So while cleaning up [https://github.com/haldean/nimage](https://github.com/haldean/nimage) I came across a rather weird bug, which results in a segfault/SIGSEV.
To reproduce, clone this repo: [https://github.com/Clyybber/nimage.git](https://github.com/Clyybber/nimage.git) and checkout the branch "crash" and do nim c tests/test1.nim and then run ./tests/test1 In this line [https://github.com/Clyybber/nimage/blob/crash/private/zutil.nim#L47](https://github.com/Clyybber/nimage/blob/crash/private/zutil.nim#L47) it will segfault. Apparently it has to do with invalid writes/reads according to valgrind: Invalid write of size 8 at 0x10E249: removeChunkFromMatrix2_XFftAAJrARamxGOKUFQy9aw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E577: getBigChunk_stlXHMKRKFIGOvq8t4ynRQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10F6CC: rawAlloc_mE4QEVyMvGRVliDWDngZCQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114251: rawNewObj_ehkAaLROrd0Hc9aLROWt1nQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114321: newObjNoInit (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114391: rawNewStringNoInit (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114A0F: setLengthStr (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x11F7CC: zuncompress_VbbZ4LVDy6izxF4k3Gvj3g (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x121798: load_idat_P5PrAgoAkNqPZC5CDuYW7A (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x12311B: load_png_ACPUuVacvyp8EtUpaNSq9cg (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x1237F7: NimMainModule (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x123699: NimMainInner (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) Address 0x17 is not stack'd, malloc'd or (recently) free'd Invalid read of size 8 at 0x10E1B4: removeChunkFromMatrix2_XFftAAJrARamxGOKUFQy9aw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E577: getBigChunk_stlXHMKRKFIGOvq8t4ynRQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E70C: getSmallChunk_0ixBBlKB5QN59bxrmztRmCw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10F3B0: rawAlloc_mE4QEVyMvGRVliDWDngZCQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114251: rawNewObj_ehkAaLROrd0Hc9aLROWt1nQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114864: newObj (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x1148FA: rawNewString (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x11798E: signalHandler (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x4A33DFF: ??? (in /usr/lib/libc-2.28.so) by 0x10E248: removeChunkFromMatrix2_XFftAAJrARamxGOKUFQy9aw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E577: getBigChunk_stlXHMKRKFIGOvq8t4ynRQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10F6CC: rawAlloc_mE4QEVyMvGRVliDWDngZCQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) Address 0xf is not stack'd, malloc'd or (recently) free'd Process terminating with default action of signal 11 (SIGSEGV): dumping core Access not within mapped region at address 0xF at 0x10E1B4: removeChunkFromMatrix2_XFftAAJrARamxGOKUFQy9aw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E577: getBigChunk_stlXHMKRKFIGOvq8t4ynRQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E70C: getSmallChunk_0ixBBlKB5QN59bxrmztRmCw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10F3B0: rawAlloc_mE4QEVyMvGRVliDWDngZCQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114251: rawNewObj_ehkAaLROrd0Hc9aLROWt1nQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x114864: newObj (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x1148FA: rawNewString (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x11798E: signalHandler (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x4A33DFF: ??? (in /usr/lib/libc-2.28.so) by 0x10E248: removeChunkFromMatrix2_XFftAAJrARamxGOKUFQy9aw (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10E577: getBigChunk_stlXHMKRKFIGOvq8t4ynRQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) by 0x10F6CC: rawAlloc_mE4QEVyMvGRVliDWDngZCQ (in /home/clyybber/projects/wyven/concept/image/nimage/tests/test1) Run **Now the weird parts:** Placing a var in front of result here: [https://github.com/Clyybber/nimage/blob/crash/private/zutil.nim#L38](https://github.com/Clyybber/nimage/blob/crash/private/zutil.nim#L38) , fixes the crash and it runs successfully. So it probably has something to do with the implicit result variable... Using nim c -d:useSysAssert tests/test1.nim to compile the programm also fixes the crash and makes the programm run successfully. But since -d:useSysAssert only enables this branch: [https://github.com/nim-lang/Nim/blob/2b5e48d80735be60c68023de114a586bbcc18360/lib/system.nim#L1660](https://github.com/nim-lang/Nim/blob/2b5e48d80735be60c68023de114a586bbcc18360/lib/system.nim#L1660) , which is never run (otherwise the programm wouldn't terminate successfully), it shouldn't affect this, right??
