In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer who uses that library to build an application or another library.

In a C struct, as with most things in C, there are no rules. Client programmers can do anything they want with that struct, and there’s no way to force any particular behaviors. For example, even though you saw in the last chapter the importance of the functions named initialize( ) and cleanup( ), the client programmer has the option not to call those functions. (We’ll look at a better approach in the next chapter.) And even though you would really prefer that the client programmer not directly manipulate some of the members of your struct, in C there’s no way to prevent it. Everything’s naked to the world.

There are two reasons for controlling access to members. The first is to keep the client programmer’s hands off tools they shouldn’t touch, tools that are necessary for the internal machinations of the data type, but not part of the interface the client programmer needs to solve their particular problems. This is actually a service to client programmers because they can easily see what’s important to them and what they can ignore.

The second reason for access control is to allow the library designer to change the internal workings of the structure without worrying about how it will affect the client programmer. In the Stack example in the last chapter, you might want to allocate the storage in big chunks, for speed, rather than creating new storage each time an element is added. If the interface and implementation are clearly separated and protected, you can accomplish this and require only a relink by the client programmer.