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17<title>Debugging with GDB: Observer Mode</title>
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66<p>
67Previous: <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> &nbsp; [<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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70<a name="Observer-Mode-1"></a>
71<h4 class="subsection">5.5.6 Observer Mode</h4>
72
73<p>If you want to build on non-stop mode and observe program behavior
74without any chance of disruption by <small>GDB</small>, you can set
75variables to disable all of the debugger&rsquo;s attempts to modify state,
76whether by writing memory, inserting breakpoints, etc.  These operate
77at a low level, intercepting operations from all commands.
78</p>
79<p>When all of these are set to <code>off</code>, then <small>GDB</small> is said to
80be <em>observer mode</em>.  As a convenience, the variable
81<code>observer</code> can be set to disable these, plus enable non-stop
82mode.
83</p>
84<p>Note that <small>GDB</small> will not prevent you from making nonsensical
85combinations of these settings. For instance, if you have enabled
86<code>may-insert-breakpoints</code> but disabled <code>may-write-memory</code>,
87then breakpoints that work by writing trap instructions into the code
88stream will still not be able to be placed.
89</p>
90<dl compact="compact">
91<dd>
92<a name="index-observer"></a>
93</dd>
94<dt><code>set observer on</code></dt>
95<dt><code>set observer off</code></dt>
96<dd><p>When set to <code>on</code>, this disables all the permission variables
97below (except for <code>insert-fast-tracepoints</code>), plus enables
98non-stop debugging.  Setting this to <code>off</code> switches back to
99normal debugging, though remaining in non-stop mode.
100</p>
101</dd>
102<dt><code>show observer</code></dt>
103<dd><p>Show whether observer mode is on or off.
104</p>
105<a name="index-may_002dwrite_002dregisters"></a>
106</dd>
107<dt><code>set may-write-registers on</code></dt>
108<dt><code>set may-write-registers off</code></dt>
109<dd><p>This controls whether <small>GDB</small> will attempt to alter the values of
110registers, such as with assignment expressions in <code>print</code>, or the
111<code>jump</code> command.  It defaults to <code>on</code>.
112</p>
113</dd>
114<dt><code>show may-write-registers</code></dt>
115<dd><p>Show the current permission to write registers.
116</p>
117<a name="index-may_002dwrite_002dmemory"></a>
118</dd>
119<dt><code>set may-write-memory on</code></dt>
120<dt><code>set may-write-memory off</code></dt>
121<dd><p>This controls whether <small>GDB</small> will attempt to alter the contents
122of memory, such as with assignment expressions in <code>print</code>.  It
123defaults to <code>on</code>.
124</p>
125</dd>
126<dt><code>show may-write-memory</code></dt>
127<dd><p>Show the current permission to write memory.
128</p>
129<a name="index-may_002dinsert_002dbreakpoints"></a>
130</dd>
131<dt><code>set may-insert-breakpoints on</code></dt>
132<dt><code>set may-insert-breakpoints off</code></dt>
133<dd><p>This controls whether <small>GDB</small> will attempt to insert breakpoints.
134This affects all breakpoints, including internal breakpoints defined
135by <small>GDB</small>.  It defaults to <code>on</code>.
136</p>
137</dd>
138<dt><code>show may-insert-breakpoints</code></dt>
139<dd><p>Show the current permission to insert breakpoints.
140</p>
141<a name="index-may_002dinsert_002dtracepoints"></a>
142</dd>
143<dt><code>set may-insert-tracepoints on</code></dt>
144<dt><code>set may-insert-tracepoints off</code></dt>
145<dd><p>This controls whether <small>GDB</small> will attempt to insert (regular)
146tracepoints at the beginning of a tracing experiment.  It affects only
147non-fast tracepoints, fast tracepoints being under the control of
148<code>may-insert-fast-tracepoints</code>.  It defaults to <code>on</code>.
149</p>
150</dd>
151<dt><code>show may-insert-tracepoints</code></dt>
152<dd><p>Show the current permission to insert tracepoints.
153</p>
154<a name="index-may_002dinsert_002dfast_002dtracepoints"></a>
155</dd>
156<dt><code>set may-insert-fast-tracepoints on</code></dt>
157<dt><code>set may-insert-fast-tracepoints off</code></dt>
158<dd><p>This controls whether <small>GDB</small> will attempt to insert fast
159tracepoints at the beginning of a tracing experiment.  It affects only
160fast tracepoints, regular (non-fast) tracepoints being under the
161control of <code>may-insert-tracepoints</code>.  It defaults to <code>on</code>.
162</p>
163</dd>
164<dt><code>show may-insert-fast-tracepoints</code></dt>
165<dd><p>Show the current permission to insert fast tracepoints.
166</p>
167<a name="index-may_002dinterrupt"></a>
168</dd>
169<dt><code>set may-interrupt on</code></dt>
170<dt><code>set may-interrupt off</code></dt>
171<dd><p>This controls whether <small>GDB</small> will attempt to interrupt or stop
172program execution.  When this variable is <code>off</code>, the
173<code>interrupt</code> command will have no effect, nor will
174<kbd>Ctrl-c</kbd>. It defaults to <code>on</code>.
175</p>
176</dd>
177<dt><code>show may-interrupt</code></dt>
178<dd><p>Show the current permission to interrupt or stop the program.
179</p>
180</dd>
181</dl>
182
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