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CHAPTER 5 DEBUGGING
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Tip You might need to extend the timeout duration for your test MySQL client. Debugging can take some
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time if you are stepping through a series of breakpoints or you are examining a lot of variables. The system is essentially in a zombie state while you are debugging. This may cause the server and the client to cease communication. Some clients are designed to terminate if they cannot communicate with the server after a period of time. If you are using the MySQL command-line client you will need to extend the timeout. You can do this by specifying the value on the command line using --connection-timeout=600. This gives you about 10 minutes to work with the debugger before the client drops the connection.
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Listing 5-14 shows how you can use the ddd debugger to debug the MySQL server. I chose the same function from earlier, the show_authors() function in the sql_show.cc source file. In this scenario, I was interested in seeing how the server handled sending information to the client. You may recall from 3 that I mentioned having an example that showed the process of returning data to the client. Listing 5-14. The show_authors Function with Highlights /*************************************************************************** ** List all Authors. ** If you can update it, you get to be in it :) ***************************************************************************/ bool mysqld_show_authors(THD *thd) { List<Item> field_list; Protocol *protocol= thd->protocol; DBUG_ENTER("mysqld_show_authors"); field_list.push_back(new Item_empty_string("Name",40)); field_list.push_back(new Item_empty_string("Location",40)); field_list.push_back(new Item_empty_string("Comment",80)); if (protocol->send_fields(&field_list, Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF)) DBUG_RETURN(TRUE); show_table_authors_st *authors; for (authors= show_table_authors; authors->name; authors++) { protocol->prepare_for_resend(); protocol->store(authors->name, system_charset_info); protocol->store(authors->location, system_charset_info); protocol->store(authors->comment, system_charset_info);
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CHAPTER 5 DEBUGGING
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if (protocol->write()) DBUG_RETURN(TRUE); } send_eof(thd); DBUG_RETURN(FALSE); } The statements in bold are the methods used to send data back to the client. The show_authors() function is perfect for demonstrating the process because it is the simplest of implementations (no complex operations just sending data). The first highlighted statement shows the declaration of a pointer to the existing threads protocol class. The protocol class encapsulates all of the lower-level communication methods (such as networking and socket control). The next set of statements builds a field list. You always send a field list to the client first. Once the field list is built, you can send it to the client with the protocol->send_fields() method. In the loop, the code is looping through a list of authors defined in a linked list of show_table_authors_st. Inside the loop are the three principal methods used to send the data to the client. The first is protocol->prepare_for_resend(), which clears the appropriate buffers and variables for sending data. The next is protocol->store(), which places information in the send buffer. You should send each field as a separate call to this method. The protocol->write() method issues the appropriate action to send the data to the client. Finally, the send_eof() method instructs the communication mechanism to send the end-of-file marker to mark the end of the data. At this point, the client displays the data. Let s see how this function works using the ddd debugger. I have built my server using the debug switches by issuing the following commands: ./configure --with-debug make make install These commands will cause the system to be compiled with the debugging information so that I can use the debugger. Once I confirm no other servers are running, I launch the ddd debugger, load my source file (sql_show.cc), set a breakpoint in the show_authors() function at line 207, and then run the program. At that point, I launch my MySQL client program, setting the connection timeout to 10 minutes, and issue the SHOW AUTHORS command. Refer back to Listing 5-12 to see the server startup sequence; Listing 5-15 shows the client startup sequence. Listing 5-15. Starting the MySQL Client for Use with the ddd Debugger Chuck@linux:~> mysql -u root -p --connection-timeout=600 Enter password: Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 1 to server version: 5.1.9-beta-debug Type 'help;' or '\h' for help. Type '\c' to clear the buffer. mysql> show authors;
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