libstdc++ search path while building GCC

["Ludovic Courtès" <ludovic.courtes@inria.fr>]

Hello,

While building GCC on a non-FHS distro, we do something like¹:

  export CPLUS_INCLUDE_PATH=/path/to/host/gcc/include/c++:/path/to/host/gcc/include
  ./configure && make

where /path/to/host/gcc points to the GCC used to build GCC.

This ensures the host GCC’s C++ headers are found by ‘xgcc’ during
build.  However, during libstdc++ configure, the host GCC’s
<bits/c++config.h> is not found, leading to misdiagnostics like this
(here we’re building GCC 10.1 with GCC 7.5):

--8<---------------cut here---------------start------------->8---
configure:19924: checking for float trig functions
configure:19948:  /tmp/guix-build-gcc-10.1.0.drv-0/build/./gcc/xgcc -shared-libgcc -B/tmp/guix-build-gcc-10.1.0.drv-0/build/./gcc -nostdinc++ -L/tmp/guix-build-gcc-10.1.0.drv-0/build/x86_64-unknown-linux-gnu/libstdc++-v3/src -L/tmp/guix-build-gcc-10.1.0.drv-0/build/x86_64-unknown-linux-gnu/libstdc++-v3/src/.libs -L/tmp/guix-build-gcc-10.1.0.drv-0/build/x86_64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs -B/gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/bin/ -B/gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/lib/ -isystem /gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/include -isystem /gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/sys-include   -fno-checking -c -fno-builtin -D_GNU_SOURCE  conftest.cpp >&5
In file included from /gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include/c++/math.h:36,
                 from conftest.cpp:122:
/gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include/c++/cmath:41:10: fatal error: bits/c++config.h: No such file or directory
   41 | #include <bits/c++config.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
configure:19948: $? = 1
configure: failed program was:

[...]

| /* end confdefs.h.  */
| #include <math.h>
| int
| main ()
| {
| acosf (0); asinf (0); atanf (0); cosf (0); sinf (0); tanf (0); coshf (0); sinhf (0); tanhf (0);
|   ;
|   return 0;
| }
configure:19962: result: no
--8<---------------cut here---------------end--------------->8---

‘c++config.h’ is not found because it lives in a different directory
that’s not searched, ‘include/c++/x86_64-unknown-linux-gnu’.
(aka. ‘GPLUSPLUS_TOOL_INCLUDE_DIR’).

So, how’s that supposed to work?  I understand this kind of issue is
unlikely to show up on an FHS distro, but still, am I missing something?

Thanks,
Ludo’.

¹ More details at <https://issues.guix.gnu.org/42392#2>.