Programming iOS 6, 3rd Edition

Files

The little dance of declaring a function before calling it (Example 1-3) may seem rather absurd, but it is of tremendous importance in the C language, because it is what allows a C program to be arbitrarily large and complex.

As your program grows, you can divide and organize it into multiple files. This kind of organization can make a large program much more maintainable — easier to read, easier to understand, easier to change without accidentally breaking things. A large C program therefore usually consists of two kinds of file: code files, whose filename extension is .c, and header files, whose filename extension is .h. The build system will automatically “see” all the files and will know that together they constitute a single program, but there is also a rule in C that code inside one file cannot “see” another file unless it is explicitly told to do so. Thus, a file itself constitutes a scope; this is a deliberate and valuable feature of C, because it helps you keep things nicely pigeonholed.

The way you tell a C file to “see” another file is with the #include directive. The hash sign in the term #include is a signal that this line is an instruction to the preprocessor. In this case, the word #include is followed by the name of another file, and the directive means that the preprocessor should simply replace the directive by the entire contents of the file that’s named.

So the strategy for constructing a large C program is something like this:

• In each .c file, put the code ...