Main image of article Developing in C/C++? Consider Clang

Summary

In order to better understand Clang, a compiler front-end for C, C++, Objective-C++, and Objective-C, you have to start with the LLVM project. LLVM is short for Low-Level Virtual Machine; it’s an intermediate compiler language (i.e., LLVM IR) similar in concept to bytecode or .il. Clang compiles the C/C++/Objective-C source code and outputs LLVM IR instructions, which are low-level RISC-like. The LLVM backend then generates the appropriate machine code for the targeted processor and system (which could include any X86, X86-64, PowerPC, PowerPC-64, ARM, Thumb, SPARC, Alpha, CellSPU, MIPS, MSP430, SystemZ, or XCore system). Although the LLVM infrastructure is open-source, the U.S. National Science Foundation, Apple, the University of Illinois, AutoESL Design Technologies, and others have all sponsored its development. Apple in particular has funded a lot of development since 2005, with Clang replacing GCC for iOS and Mac OS development in 2011. The idea with Clang and LLVM is that you can mix the compiler front-end with a targeted back-end and end up with highly portable and efficient compiler. Front-end compilers exist for C, C++, Common Lisp, ActionScript, Ada, D, Fortran, OpenGL Shading Language, Go, Haskell, Java bytecode, Julia, Objective-C, Swift, Python, R, Ruby, Rust, Scala, C# and Lua. Clang itself was open-sourced in 2007. It can perform static analysis of your code, whether in C, C++ or Objective-C. This functionality is built into the Xcode version that runs on the Mac, with instructions provided to build the static analyzer on other platforms. Static analysis is a technique whereby the program is read and ‘understood’ to the point where obvious bugs are automatically detected, such as dead code, memory leaks, double-frees, bad pointer references, and other such conditions. Clang lets you write tools that give you information about a program. There are three ways to do this:
  • Use LibCLang, a high-level C interface to Clang. This should be your first port of call, as it lets you access Clang from languages other than C++, as well as walk through the AST using a cursor. (AST is short for Abstract syntax tree, and is the output of the front end before generating the LLVM IR code that is used by the backend. Here’s a useful introduction to the Clang AST.)
  • You can write plugins for Clang to modify the compile process.
  • The third way is to use LibTooling, a C++ interface that lets you write standalone tools.
And here’s a useful set of external Clang examples: Clang has offered C++ 14 features for over a year, along with some C++ 17 (also called 1z) features; check out their current status. Note that it’s had C99 features since version 2.9. If you want to see 1z support in GCC, look here. Current release versions are GCC 5.2 and 4.9.3 with 6.0 in development. If you want to use GCC on windows, you have to use Cygwin or possibly Mingw. Cygwin and Mingw are command-line tools that let you run Unix tools on Windows, altered and recompiled for Windows. The main difference between Cygwin and Mingw is that Cygwin includes Posix support. Posix is a set of standards for Unix compatibility. (Clang can be used from within Visual Studio and does not necessarily need Cygwin or Mingw.) If you develop in C/C++ using GCC, you’re probably wondering if you should switch to Clang. While GCC is probably still best when it comes to speed, Clang is improving release by release. While I’d put my money on Clang for the long term, stick with GCC unless you have a compelling reason to do so otherwise.