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<a name="The-GDB-Agent-Expression-Mechanism"></a>
<h2 class="appendix">Appendix F The GDB Agent Expression Mechanism</h2>

<p>In some applications, it is not feasible for the debugger to interrupt
the program&rsquo;s execution long enough for the developer to learn anything
helpful about its behavior.  If the program&rsquo;s correctness depends on its
real-time behavior, delays introduced by a debugger might cause the
program to fail, even when the code itself is correct.  It is useful to
be able to observe the program&rsquo;s behavior without interrupting it.
</p>
<p>Using GDB&rsquo;s <code>trace</code> and <code>collect</code> commands, the user can
specify locations in the program, and arbitrary expressions to evaluate
when those locations are reached.  Later, using the <code>tfind</code>
command, she can examine the values those expressions had when the
program hit the trace points.  The expressions may also denote objects
in memory &mdash; structures or arrays, for example &mdash; whose values GDB
should record; while visiting a particular tracepoint, the user may
inspect those objects as if they were in memory at that moment.
However, because GDB records these values without interacting with the
user, it can do so quickly and unobtrusively, hopefully not disturbing
the program&rsquo;s behavior.
</p>
<p>When GDB is debugging a remote target, the GDB <em>agent</em> code running
on the target computes the values of the expressions itself.  To avoid
having a full symbolic expression evaluator on the agent, GDB translates
expressions in the source language into a simpler bytecode language, and
then sends the bytecode to the agent; the agent then executes the
bytecode, and records the values for GDB to retrieve later.
</p>
<p>The bytecode language is simple; there are forty-odd opcodes, the bulk
of which are the usual vocabulary of C operands (addition, subtraction,
shifts, and so on) and various sizes of literals and memory reference
operations.  The bytecode interpreter operates strictly on machine-level
values &mdash; various sizes of integers and floating point numbers &mdash; and
requires no information about types or symbols; thus, the interpreter&rsquo;s
internal data structures are simple, and each bytecode requires only a
few native machine instructions to implement it.  The interpreter is
small, and strict limits on the memory and time required to evaluate an
expression are easy to determine, making it suitable for use by the
debugging agent in real-time applications.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="General-Bytecode-Design.html#General-Bytecode-Design" accesskey="1">General Bytecode Design</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Overview of the interpreter.
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<tr><td align="left" valign="top">&bull; <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="2">Bytecode Descriptions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">What each one does.
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<tr><td align="left" valign="top">&bull; <a href="Using-Agent-Expressions.html#Using-Agent-Expressions" accesskey="3">Using Agent Expressions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How agent expressions fit into the big picture.
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<tr><td align="left" valign="top">&bull; <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="4">Varying Target Capabilities</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to discover what the target can do.
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<tr><td align="left" valign="top">&bull; <a href="Rationale.html#Rationale" accesskey="5">Rationale</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Why we did it this way.
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