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118 lines
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<title>Observer Mode - Debugging with GDB</title>
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<div class="node">
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<a name="Observer-Mode"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Interrupted-System-Calls.html#Interrupted-System-Calls">Interrupted System Calls</a>,
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Up: <a rel="up" accesskey="u" href="Thread-Stops.html#Thread-Stops">Thread Stops</a>
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</div>
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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 <span class="sc">gdb</span>, 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>When all of these are set to <code>off</code>, then <span class="sc">gdb</span> is said to
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be <dfn>observer mode</dfn>. 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>Note that <span class="sc">gdb</span> 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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<a name="index-observer-434"></a>
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<dl><dt><code>set observer on</code><dt><code>set observer off</code><dd>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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<br><dt><code>show observer</code><dd>Show whether observer mode is on or off.
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<p><a name="index-may_002dwrite_002dregisters-435"></a><br><dt><code>set may-write-registers on</code><dt><code>set may-write-registers off</code><dd>This controls whether <span class="sc">gdb</span> 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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<br><dt><code>show may-write-registers</code><dd>Show the current permission to write registers.
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<p><a name="index-may_002dwrite_002dmemory-436"></a><br><dt><code>set may-write-memory on</code><dt><code>set may-write-memory off</code><dd>This controls whether <span class="sc">gdb</span> 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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<br><dt><code>show may-write-memory</code><dd>Show the current permission to write memory.
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<p><a name="index-may_002dinsert_002dbreakpoints-437"></a><br><dt><code>set may-insert-breakpoints on</code><dt><code>set may-insert-breakpoints off</code><dd>This controls whether <span class="sc">gdb</span> will attempt to insert breakpoints.
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This affects all breakpoints, including internal breakpoints defined
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by <span class="sc">gdb</span>. It defaults to <code>on</code>.
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<br><dt><code>show may-insert-breakpoints</code><dd>Show the current permission to insert breakpoints.
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<p><a name="index-may_002dinsert_002dtracepoints-438"></a><br><dt><code>set may-insert-tracepoints on</code><dt><code>set may-insert-tracepoints off</code><dd>This controls whether <span class="sc">gdb</span> 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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<br><dt><code>show may-insert-tracepoints</code><dd>Show the current permission to insert tracepoints.
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<p><a name="index-may_002dinsert_002dfast_002dtracepoints-439"></a><br><dt><code>set may-insert-fast-tracepoints on</code><dt><code>set may-insert-fast-tracepoints off</code><dd>This controls whether <span class="sc">gdb</span> 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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<br><dt><code>show may-insert-fast-tracepoints</code><dd>Show the current permission to insert fast tracepoints.
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<p><a name="index-may_002dinterrupt-440"></a><br><dt><code>set may-interrupt on</code><dt><code>set may-interrupt off</code><dd>This controls whether <span class="sc">gdb</span> 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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<br><dt><code>show may-interrupt</code><dd>Show the current permission to interrupt or stop the program.
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</dl>
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</body></html>
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