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Client 3—Getting Connection Parameters at Runtime
Now we're ready to figure out how to do something smarter than using hardwired default connection parameters—such as letting the user specify those values at runtime. The previous client programs have a significant shortcoming in that the connection parameters are written literally into the source code. To change any of those values, you have to edit the source file and recompile it. That's not very convenient, especially if you intend to make your program available for other people to use. One common way to specify connection parameters at runtime is by using command line options. For example, the programs in the MySQL distribution accept parameters in either of two forms, as shown in the following table.
For consistency with the standard MySQL clients, our next client program, client3, will accept those same formats. It's easy to do this because the client library includes support for option processing. In addition, our client will have the ability to extract information from option files, which allows you to put connection parameters in ~/.my.cnf (that is, the .my.cnf file in your home directory) or in any of the global option files. Then you don't have to specify the options on the command line each time you invoke the program. The client library makes it easy to check for MySQL option files and pull any relevant values from them. By adding only a few lines of code to your program, you can make it option file-aware, and you don't have to reinvent the wheel by writing your own code to do it.
Before writing client3 itself, we'll develop a couple programs that illustrate how MySQL's option-processing support works. These show how option handling works fairly simply and without the added complication of connecting to the MySQL server and processing queries.
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