{"id":32,"date":"2016-05-06T16:52:03","date_gmt":"2016-05-06T08:52:03","guid":{"rendered":"http:\/\/blogs.gentoo.org\/gsoc2016-native-clang\/?p=32"},"modified":"2017-03-08T03:32:21","modified_gmt":"2017-03-07T19:32:21","slug":"build-a-gnu-free-c-program-on-gentoo","status":"publish","type":"post","link":"https:\/\/blogs.gentoo.org\/gsoc2016-native-clang\/2016\/05\/06\/build-a-gnu-free-c-program-on-gentoo\/","title":{"rendered":"Build a GNU-free C++ program on Gentoo"},"content":{"rendered":"

This post is a following-up of my previous\u00a0one:\u00a0Build a freestanding libc++<\/a>. Here I’ll demonstrate how to link a C++ program with the freestanding libc++ we just built. The resulting executable will have no dependence on glibc or any GCC component,\u00a0thus is GNU-free.<\/p>\n

As the libc++ in use is linked with musl, the C++ program to be linked with libc++ need also\u00a0be linked with musl. In the previous post we already got a musl-based toolchain via crossdev<\/code>, but its C++ compiler is g++, which unfortunately doesn’t support libc++. We’ll have to use a vanilla clang++ instead to compile our C++ program then.<\/p>\n

The problem with using a vanilla clang++ is that it’s not musl-aware, i.e. it doesn’t know where to find musl’s headers and libraries. Such information can only\u00a0be fed to clang++ via some cumbersome command-line arguments. Assume our C++ runtime libraries (including libc++) built in previous\u00a0post are installed under directory $LOCAL; the complete command for compiling a GNU-free C++ program looks like this:<\/p>\n

clang++ hello.cc \\
\n-nostdinc -isystem \/usr\/x86_64-pc-linux-musl\/usr\/include \\
\n-I $LOCAL\/include\/c++\/v1 \\
\n-L \/usr\/x86_64-pc-linux-musl\/usr\/lib -L $LOCAL\/lib \\
\n-nostartfiles \/usr\/x86_64-pc-linux-musl\/usr\/lib\/crt1.o \\
\n-Wl,-dynamic-linker,\/usr\/x86_64-pc-linux-musl\/lib\/ld-musl-x86_64.so.1 \\
\n-Wl,-rpath,\/usr\/x86_64-pc-linux-musl\/usr\/lib,-rpath,$LOCAL\/lib \\
\n-nodefaultlibs -stdlib=libc++ -lc -lc++
\n<\/code><\/p>\n

This is indeed a long command. Because clang doesn’t have something like GCC’s specs file, we have to elaborately specify every configuration on the command line. This is also what I’m going to improve this summer. Hopefully we’ll make clang more friendly to musl, so we don’t have to issue\u00a0such obscure commands when clang is eventually deployed as Gentoo’s default compiler.<\/p>\n

Now let me explain these command options in some detail:<\/p>\n