What does gcc without multilib mean?

Question

I was trying to use the omh.h header file and I realized it was missing. I tried reinstalling gcc on my mac using brew. This is the message I got at the end of the installation.

..
GCC has been built with multilib support. Notably, OpenMP may not work:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60670
If you need OpenMP support you may want to
  brew reinstall gcc --without-multilib
==> Summary
🍺  /usr/local/Cellar/gcc/4.9.2_1: 1156 files, 203M

It suggests that if I need OpenMP support I need to install brew reinstall gcc --without-multilib. I am not able to understand the meaning of --without-multilib. How is it different from simply installing gcc?

And apparently brew reinstall gcc --without-multilib takes forever to run and brew uninstall gcc && brew install gcc was lightning fast.

Answer 1

Multilib means support for multiple architectures, so you can compile binaries for them.

Given the bug you show, you should be safe as long you build binaries for AMD64 architecture (64-bit) and IA32, as that is what your machine is.

Otherwise, better to choose without multilib.