ("intermediate process terminated by signal 11") are a known compatibility issue between Mojo's process initialization and compute-sanitizer's injection libraries. They appear *before* the GPU kernel runs and do not affect the sanitizer's analysis. See the note at the end of this page for details.

## Key learning points

### Why manual bounds checking matters
Expand All @@ -200,4 +210,4 @@ expected: HostBuffer([10.0, 11.0, 12.0, 13.0])
MODULAR_DEVICE_CONTEXT_MEMORY_MANAGER_SIZE_PERCENT=0 pixi run compute-sanitizer --tool memcheck mojo your_code.mojo
```

**Note**: You may see Mojo runtime warnings in the sanitizer output. Focus on the `========= Invalid` and `========= ERROR SUMMARY` lines for actual memory violations.
**Note on Mojo + compute-sanitizer compatibility**: You may see a crash at the start of the sanitizer output — lines like `close: Bad file descriptor`, a stack dump, and `intermediate process terminated by signal 11 (Segmentation fault)`. This is a known issue where compute-sanitizer's injection libraries conflict with Mojo's process initialization. Despite the crash, the sanitizer completes its GPU kernel analysis correctly. Always use the `========= ERROR SUMMARY` line at the end as the authoritative result, and look for `========= Invalid` lines for specific memory violations.
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