Created attachment 8544 [details] Sample Code and Repro Transcript Problem: 'reinterpret_cast' in gdb seems to be doing a 'static_cast'. For example, assume we have two unrelated base classes, 'fred' and 'ginger', and another class 'hillary' that first extends 'fred' then 'ginger'. Typically, a 'hillary' object would have the same address as the inherited 'fred' object. The inherited 'ginger' object typically follows the 'fred' data. (It depends on the compiler implementation, but don't let that distract you from the description of the gdb bug.) 'reinterpret_cast' typically does not change the value of a pointer that is casts. Specifically, given 'ginger*' pointer 'g', 'reinterpret_cast<hillary*>(g)' should have the same value as 'static_cast<hillary*>(static_cast<void*>(g))' per 5.2.10.7 of the 2011 and 2014 versions of the C++ standard (N3242 and N4296). Notice that it does work properly in code generated by g++. However, in the various versions of gdb, it does not. The bug is reproduced is several GDB versions and distributions. > GNU gdb (GDB) 7.4.1-debian installed using apt-get > GNU gdb (GDB) 7.9.1 downloaded from gnu.org/software/gdb/download/ > GNU gdb (GDB) 7.10.50.20150818-cvs downloaded from gnu.org/software/gdb/download/ > GNU gdb (Gentoo 7.5.1 p2) 7.5.1 installed using portage Attached is sample code with several transcripts showing the problem.
The master branch has been updated by Hannes Domani <ssbssa@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=23cdd9431ad424b092c65419d47ef4601168a1c9 commit 23cdd9431ad424b092c65419d47ef4601168a1c9 Author: Hannes Domani <ssbssa@yahoo.de> Date: Wed Mar 20 18:02:06 2024 +0100 Fix reinterpret_cast for classes with multiple inheritance Currently a reinterpret_cast may change the pointer value if multiple inheritance is involved: ``` (gdb) p r $1 = (Right *) 0x22f75c (gdb) p reinterpret_cast<LeftRight*>(r) $2 = (LeftRight *) 0x22f758 ``` It's because value_cast is called in this case, which automatically does up- and downcasting. Fixed by simply using the target pointer type in a copy of the original value: ``` (gdb) p r $1 = (Right *) 0x3bf87c (gdb) p reinterpret_cast<LeftRight*>(r) $2 = (LeftRight *) 0x3bf87c ``` Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18861 Approved-By: Tom Tromey <tom@tromey.com>
Fixed.