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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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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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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Of course recursion is scary! At no other time in your programming career is your code more susceptible to stack overflows, which appear on the screen as ugly system errors when the code runs. I find that exciting.
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Recursion is a type of programming loop, one that’s deliberately designed to drive some programmers nuts. It’s also a great way to replace nested loops, such as those used in last week’s Lesson.
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If Arthur C. Clarke’s story The Nine Billion Names of God were true, then it seems rather pointless to plow through all 9,000,000,000 permutations to find one matching name. In fact, the exercise is more like a brute-force password cracking program than some celestial name search. What if the monks already knew the name? That would save time and effort.
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In his 1953 short story, The Nine Billion Names of God, Arthur C. Clarke writes of Tibetan llamas who seek to know all the names of God. They’ve been writing down the names for centuries, but upon the advent of the computer, they enlist western science to help them finish the task in days. You can complete the same task with your computer and the C programming language, but in hours instead of days.
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The strtok() function is the traditional C library routine used to slice up a string. It has its limitations, however, so a more recent function was included in the library on some compilers to replace it: strsep().
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