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Say we use a gcc version that (while supporting c++11) does not support c++11 by default, and needs an -std setting to enable it. If gdb would use the default AX_CXX_COMPILE_STDCXX from autoconf-archive, then we'd have: ... CXX="g++ -std=gnu++11" ... That mechanism however has the following problem (quoting from commit 0bcda685399): ... the top level Makefile passes CXX down to subdirs, and that overrides whatever gdb/Makefile may set CXX to. The result would be that a make invocation from the build/gdb/ directory would use "g++ -std=gnu++11" as expected, while a make invocation at the top level would not. ... Commit 0bcda685399 fixes this by using a custom AX_CXX_COMPILE_STDCXX which does: ... CXX=g++ CXX_DIALECT=-std=gnu++11 ... The problem reported in PR28318 is that using the custom instead of the default AX_CXX_COMPILE_STDCXX makes the configure test for std::thread support fail. We could simply add $CXX_DIALECT to the test for std::thread support, but that would have to be repeated for each added c++ support test. Instead, fix this by doing: ... CXX="g++ -std=gnu++11" CXX_DIALECT=-std=gnu++11 ... This is somewhat awkward, since it results in -std=gnu++11 occuring twice in some situations: ... $ touch src/gdb/dwarf2/read.c $ ( cd build/gdb; make V=1 dwarf2/read.o ) g++-4.8 -std=gnu++11 -x c++ -std=gnu++11 ... ... However, both settings are needed: - the switch in CXX for the std::thread tests (and other tests) - the switch in CXX_DIALECT so it can be appended in Makefiles, to counteract the fact that the top-level Makefile overrides CXX The code added in gdb/ax_cxx_compile_stdcxx.m4 is copied from the default AX_CXX_COMPILE_STDCXX from autoconf-archive. Tested on x86_64-linux. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28318
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.