malloc questions

The variables you declare are "automatic". Space is allocated in an area of memory called the stack to store these. When a function returns, its area of the stack is no longer used, and this space will be used for subsequent functions. However, there is not as much room available in this area of memory, so a huge array of structs may overfill it and your program will crash. The other section of memory where dynamically allocated items are placed is called the heap. static variables also live here so they can have one home during the whole run of your program.

When you need a very large chunk of memory or room for N items where N will only be known at runtime, you must allocate memory. Often programmers will use a "big enough" automatic array that turns out to be too small resulting in overflows, causing crashes and possibly security problems. On the flipside, proper memory management with malloc/free is easy to mess up resulting in leaks (memory the program won't release to the OS) or crashes if you try to free the same block many times.

This is one of the most important parts of dealing with C, and one of the easiest to mess up. Read the chapter a few times and ask a lot of questions.

-Lee

 

You're now moving from static memory allocation (ie memory allocation that cannot change) to dynamic memory allocation (ie memory allocation that can change).

Commercial-quality won't allocate a 5000 element array to hold DVDs because what happens if someone wants to store 5001 DVDs. With dynamic memory your program will support libraries of only a few DVDs without wasting memory while being able to grew to hold as many DVDs as will fit in the computer's memory.

In programs I write, for something like holding the library of DVDs, I'd absolutely be allocating and freeing memory (1). Generally I would use a mix of static and dynamic memory allocation. On a case-by-case basis, I'd judge whether the hassle/risk of dynamic memory allocation out weights the limitation of static memory allocations. With experience, this becomes a instinct.

(1) Not directly. I'd be using a higher-level container. There are none provided with C's standard library like there is with Objective-C and C++, though.