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| <title>Debugging with GDB: Frames</title> |
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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> |
| <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’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-number"></a> |
| <p><small>GDB</small> assigns numbers to all existing stack frames, starting with |
| zero for the innermost frame, one for the frame that called it, |
| and so on upward. These numbers do not really exist in your program; |
| they are assigned by <small>GDB</small> to give you a way of designating stack |
| frames in <small>GDB</small> commands. |
| </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">‘<samp>-fomit-frame-pointer</samp>’ |
| </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. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-frame_002c-command"></a> |
| <a name="index-current-stack-frame"></a> |
| </dd> |
| <dt><code>frame <span class="roman">[</span><var>framespec</var><span class="roman">]</span></code></dt> |
| <dd><p>The <code>frame</code> command allows you to move from one stack frame to another, |
| and to print the stack frame you select. The <var>framespec</var> may be either the |
| address of the frame or the stack frame number. Without an argument, |
| <code>frame</code> prints the current stack frame. |
| </p> |
| <a name="index-select_002dframe"></a> |
| <a name="index-selecting-frame-silently"></a> |
| </dd> |
| <dt><code>select-frame</code></dt> |
| <dd><p>The <code>select-frame</code> command allows you to move from one stack frame |
| to another without printing the frame. This is the silent version of |
| <code>frame</code>. |
| </p></dd> |
| </dl> |
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