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| <h4 class="subsection">27.1.1 Context management</h4> |
| |
| <p>In most cases when <span class="sc">gdb</span> 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 |
| <span class="sc">gdb</span> via a single terminal, so no confusion is possible as |
| to what thread and frame are the current ones. |
| |
| <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><span class="samp">--thread</span></samp>’ and |
| ‘<samp><span class="samp">--frame</span></samp>’ options, the value to each is <span class="sc">gdb</span> identifier |
| for thread and frame to operate on. |
| |
| <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 <span class="sc">gdb</span> 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><span class="samp">thread</span></samp>’ command via |
| the frontend, it is desirable to change the frontend's selected thread to the |
| one specified by user. <span class="sc">gdb</span> communicates the suggestion to |
| change current thread using the ‘<samp><span class="samp">=thread-selected</span></samp>’ notification. |
| No such notification is available for the selected frame at the moment. |
| |
| <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 <span class="sc">gdb</span> 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 <span class="sc">gdb</span>, 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><span class="samp">--thread</span></samp>’ and |
| ‘<samp><span class="samp">--frame</span></samp>’ options. |
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