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| <title>Debugging with GDB: Context management</title> |
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| <a name="Context-management"></a> |
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| Next: <a href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes" accesskey="n" rel="next">Asynchronous and non-stop modes</a>, Up: <a href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" accesskey="u" rel="up">GDB/MI General Design</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
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| <hr> |
| <a name="Context-management-1"></a> |
| <h4 class="subsection">27.1.1 Context management</h4> |
| |
| <a name="Threads-and-Frames"></a> |
| <h4 class="subsubsection">27.1.1.1 Threads and Frames</h4> |
| |
| <p>In most cases when <small>GDB</small> accesses the target, this access is |
| done in context of a specific thread and frame (see <a href="Frames.html#Frames">Frames</a>). |
| Often, even when accessing global data, the target requires that a thread |
| be specified. The CLI interface maintains the selected thread and frame, |
| and supplies them to target on each command. This is convenient, |
| because a command line user would not want to specify that information |
| explicitly on each command, and because user interacts with |
| <small>GDB</small> via a single terminal, so no confusion is possible as |
| to what thread and frame are the current ones. |
| </p> |
| <p>In the case of MI, the concept of selected thread and frame is less |
| useful. First, a frontend can easily remember this information |
| itself. Second, a graphical frontend can have more than one window, |
| each one used for debugging a different thread, and the frontend might |
| want to access additional threads for internal purposes. This |
| increases the risk that by relying on implicitly selected thread, the |
| frontend may be operating on a wrong one. Therefore, each MI command |
| should explicitly specify which thread and frame to operate on. To |
| make it possible, each MI command accepts the ‘<samp>--thread</samp>’ and |
| ‘<samp>--frame</samp>’ options, the value to each is <small>GDB</small> identifier |
| for thread and frame to operate on. |
| </p> |
| <p>Usually, each top-level window in a frontend allows the user to select |
| a thread and a frame, and remembers the user selection for further |
| operations. However, in some cases <small>GDB</small> may suggest that the |
| current thread be changed. For example, when stopping on a breakpoint |
| it is reasonable to switch to the thread where breakpoint is hit. For |
| another example, if the user issues the CLI ‘<samp>thread</samp>’ command via |
| the frontend, it is desirable to change the frontend’s selected thread to the |
| one specified by user. <small>GDB</small> communicates the suggestion to |
| change current thread using the ‘<samp>=thread-selected</samp>’ notification. |
| No such notification is available for the selected frame at the moment. |
| </p> |
| <p>Note that historically, MI shares the selected thread with CLI, so |
| frontends used the <code>-thread-select</code> to execute commands in the |
| right context. However, getting this to work right is cumbersome. The |
| simplest way is for frontend to emit <code>-thread-select</code> command |
| before every command. This doubles the number of commands that need |
| to be sent. The alternative approach is to suppress <code>-thread-select</code> |
| if the selected thread in <small>GDB</small> is supposed to be identical to the |
| thread the frontend wants to operate on. However, getting this |
| optimization right can be tricky. In particular, if the frontend |
| sends several commands to <small>GDB</small>, and one of the commands changes the |
| selected thread, then the behaviour of subsequent commands will |
| change. So, a frontend should either wait for response from such |
| problematic commands, or explicitly add <code>-thread-select</code> for |
| all subsequent commands. No frontend is known to do this exactly |
| right, so it is suggested to just always pass the ‘<samp>--thread</samp>’ and |
| ‘<samp>--frame</samp>’ options. |
| </p> |
| <a name="Language"></a> |
| <h4 class="subsubsection">27.1.1.2 Language</h4> |
| |
| <p>The execution of several commands depends on which language is selected. |
| By default, the current language (see <a href="Show.html#show-language">show language</a>) is used. |
| But for commands known to be language-sensitive, it is recommended |
| to use the ‘<samp>--language</samp>’ option. This option takes one argument, |
| which is the name of the language to use while executing the command. |
| For instance: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">-data-evaluate-expression --language c "sizeof (void*)" |
| ^done,value="4" |
| (gdb) |
| </pre></div> |
| |
| <p>The valid language names are the same names accepted by the |
| ‘<samp>set language</samp>’ command (see <a href="Manually.html#Manually">Manually</a>), excluding ‘<samp>auto</samp>’, |
| ‘<samp>local</samp>’ or ‘<samp>unknown</samp>’. |
| </p> |
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| Next: <a href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes" accesskey="n" rel="next">Asynchronous and non-stop modes</a>, Up: <a href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" accesskey="u" rel="up">GDB/MI General Design</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
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