keith gcc version 9.3.0 (Rev1, Built by MSYS2 project)
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Most include files appear to go into /mingw64/include or /mingw32/include, however flex installs its include files to /usr/include/. This path is not part of the gcc default include path, as evidenced by echo | gcc -E -Wp,-v -
flex should install its include files to /mingw64/include to avoid having to add -I/usr/include to the build.
$ gcc -v Using built-in specs. COLLECT_GCC=D:\Software\msys64\mingw64\bin\gcc.exe COLLECT_LTO_WRAPPER=D:/Software/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/9.3.0/lto-wrapper.exe Target: x86_64-w64-mingw32 Configured with: ../gcc-9.3.0/configure --prefix=/mingw64 --with-local-prefix=/mingw64/local --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --with-native-system-header-dir=/mingw64/x86_64-w64-mingw32/include --libexecdir=/mingw64/lib --enable-bootstrap --with-arch=x86-64 --with-tune=generic --enable-languages=c,lto,c++,fortran,ada,objc,obj-c++ --enable-shared --enable-static --enable-libatomic --enable-threads=posix --enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-filesystem-ts=yes --enable-libstdcxx-time=yes --disable-libstdcxx-pch --disable-libstdcxx-debug --disable-isl-version-check --enable-lto --enable-libgomp --disable-multilib --enable-checking=release --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --enable-plugin --with-libiconv --with-system-zlib --with-gmp=/mingw64 --with-mpfr=/mingw64 --with-mpc=/mingw64 --with-isl=/mingw64 --with-pkgversion='Rev1, Built by MSYS2 project' --with-bugurl=https://sourceforge.net/projects/msys2 --with-gnu-as --with-gnu-ld Thread model: posix gcc version 9.3.0 (Rev1, Built by MSYS2 project)
smcro@Cuboid MINGW64 ~ $ echo | gcc -E -Wp,-v - # 1 "<stdin>" # 1 "<built-in>" # 1 "<command-line>" # 1 "<stdin>" ignoring duplicate directory "D:/Software/msys64/mingw64/lib/gcc/../../lib/gcc/x86_64-w64-mingw32/9.3.0/include" ignoring nonexistent directory "D:/msys64/mingw64/include" ignoring nonexistent directory "/mingw64/include" ignoring duplicate directory "D:/Software/msys64/mingw64/lib/gcc/../../lib/gcc/x86_64-w64-mingw32/9.3.0/include-fixed" ignoring duplicate directory "D:/Software/msys64/mingw64/lib/gcc/../../lib/gcc/x86_64-w64-mingw32/9.3.0/../../../../x86_64-w64-mingw32/include" ignoring nonexistent directory "D:/msys64/mingw64/x86_64-w64-mingw32/include" #include "..." search starts here: #include <...> search starts here:
End of search list.
smcro@Cuboid MINGW64 ~ $