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 R

Answer 1

You might want to get a book on "embedded" programming. Such a book will likely discuss ways to keep the memory footprint down, as embedded systems are more constrained than modern desktop or server systems.

When you use "local" variables, they are saved on the stack. As long as you don't use too much stack, this is basically free memory, as when the function exits the memory is returned. How much is "too much" varies... recently I had to work on a system where there is a limit of 8 KB of data for the stack per process.

When you use "global" variables or other static variables, the memory you use is tied up for the duration of the program. Thus you should minimize your use of globals, and/or find ways to share the same memory across multiple functions in your program.

I wrote a fairly elaborate "object manager" for a project I wrote a few years ago. A function can use the "get" operation to borrow an object, and then use the "release" operation when it is done borrowing the object. This means that all the functions in the system were able to share a relatively small amount of data space by taking turns using the shared objects. It's up to you to decide whether it is worth your time to build an "object manager" sort of thing or if you have enough memory to just use simple variables.

You can get much of the benefit of an "object manager" by simply calling malloc() and free() a lot. Then the heap allocator manages the shared resource, the heap memory, for you. The reason I wrote my own "object manager" was a need for speed. My system keeps using identical data objects, and it is way faster to just keep re-using the same ones than to keep freeing them and malloc-ing them again. Also, my system can be run on a DSP chip, and malloc() can be a surprisingly slow function on some DSP architectures.

Having multiple functions using the same global variables can lead you to tricky bugs, if one function tries to hold on to a global buffer while another one is overwriting the data. So your program will likely be more robust if you use malloc() and free() as long as each function only writes into data it allocated for itself. (But malloc() and free() can introduce bugs of their own: memory leaks, double-free errors, continuing to use a pointer after the data to which it points has been freed... if you use malloc() and free() be sure to use a tool such as Valgrind to check your code.)