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<title>Debugging with GDB: Frames</title>
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<a name="Frames"></a>
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<p>
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Next: <a href="Backtrace.html#Backtrace" accesskey="n" rel="next">Backtrace</a>, Up: <a href="Stack.html#Stack" accesskey="u" rel="up">Stack</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="Stack-Frames"></a>
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<h3 class="section">8.1 Stack Frames</h3>
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<a name="index-frame_002c-definition"></a>
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<a name="index-stack-frame"></a>
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<p>The call stack is divided up into contiguous pieces called <em>stack
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frames</em>, or <em>frames</em> for short; each frame is the data associated
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with one call to one function. The frame contains the arguments given
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to the function, the function’s local variables, and the address at
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which the function is executing.
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</p>
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<a name="index-initial-frame"></a>
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<a name="index-outermost-frame"></a>
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<a name="index-innermost-frame"></a>
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<p>When your program is started, the stack has only one frame, that of the
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function <code>main</code>. This is called the <em>initial</em> frame or the
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<em>outermost</em> frame. Each time a function is called, a new frame is
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made. Each time a function returns, the frame for that function invocation
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is eliminated. If a function is recursive, there can be many frames for
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the same function. The frame for the function in which execution is
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actually occurring is called the <em>innermost</em> frame. This is the most
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recently created of all the stack frames that still exist.
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</p>
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<a name="index-frame-pointer"></a>
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<p>Inside your program, stack frames are identified by their addresses. A
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stack frame consists of many bytes, each of which has its own address; each
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kind of computer has a convention for choosing one byte whose
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address serves as the address of the frame. Usually this address is kept
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in a register called the <em>frame pointer register</em>
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(see <a href="Registers.html#Registers">$fp</a>) while execution is going on in that frame.
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</p>
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<a name="index-frame-level"></a>
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<a name="index-frame-number"></a>
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<p><small>GDB</small> labels each existing stack frame with a <em>level</em>, a
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number that is zero for the innermost frame, one for the frame that
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called it, and so on upward. These level numbers give you a way of
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designating stack frames in <small>GDB</small> commands. The terms
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<em>frame number</em> and <em>frame level</em> can be used interchangeably to
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describe this number.
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</p>
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<a name="index-frameless-execution"></a>
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<p>Some compilers provide a way to compile functions so that they operate
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without stack frames. (For example, the <small>GCC</small> option
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</p><div class="smallexample">
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<pre class="smallexample">‘<samp>-fomit-frame-pointer</samp>’
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</pre></div>
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<p>generates functions without a frame.)
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This is occasionally done with heavily used library functions to save
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the frame setup time. <small>GDB</small> has limited facilities for dealing
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with these function invocations. If the innermost function invocation
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has no stack frame, <small>GDB</small> nevertheless regards it as though
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it had a separate frame, which is numbered zero as usual, allowing
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correct tracing of the function call chain. However, <small>GDB</small> has
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no provision for frameless functions elsewhere in the stack.
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Next: <a href="Backtrace.html#Backtrace" accesskey="n" rel="next">Backtrace</a>, Up: <a href="Stack.html#Stack" accesskey="u" rel="up">Stack</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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