C program with minimum RAM

Question

I want to understand the memory management in C and C++ programming for Application Development. Application will run on the PC.

If I want to make a program which uses RAM as less as possible while running, what are the points I need to consider while programming?

Here are two points according to what I understand, but I am not sure:

(1) Use minimum local variables in main() and other functions. As local variables are saved in stack, which is RAM?

(2) Instead of local variables, use global variables on the top. As global variables are saved in the uninitialized and initialized ROM area?

Thanks.

Answer 1

1) Generally the alternative to allocating on the stack is allocating on the heap (e.g., with malloc) which actually has a greater overhead due to bookkeeping/etc, and the stack already has memory reserved for it, so allocating on the stack where possible is often preferable. On the other hand there is less space on the stack while the heap can be close to “unlimited” on modern systems with virtual memory and 64-bit address space.

2) On PCs and other non-embedded system, everything in your program goes in RAM, i.e., it is not flashed to a ROM-like memory, so global versus local does not help in that regard. Also globals† tend to “live” as long as the application is running, while locals can be allocated and freed (either on the stack or heap) as required, and are thus preferable.

  † More accurately, there can also be local variables with static duration, and variables with global scope that are pointers to dynamically allocated memory, so the terms local and global are used quite loosely here.

In general, modern desktop/laptop and even mobile operating systems are quite good at managing memory, so you probably shouldn't be trying to micro-optimize everything as you may actually do more harm than good.

If you really do need to bring down the memory footprint of your program, you must realize that everything in the program is stored in RAM, and so you need to work on reducing the number and size of the things you have, rather than trying to juggle their location. The other place where you can store things locally on a PC is the hard drive, so store large resources there and only load them as required (preferably only exactly the parts required). But remember that disk access is orders of magnitude slower than memory access, and that the operating system can also swap things out to disk if its memory gets full.

The program code itself is also stored in RAM, so have your compiler optimize for size (-Os or /Os option in many common compilers). Also remember that if you save a bit of space in variables by writing more complex code, the effort may be undone by the increased code size; save your optimizations for big wins (e.g., compressing large resources will require the added decompression code, but may still yield a large net win). Use of dynamically linked libraries (and other resources) also helps the overall memory footprint of the system if the same library is used by multiple programs running at the same time.

(Note that some of the above does not apply in embedded development, e.g., code and static constants may be indeed be stored in flash instead of RAM, etc.)