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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The code to recursively plow a directory tree presented in last week’s Lesson could be improved upon. Primarily, it relies upon the .. shortcut to jump back to the parent directory. This method works only some of the time.
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Exploring a folder tree — I mean directory tree — is a procedure found in many file and media utilities. From an original directory, you scan the list of files looking for a subdirectory. When it’s found, you open it and recursively continue the scan.
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