Common mistakes with memory allocation

Comments and questions to John Rowe.

Bad	Good
Dereferencing a pointer without allocating some memory for it This is the ultimate memrory allocation error and will lead to disaster.

Not checking for NULL return from `malloc()`
Bad	Good

	Better

Poor	Better
Using `malloc()` instead of an ordinary array when the array is not needed after the function returns Ordinary array declaration is fast and the memory is automatically released when the function returns. Dynamic memory allocation is slow and is permenant until freed. So: Use normal arrays when the array is not needed after the function returns. Use dynamic allocation when it is.

The first version has more ways to go wrong, we might forget to call `free()`, and is also slower. (However, until C99 programmers had no choice as array sizes had to be declared at compile time.) It's not unknown for the speed of some programs to be dominated by calls to `malloc()`.
Using an ordinary array instead of `malloc()` when the array is needed after the function returns This is the reverse of the previous one but is much more serious as the program will give the wrong answers or crash.
Bad	Better

Dereferencing a pointer without allocating some memory for it