C Memory Management

Question

I\'ve always heard that in C you have to really watch how you manage memory. And I\'m still beginning to learn C, but thus far, I have not had to do any memory managing rela

Answer 1

I think the most concise way to answer the question in to consider the role of the pointer in C. The pointer is a lightweight yet powerful mechanism that gives you immense freedom at the cost of immense capacity to shoot yourself in the foot.

In C the responsibility of ensuring your pointers point to memory you own is yours and yours alone. This requires an organized and disciplined approach, unless you forsake pointers, which makes it hard to write effective C.

The posted answers to date concentrate on automatic (stack) and heap variable allocations. Using stack allocation does make for automatically managed and convenient memory, but in some circumstances (large buffers, recursive algorithms) it can lead to the horrendous problem of stack overflow. Knowing exactly how much memory you can allocate on the stack is very dependent on the system. In some embedded scenarios a few dozen bytes might be your limit, in some desktop scenarios you can safely use megabytes.

Heap allocation is less inherent to the language. It is basically a set of library calls that grants you ownership of a block of memory of given size until you are ready to return ('free') it. It sounds simple, but is associated with untold programmer grief. The problems are simple (freeing the same memory twice, or not at all [memory leaks], not allocating enough memory [buffer overflow], etc) but difficult to avoid and debug. A hightly disciplined approach is absolutely mandatory in practive but of course the language doesn't actually mandate it.

I'd like to mention another type of memory allocation that's been ignored by other posts. It's possible to statically allocate variables by declaring them outside any function. I think in general this type of allocation gets a bad rap because it's used by global variables. However there's nothing that says the only way to use memory allocated this way is as an undisciplined global variable in a mess of spaghetti code. The static allocation method can be used simply to avoid some of the pitfalls of the heap and automatic allocation methods. Some C programmers are surprised to learn that large and sophisticated C embedded and games programs have been constructed with no use of heap allocation at all.