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<title>Debugging with GDB: Frames</title>

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<a name="Frames"></a>
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<a name="Stack-Frames"></a>
<h3 class="section">8.1 Stack Frames</h3>

<a name="index-frame_002c-definition"></a>
<a name="index-stack-frame"></a>
<p>The call stack is divided up into contiguous pieces called <em>stack
frames</em>, or <em>frames</em> for short; each frame is the data associated
with one call to one function.  The frame contains the arguments given
to the function, the function&rsquo;s local variables, and the address at
which the function is executing.
</p>
<a name="index-initial-frame"></a>
<a name="index-outermost-frame"></a>
<a name="index-innermost-frame"></a>
<p>When your program is started, the stack has only one frame, that of the
function <code>main</code>.  This is called the <em>initial</em> frame or the
<em>outermost</em> frame.  Each time a function is called, a new frame is
made.  Each time a function returns, the frame for that function invocation
is eliminated.  If a function is recursive, there can be many frames for
the same function.  The frame for the function in which execution is
actually occurring is called the <em>innermost</em> frame.  This is the most
recently created of all the stack frames that still exist.
</p>
<a name="index-frame-pointer"></a>
<p>Inside your program, stack frames are identified by their addresses.  A
stack frame consists of many bytes, each of which has its own address; each
kind of computer has a convention for choosing one byte whose
address serves as the address of the frame.  Usually this address is kept
in a register called the <em>frame pointer register</em>
(see <a href="Registers.html#Registers">$fp</a>) while execution is going on in that frame.
</p>
<a name="index-frame-level"></a>
<a name="index-frame-number"></a>
<p><small>GDB</small> labels each existing stack frame with a <em>level</em>, a
number that is zero for the innermost frame, one for the frame that
called it, and so on upward.  These level numbers give you a way of
designating stack frames in <small>GDB</small> commands.  The terms
<em>frame number</em> and <em>frame level</em> can be used interchangeably to
describe this number.
</p>
<a name="index-frameless-execution"></a>
<p>Some compilers provide a way to compile functions so that they operate
without stack frames.  (For example, the <small>GCC</small> option
</p><div class="smallexample">
<pre class="smallexample">&lsquo;<samp>-fomit-frame-pointer</samp>&rsquo;
</pre></div>
<p>generates functions without a frame.)
This is occasionally done with heavily used library functions to save
the frame setup time.  <small>GDB</small> has limited facilities for dealing
with these function invocations.  If the innermost function invocation
has no stack frame, <small>GDB</small> nevertheless regards it as though
it had a separate frame, which is numbered zero as usual, allowing
correct tracing of the function call chain.  However, <small>GDB</small> has
no provision for frameless functions elsewhere in the stack.
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