include-what-you-use with non-default g++ version

Question

I'm trying to get include-what-you-use working with my codebase for the first time.

My CentOS6 system has its default of g++ 4.4.7 installed. I use g++ 4.9.3 for my C++11 project. Those four choices are inflexible.

I effectively build the cmake database for my project like this:

%> export CXX=/path/to/gcc-4.9.3/bin/g++
%> export CC=/path/to/gcc-4.9.3/bin/gcc
%> mkdir $HOME/dev/build
%> cd $HOME/dev/build
%> cmake .. -GNinja -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=/path/to/iwyu-6/bin/include-what-you-use

When I compile my project, I get output like this:

...
Warning: include-what-you-use reported diagnostics:
In file included from ../dev/src/whatever/foo.cc:13:
In file included from ../dev/src/whatever/foo.h:17:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/string:42:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/char_traits.h:41:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algobase.h:61:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/cstddef:44:10: fatal error: 'stddef.h' file not found
#include <stddef.h>
         ^~~~~~~~~~

../dev/src/whatever/foo.h should add these lines:

../dev/src/whatever/foo.h should remove these lines:
- #include <deque>  // lines 18-18

The full include-list for ../dev/src/trekcc/pss_dsl/pssTypes.h:
#include <string>  // for string
...

Great! I don't need deque.h in that file, so the tool is somewhat working!

The warnings are discouraging though... Of course stddef.h is found by my actual compilation because it is actually defined below gcc-4.9.3 at:

/path/to/gcc-4.9.3/lib/gcc/x86_64-unknown-linux-gnu/4.9.3/include/stddef.h

Q: How can I change my ${CMAKE_CXX_INCLUDE_WHAT_YOU_USE} variable so that the tool looks below gcc-4.9.3 instead of the default of gcc-4.4.7?

Note that if I ask cmake to export the JSON compile commands, this entry looks something like:

{
  "directory": "/home/me/dev/build",
  "command": "/tools/gnu/gcc-4.9.3/bin/g++ -I../dev/foo -I../dev/bar -I/path/to/boost_1_66_0/include -I/path/to/graphviz-2.38.0/include -I/path/to/graphviz-2.38.0/include/graphviz -I../include -Wall --std=c++11 -Wno-parentheses -fPIC -D_GLIBCXX_USE_CXX11_ABI=0 -g -o dev/whatever/foo.cc.o -c /home/me/dev/src/whatever/foo.cc",
  "file": "/home/me/dev/src/whatever/foo.cc"
}

Thanks!

---

edit:

Per @KamilCuk's suggestion below, adding a sysroot to the iwyu command seems to make the 4.4.7 warning go away.

%> cmake .. -GNinja -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="/path/to/iwyu-6/bin/include-what-you-use;-Xiwyu;--mapping_file=../iwyu.imp;--sysroot;/path/to/gcc-4.9.3/lib/gcc/x86_64-unknown-linux-gnu/4.9.3"

It cannot find basics like <map> or <vector> now, but the 4.4.7 warning has gone away, which tells me we're on the right track!

Using --sysroot /path/to/gcc-4.9.3/include/c++/4.9.3 doesn't solve that problem. I get many errors like this, for many common files:

Warning: include-what-you-use reported diagnostics:
../dev/whatever/foo.cc:12:10: fatal error: 'sstream' file not found
#include <sstream>
         ^~~~~~~~~

Answer 1

Probably you should just use a toolchain file. Often in my project (especially for cross-compiling), I will make a dedicated folder where I put all of my toolchains and other cmake commands.

my_proj
    cmake 
        other_gcc.cmake
    CMakeLists.txt
    ...

For example, here is one version of other_gcc.cmake I use for cross-compiling:

# Define our host system
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_VERSION 1)
set(CMAKE_SYSTEM_PROCESSOR aarch64)

# Define the cross compiler locations
set(CMAKE_C_COMPILER aarch64-linux-gnu-gcc)
set(CMAKE_CXX_COMPILER aarch64-linux-gnu-g++)

# Define the sysroot path for the RaspberryPi distribution in our tools folder
#set(CMAKE_FIND_ROOT_PATH $ENV{AARCH64_LINUX_TOOLS_DIR}/aarch64-linux-gnu/sysroot)

# Use our definitions for compiler tools
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)

This is relevant for you because you also need to change the compiler. Probably you can comment off those first 3 CMAKE_SYSTEM_ commands for your use case.

Then during the configure step, you just tell Cmake that you want to use this toolchain file:

cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/other_gcc.cmake ..