Hello World - installing Clang/LLVM on RHEL 7

Introduction and Prerequisites

In this tutorial, you will install the Clang/LLVM 7.0 Toolset and build a simple Clang/LLVM Hello World application. This tutorial should take less than 30 minutes to complete.

Before you begin, you will need a current Red Hat Enterprise Linux 7 workstation or server subscription that allows you to download software and get updates from Red Hat. If you don’t have an active subscription, register and obtain the RHEL Developer subscription (includes RHEL server) from here.

NOTE: The Clang/LLVM Compiler naming convention has changed so that the version number reflects the community version. The previous had the installation command of llvm-toolset-7 which was based on Clang/LLVM 6.0. 

If you encounter difficulties at any point, see Troubleshooting and FAQ.

Enable necessary software repositories

In this step, you will configure and “register” your system to obtain software from the devtools repository by using the command line interface. 

Note: registering a subscription (the steps to follow) is different than registering for the Red Hat Developer Program (which you’ve likely done by now).

You can add or remove software repositories from the command line using the subscription-manager tool as the root user. Use the --list option to view the available software repositories and verify that you have access to the devtools repo.

$ su -
# subscription-manager repos --list | egrep devtools

If you don’t see any devtools repositories in the list, your subscription might not include it. See Troubleshooting and FAQ for more information.

If you are using a desktop edition of Red Hat Enterprise Linux, change -server- to -desktop- in the following commands:

# subscription-manager repos --enable rhel-7-server-devtools-rpms
# subscription-manager repos --enable rhel-server-rhscl-7-rpms

Add the Red Hat Developer Tools key to your system:

# cd /etc/pki/rpm-gpg
# wget -O RPM-GPG-KEY-redhat-devel https://www.redhat.com/security/data/a5787476.txt
# rpm --import RPM-GPG-KEY-redhat-devel

Setup your development environment

In this next step you will use a single command to download the Clang/LLVM Toolset. The length of time this step takes depends on the speed of your Internet connection and your system. With a reasonably fast connection, this step should complete in well under 5 minutes.

$ su -
# yum install llvm-toolset-7.0

This installs clang and all development and debugging tools, and other dependent packages to the system.

Hello World and your first application

To compile a C++ program on the command line, run the clang++ compiler as follows:

$ scl enable llvm-toolset-7.0 'clang++ -o output_file source_file...'

This creates a binary file named output_file in the current working directory. If the -o option is omitted, the clang++ compiler creates a file named a.out by default.

When you are working on a project that consists of several source files, it is common to compile an object file for each of the source files first and then link these object files together. This way, when you change a single source file, you can recompile only this file without having to compile the entire project. To compile an object file on the command line:

$ scl enable llvm-toolset-7.0 'clang++ -o object_file -c source_file'

This creates an object file named object_file. If the -o option is omitted, the clang++ compiler creates a file named after the source file with the .o file extension. To link object files together and create a binary file:

$ scl enable llvm-toolset-7.0 'clang++ -o output_file object_file...'

Note that you can execute any command using the scl utility, causing it to be run with the LLVM Toolset binaries available. This allows you to run a shell session with LLVM Toolset clang directly available:

$ scl enable llvm-toolset-7.0 'bash'

⁠

Compiling a C++ Program on the Command Line

Consider a source file named hello.cpp with the following contents:

#include <iostream>



using namespace std;



int main(int argc, char *argv[]) {

 cout << "Hello, World!" << endl;

 return 0;

}

To compile this source code on the command line by using the clang++ compiler from LLVM Toolset, type:

$ scl enable llvm-toolset-7.0 'clang++ -o hello hello.cpp'

This creates a new binary file called hello in the current working directory.

Running a C++ Program

When clang++ compiles a program, it creates an executable binary file. To run this program on the command line, change to the directory with the executable file and type:

./file_name

⁠

Running a C++ Program on the Command Line

Assuming that you have successfully compiled the hello binary file, you can run it by typing the following at a shell prompt:

$ ./hello
Hello, World!

To run a C program, see the documentation instructions.

Troubleshooting and FAQ

1. As a developer, how can I get a Red Hat Enterprise Linux subscription that includes the Clang/LLVM, Go, or Rust Compilers?

Developers can get a no-cost Red Hat Enterprise Linux Developer subscription for development purposes by registering and downloading through developers.redhat.com. We recommend you follow our Getting Started Guide which covers downloading and installing Red Hat Enterprise Linux on a physical system or virtual machine (VM) using your choice of VirtualBox, VMware, Microsoft Hyper-V, or Linux KVM/Libvirt. For more information, see Frequently asked questions: no-cost Red Hat Enterprise Linux Developer subscription.

2. I can’t find the devtools or RHSCL repository on my system.

Some Red Hat Enterprise Linux subscriptions do not include access to Software Collections or development tools. For a list of what subscriptions are included see How to use Red Hat Software Collections (RHSCL), Red Hat Developer Toolset (DTS, etc.).

The name of the repository depends on whether you have a server or workstation version of Red Hat Enterprise Linux installed. You can use subscription-manager to view the available software repositories and verify that you have access to RHSCL and devtools:

$ su -
# subscription-manager repos --list | egrep rhscl
# subscription-manager repos --list | egrep devtools

3. When I run yum install package (go-toolset-7, llvm-toolset-7, rust-toolset-7), it fails due to a missing dependency.

Some software collections require packages that are in the devtools RPMs repository, which is not enabled by default. See Step 1 above, for how to enable both the applicable RPMs and repositories.

4. How can I view the Go manual pages?

The Go compiler help command provides information on its usage. To show the help index:

$ scl enable go-toolset-7 'go help'

5. How can I find out what RHSCL packages are installed?

scl --list will show the list of RHSCL packages that have been installed, whether they are enabled or not.

$ scl --list