Programming iOS 6, 3rd Edition

The Standard Library

You also have at your disposal a large collection of built-in C library files. A library file is a centrally located collection of C functions, along with a .h file that you can include so as to make those functions available to your code.

For example, suppose you want to round a float up to the next highest integer. The way to do this is to call some variety of the ceil function. You can read the ceil man page by typing man ceil in the Terminal. The documentation tells you what #include to use to incorporate the correct header and also shows you the function declarations and tells you what those functions do. A small pure C program might thus look like this:

#include <math.h>
float f = 4.5;
int i = ceilf(f); // now i is 5

In your iOS programs, math.h is included for you as part of the massive UIKit #import, so there’s no need to include it again. But some library functions might require an explicit #import.

The standard library is discussed in K&R Appendix B. But the modern standard library has evolved since K&R; it is a superset of K&R’s library. The ceil function, for example, is listed in K&R appendix B, but the ceilf function is not. Similarly, if you wanted to generate a random number (which is likely if you’re writing a game program that needs to incorporate some unpredictable behavior), you probably wouldn’t use the rand function listed in K&R; you’d use the random function, which supersedes it.

Forgetting that Objective-C is C and that the C library functions are available to your code is a common beginner mistake.