Programming iOS 6, 3rd Edition

Chapter 1. Just Enough C

Do you believe in C? Do you believe in anything that has to do with me?

—Leonard Bernstein and Stephen Schwartz, Mass

To program for iOS, you need to speak to iOS. Everything you say to iOS will be in accordance with the iOS API. (An API, for application programming interface, is a list or specification of things you are allowed to say when communicating.) Therefore, you will need some knowledge of the C programming language, for two reasons:

• Most of the iOS API involves the Objective-C language, and most of your iOS programming will be in the Objective-C language; and Objective-C is a superset of C. This means that Objective-C presupposes C; everything that is true of C trickles up to Objective-C. A common mistake is to forget that “Objective-C is C” and to neglect a basic understanding of C.
• Some of the iOS API involves C rather than Objective-C. Even in Objective-C code, you often need to use C data structures and C function calls. For example, a rectangle is represented as a CGRect, which is a C struct, and to create a CGRect from four numbers you call CGRectMake, which is a C function. The iOS API documentation will very often show you C expressions and expect you to understand them.

The best way to learn C is to read The C Programming Language (PTR Prentice Hall, 1988) by Brian W. Kernighan and Dennis M. Ritchie, commonly called K&R (Ritchie was the creator of C). It is one of the best computer books ever written: brief, dense, and stunningly precise and clear. K&R is so important for effective iOS (and Mac OS X) programming that I keep a physical copy beside me at all times while coding, and I recommend that you do the same. Another useful manual is The C Book, by Mike Banahan, Declan Brady and Mark Doran, available online at http://publications.gbdirect.co.uk/c_book/.

It would be impossible, and unnecessary, for me to describe all of C in a single chapter. C is not a large or difficult language, but it has some tricky corners and can be extremely subtle, powerful, and low-level. Moreover, since C is described fully and correctly in the manuals I’ve just mentioned, it would be a mistake for me to repeat what they can tell you better than I.

You don’t have to know all about C in order to use Objective-C effectively, though; so my purpose in this chapter is to outline those aspects of C that are important for you to understand at the outset, before you even start using Objective-C for iOS programming. That’s why this chapter is “Just Enough C”: it’s just enough to get you going, comfortably and safely. Just keep in mind that this chapter is not a technical manual of C.

If you know no C at all, I suggest that, as an accompaniment to this chapter, you also read select parts of K&R (think of this as “C: The Good Parts Version”). Here’s my proposed K&R syllabus:

• Quickly skim K&R Chapter 1, the tutorial.
• Carefully read K&R Chapters 2 through 4.
• Read the first three sections of K&R Chapter 5 on pointers and arrays. You don’t need to read the rest of Chapter 5 because you won’t typically be doing any pointer arithmetic, but you do need to understand clearly what a pointer is, as Objective-C is all about objects, and every reference to an object is a pointer; you’ll be seeing and using that * character constantly.
• Read also the first section of K&R Chapter 6, on structures (structs); as a beginner, you probably won’t define any structs, but you will use them quite a lot, so you’ll need to know the notation (for example, as I’ve already said, a CGRect is a struct).
• Glance over K&R Appendix B, which covers the standard library, because you may find yourself making certain standard library calls, such as the mathematical functions; forgetting that the library exists is a typical beginner mistake.

Just to make things a little more confusing, the C defined in K&R is not precisely the C that forms the basis of Objective-C. Developments subsequent to K&R have resulted in further C standards (ANSI C, C89, C99), and the Xcode compiler extends the C language in its own ways. By default, Xcode projects are treated as GNU99, which is itself an extension of C99 (though you could specify another C standard if you really wanted to). Fortunately, the most important differences between K&R’s C and Xcode’s C are small, convenient improvements that are easily remembered, so K&R remains the best and most reliable C reference.