AKMA’s Random Thoughts

Been out of touch too long, I guess. Just wanted to add my belated congratulations. Good for you and for Seabury.

This back and forth is an important concept to understand in C programming, especially on the Mac's RISC architecture. Almost every variable you work with can be represented in 32 bits of memory: thirty-two 1s and 0s define the data that a simple variable can hold. There are exceptions, like on the new 64-bit G5s and in the 128-bit world of AltiVec

We can see an example of this in our code we've written so far. In each function's block, we declare variables that hold our data. When each function ends, the variables within are disposed of, and the space they were using is given back to the computer to use. The variables live in the blocks of conditionals and loops we write, but they don't cascade into functions we call, because those aren't sub-blocks, but different sections of code entirely. Every variable we've written has a well-defined lifetime of one function.

Each Stack Frame represents a function. The bottom frame is always the main function, and the frames above it are the other functions that main calls. At any given time, the stack can show you the path your code has taken to get to where it is. The top frame represents the function the code is currently executing, and the frame below it is the function that called the current function, and the frame below that represents the function that called the function that called the current function, and so on all the way down to main, which is the starting point of any C program.

We can see an example of this in our code we've written so far. In each function's block, we declare variables that hold our data. When each function ends, the variables within are disposed of, and the space they were using is given back to the computer to use. The variables live in the blocks of conditionals and loops we write, but they don't cascade into functions we call, because those aren't sub-blocks, but different sections of code entirely. Every variable we've written has a well-defined lifetime of one function.

But some variables are immortal. These variables are declared outside of blocks, outside of functions. Since they don't have a block to exist in they are called global variables (as opposed to local variables), because they exist in all blocks, everywhere, and they never go out of scope. Although powerful, these kinds of variables are generally frowned upon because they encourage bad program design.

This code should compile and run just fine, and you should see no changes in how the program works. So why did we do all of that?