| <html lang="en"> |
| <head> |
| <title>Context management - Debugging with GDB</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Debugging with GDB"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design" title="GDB/MI General Design"> |
| <link rel="next" href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes" title="Asynchronous and non-stop modes"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988-2019 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software'' and ``Free Software Needs |
| Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,'' |
| and with the Back-Cover Texts as in (a) below. |
| |
| (a) The FSF's Back-Cover Text is: ``You are free to copy and modify |
| this GNU Manual. Buying copies from GNU Press supports the FSF in |
| developing GNU and promoting software freedom.'' |
| --> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Context-management"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Asynchronous-and-non_002dstop-modes.html#Asynchronous-and-non_002dstop-modes">Asynchronous and non-stop modes</a>, |
| Up: <a rel="up" accesskey="u" href="GDB_002fMI-General-Design.html#GDB_002fMI-General-Design">GDB/MI General Design</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">27.1.1 Context management</h4> |
| |
| <h5 class="subsubsection">27.1.1.1 Threads and Frames</h5> |
| |
| <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> global |
| 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 or frame 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>’ or |
| ‘<samp><span class="samp">frame</span></samp>’ commands via the frontend, it is desirable to change the |
| frontend's selection to the one specified by user. <span class="sc">gdb</span> |
| communicates the suggestion to change current thread and frame using the |
| ‘<samp><span class="samp">=thread-selected</span></samp>’ notification. |
| |
| <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. |
| |
| <h5 class="subsubsection">27.1.1.2 Language</h5> |
| |
| <p>The execution of several commands depends on which language is selected. |
| By default, the current language (see <a href="show-language.html#show-language">show language</a>) is used. |
| But for commands known to be language-sensitive, it is recommended |
| to use the ‘<samp><span class="samp">--language</span></samp>’ option. This option takes one argument, |
| which is the name of the language to use while executing the command. |
| For instance: |
| |
| <pre class="smallexample"> -data-evaluate-expression --language c "sizeof (void*)" |
| ^done,value="4" |
| (gdb) |
| </pre> |
| <p>The valid language names are the same names accepted by the |
| ‘<samp><span class="samp">set language</span></samp>’ command (see <a href="Manually.html#Manually">Manually</a>), excluding ‘<samp><span class="samp">auto</span></samp>’, |
| ‘<samp><span class="samp">local</span></samp>’ or ‘<samp><span class="samp">unknown</span></samp>’. |
| |
| </body></html> |
| |
