Isomalloc: fix macOS 26+ segfaults - #3996
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bad_location() probed candidate addresses with mmap(..., MAP_FIXED), which silently replaces whatever is mapped there. On macOS 26 the system allocator's zones lie in the range that find_largest_free_region() probes, so the probe unmapped live heap memory and every Charm++ program crashed a few percent of the time. Add call_mmap_probe(), which never replaces an existing mapping: MAP_FIXED_NOREPLACE on Linux, MAP_FIXED | MAP_EXCL on FreeBSD, and a hint-only mmap elsewhere. bad_location() unmaps whatever the probe returned and treats a returned address that differs from the hint as "in use". Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
ritvikrao
self-requested a review
September 29, 2026 14:45
ritvikrao
approved these changes
Sep 29, 2026
adityapb
approved these changes
Sep 29, 2026
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Fixes #3995. This fix was suggested by Claude. I don't have enough experience with this part of the code to properly review it, but it did fix the random segfaults that I was encountering on macOS, so I thought I'd at least show you the potential fix. Please feel free to either merge it, merge it with changes, or discard this PR.