compilers

programs that translate C++ to machine code (g++, clang)

In programming, the term compiler refers to a program that:

• takes as its input a program written in source code as input (e.g. a program in C++, Python, Java)
• produces as output that program in “object code” as output.

For C++, the two main compilers we use are g++ and clang++. Both are free and open source:

• g++ is the default compiler used by the make program on CSIL, and the most commonly used compiler. It comes from the gcc family of compilers, supported by GNU, a project of the Free Software Foundation. It is open source under the GNU public license.
• clang++ is an alternative compiler available on the CSIL machines. clang++ is a part of the LLVM family of compilers, which began as a research project at the Univ. of Illinois, and is open source under the BSD Open Source license.

Why would I want to use clang++ instead of g++ (or vice-versa)?

Both compilers are good choices—there is no particular reason for us to recommend one over the other.

However, there are some circumstances where you might want to switch compilers:

• Error messages: if you have a syntax error and the error message from the compiler is difficult to understand, try switching to the other compiler. For each of the two compilers, there are special cases where the error message from one is easier to understand than the other.
• Debugging: Sometimes a C++ program will have a bug because of code that has undefined behavior. In this case, running with both compilers can help you determine the source of the problem.
  • In this case undefined behavior is a technical term meaning that the program code is written in a way that is behavior cannot be accurately predicted.
  • Examples include printing the value of an uninitalized variable, or indexing into an array with an uninitialized variable.
  • Since an uninitialized variable can contain many different values (e.g. for a 32-bit integer, over 4 billion values), there could be many different possible outcomes.
  • It is often the case that for such code, running the same program compiled with g++ and clang++ produces different results.
  • Trying your code with both compilers before turning it in, or if/when your code is misbehaving, is often a helpful way to uncover what might be going wrong.

How do I switch between using g++ and clang++?

On CSIL, by default, the compiler used by make is g++. However, you can change the default compiler to clang++ by defining the environment variable CXX, as shown in this transcript:

-bash-4.3$ hostname
csil.cs.ucsb.edu
-bash-4.3$ cat hello.cpp
#include <iostream>
using namespace std;

int main(void) {
  cout << "Hello, World!" << endl;
}
-bash-4.3$ make hello
g++     hello.cpp   -o hello
-bash-4.3$ ./hello
Hello, World!
-bash-4.3$ rm hello
-bash-4.3$ export CXX=clang++
-bash-4.3$ make hello
clang++     hello.cpp   -o hello
-bash-4.3$ ./hello
Hello, World!
-bash-4.3$ export CXX=g++
-bash-4.3$ rm hello
-bash-4.3$ make hello
g++     hello.cpp   -o hello
-bash-4.3$