89 lines
3.6 KiB
Plaintext
89 lines
3.6 KiB
Plaintext
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<sect1 id="gdb"><title>Debugging Cygwin Programs</title>
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<para>When your program doesn't work right, it usually has a "bug" in
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it, meaning there's something wrong with the program itself that is
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causing unexpected results or crashes. Diagnosing these bugs and
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fixing them is made easy by special tools called
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<emphasis>debuggers</emphasis>. In the case of Cygwin, the debugger
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is GDB, which stands for "GNU DeBugger". This tool lets you run your
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program in a controlled environment where you can investigate the
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state of your program while it is running or after it crashes.
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Crashing programs sometimes create "core" files. In Cygwin these are
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regular text files that cannot be used directly by GDB.
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</para>
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<para>Before you can debug your program, you need to prepare your
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program for debugging. What you need to do is add
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<literal>-g</literal> to all the other flags you use when compiling
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your sources to objects.</para>
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<example id="gdb-g"><title>Compiling with -g</title>
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<screen>
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<prompt>$</prompt> gcc -g -O2 -c myapp.c
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<prompt>$</prompt> gcc -g myapp.c -o myapp
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</screen>
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</example>
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<para>What this does is add extra information to the objects (they get
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much bigger too) that tell the debugger about line numbers, variable
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names, and other useful things. These extra symbols and debugging
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information give your program enough information about the original
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sources so that the debugger can make debugging much easier for
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you.</para>
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<para>In Windows versions of GNUPro, GDB comes with a full-featured
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graphical interface. In Cygwin Net distributions, GDB is only
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available as a command-line tool. To invoke GDB, simply type
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<command>gdb myapp.exe</command> at the command prompt. It will
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display some text telling you about itself, then
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<literal>(gdb)</literal> will appear to prompt you to enter commands.
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Whenever you see this prompt, it means that gdb is waiting for you to
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type in a command, like <command>run</command> or
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<command>help</command>. Oh <literal>:-)</literal> type
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<command>help</command> to get help on the commands you can type in,
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or read the <citation>GDB User's Manual</citation> for a complete
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description of GDB and how to use it.</para>
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<para>If your program crashes and you're trying to figure out why it
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crashed, the best thing to do is type <command>run</command> and let
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your program run. After it crashes, you can type
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<command>where</command> to find out where it crashed, or
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<command>info locals</command> to see the values of all the local
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variables. There's also a <command>print</command> that lets you look
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at individual variables or what pointers point to.</para>
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<para>If your program is doing something unexpected, you can use the
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<command>break</command> command to tell gdb to stop your program when it
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gets to a specific function or line number:</para>
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<example id="gdb-break"><title>"break" in gdb</title>
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<screen>
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<prompt>(gdb)</prompt> break my_function
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<prompt>(gdb)</prompt> break 47
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</screen>
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</example>
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<para>Now, when you type <command>run</command> your program will stop
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at that "breakpoint" and you can use the other gdb commands to look at
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the state of your program at that point, modify variables, and
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<command>step</command> through your program's statements one at a
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time.</para>
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<para>Note that you may specify additional arguments to the
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<command>run</command> command to provide command-line arguments to
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your program. These two cases are the same as far as your program is
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concerned:</para>
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<example id="gdb-cliargs"><title>Debugging with command line arguments</title>
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<screen>
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<prompt>$</prompt> myprog -t foo --queue 47
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<prompt>$</prompt> gdb myprog
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<prompt>(gdb)</prompt> run -t foo --queue 47
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</screen>
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</example>
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</sect1>
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