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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Computer games were simple back in the early days. Output was printed on a teletype or displayed on a text-only CRT. Input wasn’t interactive or real-time. These games were fun to play back then, but are kind of lame now. They still exist with regards to simple programming exercises. In fact, you can pound out a older type computer game in a few minutes if you know the basics of the C language.
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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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One of the things I enjoy about coding defensive input is that I get to think about all the oddball possibilities. Users can type anything, which is something you must always anticipate. For typing in a percentage value, the door for whacky input is left wide open.
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The three basic bit manipulation operations are:
The standard C library lacks specific functions to carry out these bit manipulations, and I haven’t checked to see whether a third party library is available. That’s because you can easily code these operations on your own.
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Here’s a thought experiment for you: How can you prompt a user to properly input a percentage value?
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One of the C language’s strengths is that it can venture down into the depths of a computer’s inner workings. It loves to play in those dark, subterranean pools of raw data, bits, ports, and other primitive places that few high-level languages dare to tread.
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