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C, the Foundation of Objective-C

Dec 6, 2012

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This chapter is from the book 

Learning Objective-C 2.0: A Hands-on Guide to Objective-C for Mac and iOS Developers, 2nd Edition

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Command Line Compiling and Debugging

When you write programs for Mac OS X or iOS, you should write, compile, and debug your programs using Xcode, Apple’s integrated development environment (IDE). You’ll learn how to set up a simple Xcode project in Chapter 4, “Your First Objective-C Program.” However, for the simple C programs required in the exercises in this chapter and the next chapter, you may find it easier to write the programs in your favorite text editor and then compile and run them from a command line.

Compilers and Debuggers

Historically, Apple has used the open-source GNU compiler, gcc, for building iOS and OS X programs. However, in the past several years they have transitioned to using compilers from the open-source LLVM (Low Level Virtual Machine) project. LLVM is not a single compiler; it is a set of modules that can be used to build compilers, debuggers, and related tools. In the first part of the transition Apple used a compiler called llvm-gcc-4.2 that combined the front end from gcc 4.2 with the LLVM code generator and optimizer. The current compiler is the Clang compiler, which combines a new unified parser for C, Objective C, C++, and Objective C++ with the LLVM code generator and optimizer.

Note that Apple refers to the Clang compiler as “LLVM Compiler N,” where N is the current version number—4.0 as of Xcode 4.4.

The Clang compiler has many advantages over llvm-gcc-4.2:

Clang provides much more informative error messages when you make a mistake.
Clang is faster than gcc.
Certain newer features of Objective-C, such as automatic reference counting (ARC), are available only by using the Clang compiler.

Clang is the default compiler in the current version of Xcode. Apple has announced that llvm-gcc-4.2 is “frozen” (no new features or bug fixes) as of Xcode 4.4 and will be removed in a future version of Xcode.

The LLVM project also includes LLDB, a new debugger.

You can find more information on Clang, LLDB, and the LLVM project on the LLVM website: http://llvm.org.

To compile from the command line, you will need:

A terminal window. You can use the Terminal app (/Applications/Utilities/Terminal) that comes with Mac OS X. If you are coming from another Unix environment, and you are used to xterms, you may prefer to download and use iTerm, an OS X native terminal application that behaves similarly to an xterm (http://iterm.sourceforge.net/).
A text editor. Mac OS X comes with both vi and emacs, or you can use a different editor if you have one.
The Apple Developer tools. You can get the current version of Xcode and the Developer tools from the OS X App Store (they’re free).

The first step is to check whether the command line tools are installed on your system. At the command prompt type

which clang

If the response is

/usr/bin/clang

your command line tools are already installed. If the response is

clang: Command not found.

the command line tools are not installed on your system and you must use Xcode to install them. To install the command line tools:

Open Xcode.
Choose Xcode > Preferences... from the menu.
When the Preferences panel opens, click the Downloads tab and then click on Components.
Finally, click the Install button for Command Line Tools.
When the installation is finished, you may close Xcode.

You are now ready to compile. If your source code file is named MyCProgram.c, you can compile it by typing the following at the command prompt:

clang -o MyCProgram MyCProgram.c

The -o flag allows you to give the compiler a name for your final executable. If the compiler complains that you have made a mistake or two, go back to fix them, and then try again. When your program compiles successfully, you can run it by typing the executable name at the command prompt:

MyCProgram

If you want to debug your program using gdb, the GNU debugger, or LLDB, you must use the -g flag when you compile:

clang -g -o MyCProgram MyCProgram.c

The -g flag causes clang to attach debugging information to the final executable.

To use gdb to debug a program, type gdb followed by the executable name:

gdb MyCProgram

Similarly, to use lldb you type lldb followed by the executable name:

lldb MyCProgram

Documentation for gdb is available at the GNU website, www.gnu.org/software/gdb/, or from Apple at http://developer.apple.com/mac/library/#documentation/DeveloperTools/gdb/gdb/gdb_toc.html. In addition, there are many websites with instructions for using gdb. Search for “gdb tutorial.”

You can learn more about LLDB by watching the Apple video at http://devimages.apple.com/llvm/videos/LLDB_Debugging_Infrastructure.mov or by going to the LLDB website, http://lldb.llvm.org/.

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