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CHAPTER 5 DEBUGGING
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Notice in Listing 5-1 that the first inline debugging statements code documents the arrival of the system at this function, or its state, by indicating the name of the function. Notice also that each exit point of the function is documented along with the return value of the function. An excerpt from a trace file running the SHOW AUTHORS command is shown in Listing 5-2. I ve omitted a large section of the listing in order to show you how the trace file works for a successful execution of the SHOW AUTHORS command. Listing 5-2. Sample Trace File T@6 ... T@6 T@6 05 00 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 T@6 : : 00 : : : : : : : : : : : : : : | | | | 50 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | >send_eof | | | packet_header: Memory: 0x9b6ead8 | | | | | | | | | | | | | | : | | | >mysqld_show_authors
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Bytes: (4)
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| | >net_flush | | | >vio_is_blocking | | | | exit: 1 | | | <vio_is_blocking | | | >net_real_write | | | | >vio_write | | | | | enter: sd: 17776, buf: 0x0734D278, size: 5029 | | | | | exit: 5029 | | | | <vio_write | | | <net_real_write | | <net_flush | | info: EOF sent, so no more error sending allowed | <send_eof <mysqld_show_authors
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Note These inline debug statements are turned off by default. You can turn them on by compiling the
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server with debug and running the server in debug mode using the --debug command-line switch. This creates a trace file with all of the debug statements. On Linux, the trace file is stored in /tmp/mysqld.trace and on Windows, the file is stored in c:\mysqld.trace. These files can become quite large as all of the functions in MySQL are written using inline debugging statements.
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This technique, while simple, is a versatile tool. When you examine the flow of the system by inspecting the trace file, you can easily discover a starting point for further investigation. Sometimes just knowing where to look can be the greatest challenge.
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CHAPTER 5 DEBUGGING
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Error Handlers
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Have you ever encountered an error message while using software Whether you re using something created in the Pacific Northwest or created by the global community of developers, chances are you ve seen the end result of an error handler. You may be wondering why I would include error handlers as a debugging technique. That s because a good error handler presents the cause of the problem along with any possible corrective options. Good error handlers provide developers with enough information to understand what went wrong and how they might overcome the problem, and in some cases, include additional information that can assist them in diagnosing the problem. That last bit can sometimes go too far. Too many of us have seen dialog boxes containing terse error messages with confusing resolution options like the one shown in Figure 5-1.
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Figure 5-1. Poor error handler example As humorous as this example may be, messages like it are seen by users every day. Developers who write error messages like this are not making themselves clear. Statements that may be perfectly understandable for developers of a system could be gibberish for its users. The best policy is to create error messages that explain what has gone wrong and offer the user a resolution if one exists or at least a means to report the problem. It is also a good idea to provide a way to record the information a developer needs to diagnose the problem. This could be done via logging, a system state dump, or an auto-generated report. Figure 5-2 depicts a better example of how to present errors to the user.
Figure 5-2. Better error handler example Error handlers aren t just for reporting errors. There is another construct that is also called an error handler. This construct is simply the code used to trap and process (handle) errors. Perhaps you are familiar with the C++ try...catch block. This is an excellent example of an error handler as the language has been modified to include the construct in its syntax. Listing 5-3 depicts a typical try...catch block. The example shows the basic syntax for the C++ error handler (also called an exception handler).
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