From last week’s Lesson, the code I wrote for my clock program outputs the current hour and minute. I also want it to update, to keep running and output the next minute and so on. To me, that’s the difference between a clock program and code that just outputs the current time.
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The other day I had this strong urge to write myself a text mode clock program, one that runs at the command prompt. This desire harkens back to my days programming text mode computers: I don’t need a clock, I just wanted to see if I could code one.
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I’ve often wondered how it works. The printf() function has one argument minimum, a formatting string. Yet how does the compiler know how many other arguments are required? It’s not really a mystery, once you understand the concept of variable argument lists.
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With the demise of the gets() function, fgets() remains the top C language text-input function. Whether reading from a file or from standard input, the function is quite useful, but it’s not without some quirks.
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For pointer structures, the process of writing the structure to a file works similarly to writing a non-pointer structure, but it’s not without some pratfalls. Further, if you have a structure that contains a pointer, things get hinkey quickly.
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You need not read all structures from a file when you know the exact one you want. To fetch that record, you use the fseek() function. This function manipulates the file position indicator, allowing for random access to a file’s data.
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Sequential file access works like a tape recorder — if you remember using one. Data is read from byte 0 through the last byte in the file, one after the other. Random file access can be sequential, but you can also hop around within the file, reading a chunk here, writing a chunk there. The secret has to do with the how the file position indicator is manipulated.
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From your early C language training, you should know the difference between 1088 as a string and 1088 as an integer. They may look the same to human eyes, but inside the computer they’re completely different.
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Writing a value to a file and reading it from a file work exactly like reading and writing values from standard input and output. File-based versions of standard I/O functions are used, so the process should be familiar to you. Still, there’s an interesting catch.
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Unless your program also created the file it’s reading, you have no guarantee how must text lurks inside. It could be a single character or the entire works of Shakespeare. Dealing with an unknown quantity of text it a file-reading challenge.
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