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This commit adds a new maintenance feature, the ability to print a (limited) backtrace if GDB dies due to a fatal signal. The backtrace is produced using the backtrace and backtrace_symbols_fd functions which are declared in the execinfo.h header, and both of which are async signal safe. A configure check has been added to check for these features, if they are not available then the new code is not compiled into GDB and the backtrace will not be printed. The motivation for this new feature is to aid in debugging GDB in situations where GDB has crashed at a users site, but the user is reluctant to share core files, possibly due to concerns about what might be in the memory image within the core file. Such a user might be happy to share a simple backtrace that was written to stderr. The production of the backtrace is on by default, but can switched off using the new commands: maintenance set backtrace-on-fatal-signal on|off maintenance show backtrace-on-fatal-signal Right now, I have hooked this feature in to GDB's existing handling of SIGSEGV only, but this will be extended to more signals in a later commit. One additional change I have made in this commit is that, when we decide GDB should terminate due to the fatal signal, we now raise the same fatal signal rather than raising SIGABRT. Currently, this is only effecting our handling of SIGSEGV. So, previously, if GDB hit a SEGV then we would terminate GDB with a SIGABRT. After this commit we will terminate GDB with a SIGSEGV. This feels like an improvement to me, we should still get a core dump, but in many shells, the user will see a more specific message once GDB exits, in bash for example "Segmentation fault" rather than "Aborted". Finally then, here is an example of the output a user would see if GDB should hit an internal SIGSEGV: Fatal signal: Segmentation fault ----- Backtrace ----- ./gdb/gdb[0x8078e6] ./gdb/gdb[0x807b20] /lib64/libpthread.so.0(+0x14b20)[0x7f6648c92b20] /lib64/libc.so.6(__poll+0x4f)[0x7f66484d3a5f] ./gdb/gdb[0x1540f4c] ./gdb/gdb[0x154034a] ./gdb/gdb[0x9b002d] ./gdb/gdb[0x9b014d] ./gdb/gdb[0x9b1aa6] ./gdb/gdb[0x9b1b0c] ./gdb/gdb[0x41756d] /lib64/libc.so.6(__libc_start_main+0xf3)[0x7f66484041a3] ./gdb/gdb[0x41746e] --------------------- A fatal error internal to GDB has been detected, further debugging is not possible. GDB will now terminate. This is a bug, please report it. For instructions, see: <https://www.gnu.org/software/gdb/bugs/>. Segmentation fault (core dumped) It is disappointing that backtrace_symbols_fd does not actually map the addresses back to symbols, this appears, in part, to be due to GDB not being built with -rdynamic as the manual page for backtrace_symbols_fd suggests, however, even when I do add -rdynamic to the build of GDB I only see symbols for some addresses. We could potentially look at alternative libraries to provide the backtrace (e.g. libunwind) however, the solution presented here, which is available as part of glibc is probably a good baseline from which we might improve things in future.
README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.