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<title>Debugging with GDB: Observer Mode</title>
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<a name="Observer-Mode"></a>
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<div class="header">
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<p>
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Previous: <a href="Interrupted-System-Calls.html#Interrupted-System-Calls" accesskey="p" rel="previous">Interrupted System Calls</a>, Up: <a href="Thread-Stops.html#Thread-Stops" accesskey="u" rel="up">Thread Stops</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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<a name="Observer-Mode-1"></a>
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<h4 class="subsection">5.5.6 Observer Mode</h4>
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<p>If you want to build on non-stop mode and observe program behavior
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without any chance of disruption by <small>GDB</small>, you can set
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variables to disable all of the debugger’s attempts to modify state,
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whether by writing memory, inserting breakpoints, etc. These operate
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at a low level, intercepting operations from all commands.
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</p>
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<p>When all of these are set to <code>off</code>, then <small>GDB</small> is said to
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be <em>observer mode</em>. As a convenience, the variable
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<code>observer</code> can be set to disable these, plus enable non-stop
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mode.
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</p>
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<p>Note that <small>GDB</small> will not prevent you from making nonsensical
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combinations of these settings. For instance, if you have enabled
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<code>may-insert-breakpoints</code> but disabled <code>may-write-memory</code>,
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then breakpoints that work by writing trap instructions into the code
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stream will still not be able to be placed.
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</p>
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<dl compact="compact">
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<dd>
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<a name="index-observer"></a>
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</dd>
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<dt><code>set observer on</code></dt>
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<dt><code>set observer off</code></dt>
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<dd><p>When set to <code>on</code>, this disables all the permission variables
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below (except for <code>insert-fast-tracepoints</code>), plus enables
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non-stop debugging. Setting this to <code>off</code> switches back to
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normal debugging, though remaining in non-stop mode.
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</p>
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</dd>
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<dt><code>show observer</code></dt>
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<dd><p>Show whether observer mode is on or off.
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</p>
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<a name="index-may_002dwrite_002dregisters"></a>
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</dd>
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<dt><code>set may-write-registers on</code></dt>
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<dt><code>set may-write-registers off</code></dt>
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<dd><p>This controls whether <small>GDB</small> will attempt to alter the values of
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registers, such as with assignment expressions in <code>print</code>, or the
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<code>jump</code> command. It defaults to <code>on</code>.
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</p>
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</dd>
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<dt><code>show may-write-registers</code></dt>
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<dd><p>Show the current permission to write registers.
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</p>
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<a name="index-may_002dwrite_002dmemory"></a>
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</dd>
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<dt><code>set may-write-memory on</code></dt>
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<dt><code>set may-write-memory off</code></dt>
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<dd><p>This controls whether <small>GDB</small> will attempt to alter the contents
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of memory, such as with assignment expressions in <code>print</code>. It
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defaults to <code>on</code>.
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</p>
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</dd>
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<dt><code>show may-write-memory</code></dt>
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<dd><p>Show the current permission to write memory.
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</p>
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<a name="index-may_002dinsert_002dbreakpoints"></a>
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</dd>
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<dt><code>set may-insert-breakpoints on</code></dt>
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<dt><code>set may-insert-breakpoints off</code></dt>
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<dd><p>This controls whether <small>GDB</small> will attempt to insert breakpoints.
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This affects all breakpoints, including internal breakpoints defined
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by <small>GDB</small>. It defaults to <code>on</code>.
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</p>
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</dd>
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<dt><code>show may-insert-breakpoints</code></dt>
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<dd><p>Show the current permission to insert breakpoints.
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</p>
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<a name="index-may_002dinsert_002dtracepoints"></a>
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</dd>
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<dt><code>set may-insert-tracepoints on</code></dt>
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<dt><code>set may-insert-tracepoints off</code></dt>
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<dd><p>This controls whether <small>GDB</small> will attempt to insert (regular)
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tracepoints at the beginning of a tracing experiment. It affects only
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non-fast tracepoints, fast tracepoints being under the control of
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<code>may-insert-fast-tracepoints</code>. It defaults to <code>on</code>.
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</p>
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</dd>
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<dt><code>show may-insert-tracepoints</code></dt>
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<dd><p>Show the current permission to insert tracepoints.
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</p>
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<a name="index-may_002dinsert_002dfast_002dtracepoints"></a>
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</dd>
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<dt><code>set may-insert-fast-tracepoints on</code></dt>
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<dt><code>set may-insert-fast-tracepoints off</code></dt>
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<dd><p>This controls whether <small>GDB</small> will attempt to insert fast
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tracepoints at the beginning of a tracing experiment. It affects only
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fast tracepoints, regular (non-fast) tracepoints being under the
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control of <code>may-insert-tracepoints</code>. It defaults to <code>on</code>.
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</p>
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</dd>
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<dt><code>show may-insert-fast-tracepoints</code></dt>
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<dd><p>Show the current permission to insert fast tracepoints.
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</p>
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<a name="index-may_002dinterrupt"></a>
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</dd>
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<dt><code>set may-interrupt on</code></dt>
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<dt><code>set may-interrupt off</code></dt>
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<dd><p>This controls whether <small>GDB</small> will attempt to interrupt or stop
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program execution. When this variable is <code>off</code>, the
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<code>interrupt</code> command will have no effect, nor will
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<kbd>Ctrl-c</kbd>. It defaults to <code>on</code>.
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</p>
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</dd>
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<dt><code>show may-interrupt</code></dt>
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<dd><p>Show the current permission to interrupt or stop the program.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Interrupted-System-Calls.html#Interrupted-System-Calls" accesskey="p" rel="previous">Interrupted System Calls</a>, Up: <a href="Thread-Stops.html#Thread-Stops" accesskey="u" rel="up">Thread Stops</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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