To launch and run another program from within your code, use the system() function. When your code must examine or save that program’s output, use the popen() function.
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The system() function allows you to run one program from within another. If it’s your desire to launch another program and have your program quit, you can immediately follow system() with an exit() function. Or you can go out of your way and use the oddball execl() function.
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Suppose that you’re testing code and need to run a program six times in a row. You could keep repeating the program, running it multiple times, but why not write a utility that does the same thing? That would be a good and practical way to put the system() function to work.
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One thing that continues to puzzle me about learning the C language is how frequently beginners use the system() function. Is it a crutch? Is it a necessity? What’s the allure of this function that makes it show up in beginner code?
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The standard C library contains a lot of interesting and unusual routines. Some can really put the fun into function. One of them I’ve rarely used, but which can be extremely handy, is atexit().
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I was a bit surprised the other day. I’d written code on my Macintosh and tried to run it on the PC. Because I usually write generic stuff, I figured that the code would compile and run on both systems. But it didn’t compile.
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Command line options often sport their own options or settings. For example, tab width might be set by specifying the tabwidth option followed by a value. No hard and fast rules exist for how such an option is set, but I’ve seen it happen in one of three ways:
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Your code must consider whether or not command line options are present, that the user may type the options in any order, and that some or all of the option could be incorrect. That makes for a lot of processing, but it’s also forgiving and users prefer the flexibility.
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When processing the main() function’s arguments, you must keep in mind the possibilities. Options are, after all, options. They might be there, they might not.
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About a year ago, I wrote a post on reading the command line arguments. It’s a process that takes place all the time, not only when running programs in a terminal window but also for graphical operating systems. Knowing how to manipulate command line arguments is important.
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