Date: Wed, 13 Feb 2013 17:18:25 +0100 From: <natris@centrum.cz> To: <freebsd-hackers@freebsd.org> Subject: =?utf-8?q?SIGSEGV=2FSIGBUS_when_accessing_after_end_of_mmapped_file=3B_why_it_differs_with_GCC=3F?= Message-ID: <20130213171825.76D3A9DC@centrum.cz>
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Hello, I am porting an application which maps files into the memory and works directly with the memory. When doing this, it can happen that when someone resizes the file so that part of the previously mapped region is no longer backed by the file, synchronous signal is sent to the process which needs to be handled. On all other platforms than FreeBSD I have tested (Solaris, Linux, Darwin, HP-UX) the signal in question is SIGBUS. However on FreeBSD, depending on the >>compiler<< used, it is either SIGBUS or SIGSEGV. When I compile the binary with "gcc version 4.2.1 20070831 patched [FreeBSD], amd64", the signal is SIGBUS, when i use "gcc version 4.7.3 20121103 (prerelease) (FreeBSD Ports Collection)", the signal is SIGSEGV. Please note that one of the versions of gcc between 4.2 and 4.7 also caused SIGSEFV to be sent but this did not matter for me as I did not need to use that version; I however need gcc 4.7 to work because of c++11 stuff that my project has recently started to use. Unfortunately registering signal handler on SIGSEGV is very inconvenient for me; I would prefer to somehow switch the behavior to be sane. Please anyone has an idea whether or how this could be achieved? I have tried to find out why, on single machine, just because of different gcc version, kernel sends different signal but I have never worked with fbsd kernel before and so my search did not succeed so far. Machine in question runs amd64 FreeBSD 9.1-RC2, but this has also happened to me with older version of FreeBSD. gcc 4.2 (same happens also with libstdc++ and co. from gcc 4.2): Program received signal SIGBUS, Bus error. (gdb) i shared From To Syms Read Shared Object Library 0x0000000800f2ef70 0x0000000800f3ee68 Yes (*) /libexec/ld-elf.so.1 0x000000080114d710 0x0000000801159748 Yes (*) /lib/libthr.so.3 0x00000008013c2d30 0x000000080142c656 Yes /usr/local/lib/gcc47/libstdc++.so.6 0x00000008016737c0 0x00000008016891b8 Yes (*) /lib/libm.so.5 0x0000000801893930 0x00000008018a3088 Yes /usr/local/lib/gcc47/libgcc_s.so.1 0x0000000801ad71d0 0x0000000801ba9358 Yes (*) /lib/libc.so.7 gcc 4.7: Program received signal SIGSEGV, Segmentation fault. (gdb) i shared From To Syms Read Shared Object Library 0x0000000800f5ef70 0x0000000800f6ee68 Yes (*) /libexec/ld-elf.so.1 0x000000080117d710 0x0000000801189748 Yes (*) /lib/libthr.so.3 0x00000008013f2d30 0x000000080145c656 Yes /usr/local/lib/gcc47/libstdc++.so.6 0x00000008016a37c0 0x00000008016b91b8 Yes (*) /lib/libm.so.5 0x00000008018c3930 0x00000008018d3088 Yes /usr/local/lib/gcc47/libgcc_s.so.1 0x0000000801b071d0 0x0000000801bd9358 Yes (*) /lib/libc.so.7 I would be glad for any hint or information. Kind Regards, Ondrej Kolacek
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