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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Sending text to standard output is something you learn with your first C program. The common functions are putchar(), puts(), printf(), and so on. When you explore file access, you’ll find familiarity in those functions because they’re quite similar to the standard I/O functions.
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I had such a struggle with file I/O when I first learned computer programming. I knew what a file was and how to create it in an application. With computer programming, however, you enter a lower-level realm that requires more knowledge of file access.
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Your probably familiar with the #include and #define preprocessor directives. You may know a few more, which come in handy for certain coding issues. I have a page on this blog, which goes over some of the common directives, but it doesn’t cover them all. Specifically, it doesn’t cover the preprocessor names.
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