I know C programmers who make it their goal to avoid pointers — even where necessary. It’s possible to do so, and many have cheat sheets to help them. Then comes the ** pointer monster, which can be avoided altogether . . . until you need to manipulate a pointer variable’s own address within a function. Then you’re cursed.
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When a function must manipulate a pointer’s address, the argument passed is a pointer-pointer, not a pointer. Confused? Oh, I’m just getting started . . .
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It’s common to pass a pointer to a function. Within the function, you can manipulate the data the pointer references without having to return that data from the function. This aspect of a pointer is what makes C a powerful — and scary — programming language. But what about when you need to manipulate the pointer’s address in the function?
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Logic can be frustrating, especially for non-Vulcans. The computer is kind of a Vulcan, so logic comes naturally to the machine. Programming languages are packed with logical expressions. A programmer’s duty is to convert human thoughts into the raw, powerful logic the computer understands — and end up with the desired result.
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In last week’s Lesson, I showed a program that translates a string of hex values into ASCII text. This code was to sate the nerd in me so that when another nerd writes 48 65 6c 6c 6f 2c 20 66 65 6c 6c 6f 77 20 6e 65 72 64 21, I can respond accordingly. But to respond in hex, a second program is required, one that translates ASCII text into a string of hex values.
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Do you speak hex? As a programmer, do you look at 0xF and see 1111 in binary? Do you see 15 decimal? What about the ASCII code? Do you know the letter for code 0x41? Are you that good?
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Pulling in all the stack items mentioned in last week’s Lesson, I present to you a C program that emulates stack storage. I’m certain this presentation will be one of the highlights of your programming career.
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To simulate stack storage in a C program, you need to emulate the four basic parts of a stack:
Further, you need to implement controls that prevent a stack overflow or underflow. And you can get real fancy, but those four items are the basics.
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Whether you simulate a stack in your C code or just ignore the concept altogether, your program constantly uses the processor’s stack. Specifically, if the code contains a function, it uses this stack not only to call and return from the function, but to pass and return arguments as well.
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At the lowest programming level, you encounter machine code. This is the language of the processor itself. Machine code directs the processor to do things, such as read from or write to memory, store data, perform math, and other tasks. And if you dare to program a processor directly, you’ll encounter something called the stack pointer register.
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