Some C coders really go nuts with macros. I’m not a big fan of those people.
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When you desire to do something specific in your program, or have a chunk of code that’s duplicated elsewhere, you craft a function. That’s the traditional, problem-solving approach, and the C language is full of functions. Yet sometimes, programmers build macros instead of functions.
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Writing a chunk of bits into an integer at a specific position requires major application of binary manipulation. That sounds impressive and mystical. To put it another way, writing a bit field is like using a stencil to paint a wall: You create the stencil, set it in place, then you slap down the paint.
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The same programming kung fu used to read a single bit also applies to wide bit fields. The process involves masking and shifting: You must know the bit field’s width and position in an integer value. Once those values are obtained — no matter how wide the bit field — a single function handles the job.
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An on-off bit field is pretty common; you’ll find them all over. Also common are wide bit fields, which can hold values greater than one or zero. These wide bit fields are often mixed with single-bit fields making for a wonderful stew of binary data all held within a single integer value.
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To assemble all the bit field manipulation functions into one program, I present the status portion of what could be a complex computer game. The bit fields represent the current conditions of the game’s starship. The fields are adjusted as the game progresses and various things happen.
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Once you break out the tools, bit manipulation in C can be a fascinating and useful thing. Setting a bit, covered in last week’s Lesson, is a big first step. Once you’ve done that, the functions to read and set a bit fall into place naturally.
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