[LLVMdev] Building llvm from source with clang instead of gcc

Óscar Fuentes ofv at wanadoo.es
Sun Jan 6 10:36:03 PST 2013

Edward Diener <eldlistmailingz at tropicsoft.com> writes:

>> Clang will attempt to use your installed system libraries, usually
>> libstdc++ and glibc on a Linux distribution.  We could probably maintain
>> a libstdc++ compatibility table that says clang x.y is compatible with
>> libstdc++ up to z.w.  In the vast majority of cases, this just works if
>> you install a somewhat-newer version of clang.  The problems arise when
>> you use an older clang with a newer libstdc++ (e.g. newer gcc and/or
>> linux distribution).
> This is what evidently happens. A clang binary distribution gets
> created for a particular Linux distro but there is no compatibility in
> the package which states that it only works with some version of
> libstdc++ or glibc or earlier. Then a later version of gcc gets
> installed and with it a later version of libstdc++ and glibc as a
> dependency. Now the binary version of clang no longer works on that
> system because of the updates to the C/C++ standard library.

Then, the package system of your Linux distribution is broken. Or the
package containing the Clang binaries does not correctly state its


>> Generally, it is recommended to bootstrap clang:
>> 1. Build llvm/clang with system gcc
>> 2. Build llvm/clang with new clang
>> 3. Install new clang somewhere (which will automatically use system
>> libstdc++/glibc).
>> This should solve your problem.

[I think Justin's recipe is overkill, but whatever]

> OK, this sounds good. Can this process not be explained on the clang
> Getting Started page for those who are building clang from source. I
> am not sure how step 3) is supposed to go. Do I just copy the built
> bin directory somewhere else and then put it first in my PATH, before
> running llvm/configure ?

You can install on a temporary directory or directly use the newly
created clang binaries, setting the environment variables CC and CXX
with full paths like this:

CC=/path/to/clang CXX=/path/to/clang++ configure [configure options] && make

Changing the PATH should work, but remeber to undo the change

> Or is "Install new clang" more complicated than that ?

It is simply `sudo make install' (which installs on system-level default
directories.) You can also install on a custom directory, using the
parameter `--prefix=' in the `configure' invocation.

> Originally I thought I could have both an earlier binary version of
> clang as well as the latest built version of clang in a Linux distro
> so that I could test code on both for compatibility of my library for
> end-users. But with the fairly common breakage of the binary version
> package of clang when libstdc++/glibc gets updated, this has become
> much less of a reality.

See above. You can have as much Clang versions as you please. Just be
sure to not uninstall the required gcc version for each Clang version.

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