clang 3.8+ -fopenmp on linux: ld cannot find -lomp - linux

Question

I have installed clang 3.8 from the base repositories for both Debian Jessie and Fedora 24. When I try to compile a simple HelloWorld.cpp test program with clang++, and i pass the -fopenmp flag, in both cases i get the same error:
/usr/bin/ld: cannot find -lomp
clang-3.8: error: linker command failed with exit code 1 (use -v to see invocation)
I see that if I instead pass -fopenmp=libgomp, it works. However, the Clang OpenMP website says that the OpenMP runtime is shipped with Clang 3.8. Why, then, can it not find the default libomp library? I do not see this library anywhere on my system.

Answer 1

There is good chances that the OpenMP development package is missing on your system.
On Ubuntu: sudo apt install libomp-dev

Answer 2

If you have libomp installed correctly you will need to use -fopenmp=libomp. libgomp is for gcc. You might check that clang isn't symbollically linked to gcc on your computer.

Answer 3

TL;DR
If you have libomp.so for llvm in somewhere like /usr/lib/llvm-12/lib make file /etc/ld.so.conf.d/libomp.conf with the line /usr/lib/llvm-12/lib in it, then run sudo ldconfig.
Intro
In my case, I had libomp-12-dev installed, but it was not in my linker's library path. See the footnote on how I found the library. There are a couple solutions in this scenario:
Add library path with ldconfig
If you want this in your default library path, consider using ldconfig [man page].
This will look for files in /etc/ld.so.conf. For me, running Ubuntu 20.04, this file only points to including files in the directory /etc/ld.so.conf.d.
$ cat /etc/ld.so.conf
include /etc/ld.so.conf.d/*.conf
As such, I made a config llvm-libomp-12 in my /etc/ld.so.conf.d directory that looks like this:
$ cat /etc/ld.so.conf.d/libomp.conf
# libomp.so for llvm
/usr/lib/llvm-12/lib
Then I asked ldconfig to update the paths with sudo ldconfig. You can add the -v flag and it will print all libraries and paths it is aware of.
Add library to environment variable
We can also direct the linker to our library using the $LD_LIBRARY_PATH environment variable
This may be advantageous if you're on a multiuser system and don't want to impact others, or if you have temporary changes to your library paths you would like to make in your shell.
See what your current $LD_LIBRARY_PATH is with echo $LD_LIBRARY_PATH. You may not have this set by default. Add paths to this variable, each delimited by a colon.
For your current shell session, simply append or prepend to your $LD_LIBRARY_PATH like this (assuming bash, zsh, or fish >v3.0):
export "$LD_LIBRARY_PATH:/path/to/lib"
Or for a more permanent change limited to your user, add the above export to your shell's config file (e.g. ~/.bashrc).
Manually specify library path(s) in compiler flags
Nice for a one-off specific library that you don't always want in your default library paths. Specify the path to the library as a flag like this:
-L/path/to/lib
For example:
clang++ -L/usr/lib/llvm-12/lib [...]
make -L/usr/lib/llvm-12/lib
Footnotes
On searching
If you don't know where a given library you need is, you can use things like find. Personally though, I used a package called mlocate that indexes files on my machine and allows you to search them.
Installing mlocate
sudo apt install mlocate
Updating the indexes
sudo updatedb
Searching for a substring
locate libomp.so
When I searched for where my libomp libraries were, I did this:
$ locate libomp.so
/usr/lib/llvm-12/lib/libomp.so
/usr/lib/llvm-12/lib/libomp.so.5
/usr/lib/x86_64-linux-gnu/libomp.so.5
Notably it seemed like clang was using the libomp.so.5 in the linux-gnu directory, but I needed it to be using the llvm library.
Environment used in this post
$ lsb_release --all
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.2 LTS
Release: 20.04
Codename: focal
$ uname -a
Linux bip 5.8.0-48-generic #54~20.04.1-Ubuntu SMP Sat Mar 20 13:40:25 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
TODO
Some notes that could be added to this question:
Confirm and list priority of env vars vs config files vs flags (does this vary between compilers and linkers?)
Ordering library paths when using multiple config files (can we prefix with numbers to ensure the order libraries are parsed?)