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if (protocol->send_fields(&field_list, Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF)) DBUG_RETURN(TRUE); show_privileges_st *privilege= sys_privileges; for (privilege= sys_privileges; privilege->privilege ; privilege++) { protocol->prepare_for_resend(); protocol->store(privilege->privilege, system_charset_info); protocol->store(privilege->context, system_charset_info); protocol->store(privilege->comment, system_charset_info); if (protocol->write()) DBUG_RETURN(TRUE); } send_eof(thd); DBUG_RETURN(FALSE); } The list of debug tags is quite comprehensive. The DBUG_ENTER and DBUG_RETURN tags are some of the most useful because they allow you to record a trace of the execution of the system throughout all of the functions called. It is especially important to point out that all the functions in the MySQL source code include these tags on entry and exit, respectively. Should you add your own functions, you should do the same and record the entry and exit(s) of your functions. These tags are written to a trace file stored in /tmp/mysqld.trace on Linux and c:\mysqld.trace on Windows. It should be noted that the trace file created can become very large. Fortunately, you can control which tags are written to the trace file by supplying them on the command line. For example, to limit the trace file to display the more interesting debug tags, you can use a command like the following. The general format of the switches is a:b:c for turning on switches a, b, and c. Any switches that take parameters are separated by commas. mysqld-debug --debug=d,info,error,query,general,where:t:L:g:O, /tmp/mysqd.trace -u root The previous command runs the MySQL server that is compiled with debug enabled (mysqld-debug). The command line parameter --debug=d,info,error,query,general, where:t:L:g:O,/tmp/mysqd.trace instructs the DBUG system to enable output from the DBUG_INFO, DBUG_ERROR, DBUG_QUERY, and DBUG_WHERE macros, turns on the trace lines for enter/exit of functions, includes the line number of the source code for the debug statement, enables profiling, and writes the file to /tmp/mysqld.trace. The -u root parameter passes the username root to the server for execution. Many more options are available; some common options are shown in Table 5-2.2
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2. A complete list of the commonly used DBUG switches can be found in the MySQL reference manual in the appendix titled Porting to Other Systems, under the subheading The DBUG Package.
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Table 5-2. List of Commonly Used DBUG Switches
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Turns on the output for the DBUG tags specified in the parameters. An empty list causes output for all tags. Performs a delay after each output. The parameter specifies the number of tenths of seconds to delay. For example, D,40 will cause a delay of 4 seconds. Limits the recording of debugging, tracing, and profiling to the list specified with d. Outputs the name of the source file for every line of debug or trace recorded. Outputs the process ID or thread ID for every line of debug or trace recorded. Turns on profiling. The parameters specify the keywords for those items to be profiled. An empty list implies all keywords are profiled. Outputs the source code line number for each line recorded. Sets the nesting depth for each line of output. This can help make the output more readable. Places sequential numbers on each line recorded. Saves the output to the file specified in the parameter. The default is written to stderr. Saves the output to the file specified in the parameter. The default is written to stderr. Flushes the file between each write. Outputs the current process name for each line recorded. Turns on function call/exit trace lines (represented as a vertical bar).
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Listing 5-9 shows an excerpt of a trace run while executing the show authors; command. You can see the entire trace of the system as it runs the command and returns data (I have omitted many lines as this list was generated with the default debug switches). I ve highlighted the most interesting lines. Notice also the trace lines that run down the lines of output. This allows you to follow the flow of the execution more easily. If you write your own functions in MySQL, you can use the DBUG tags to record your own information to the trace file. This file can prove to be helpful in the event that your code causes unpredictable or unexpected behavior. Listing 5-9. Sample Trace of the Show Privileges Command 338: | | | >mysqld_show_privileges 171: 220: 171: 220: 171: 220: 171: | | | | | | | | | | | | | | | | | | | | | | | | | | | | >alloc_root <alloc_root >alloc_root <alloc_root >alloc_root <alloc_root >alloc_root
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