Best way to allocate memory to a two-dimensional array in C?

Question

What is the best way to allocate memory to a two-d array in C,from both the perspectives : memory-management and speed ?

Answer 1

Given a fixed size, you can simply say twoDimArray[100][100], which will allocate it on the stack. When allocating on the heap, however, (whether because the size is very large or because the size is dynamic) you have more options.

You could allocate an array of pointers, then loop through allocating memory for each row. This is problematic for cache locality, but very good if the size is very large and your access is sequential; it allows a reasonable amount of fragmentation without a massive impact on performance, because the array of arrays can be separate from the arrays themselves, which can each be separate from each other. In a linear access scenario, you will mostly not be jumping between memory regions; rather, you'll access across a whole line before even possibly moving to a new region.

The second way is to linearize the access and allocate it all at once; i.e., allocate enough memory for sizex * sizey and then index it with (positiony * sizex) + positionx; that is, count down some rows and then across some columns. This is great for random access and improves cache locality because the memory is contiguous, but it might fail if there is not enough contiguous memory available (and the cache locality benefit is not applicable if you need more memory than there is cache).