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95 lines
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<title>Frames - Debugging with GDB</title>
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<a name="Frames"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Backtrace.html#Backtrace">Backtrace</a>,
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Up: <a rel="up" accesskey="u" href="Stack.html#Stack">Stack</a>
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</div>
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<h3 class="section">8.1 Stack Frames</h3>
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<p><a name="index-frame_002c-definition-504"></a><a name="index-stack-frame-505"></a>The call stack is divided up into contiguous pieces called <dfn>stack
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frames</dfn>, or <dfn>frames</dfn> 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><a name="index-initial-frame-506"></a><a name="index-outermost-frame-507"></a><a name="index-innermost-frame-508"></a>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 <dfn>initial</dfn> frame or the
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<dfn>outermost</dfn> 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 <dfn>innermost</dfn> frame. This is the most
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recently created of all the stack frames that still exist.
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<p><a name="index-frame-pointer-509"></a>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 <dfn>frame pointer register</dfn>
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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><a name="index-frame-level-510"></a><a name="index-frame-number-511"></a><span class="sc">gdb</span> labels each existing stack frame with a <dfn>level</dfn>, 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 <span class="sc">gdb</span> commands. The terms
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<dfn>frame number</dfn> and <dfn>frame level</dfn> can be used interchangeably to
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describe this number.
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<!-- The -fomit-frame-pointer below perennially causes hbox overflow -->
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<!-- underflow problems. -->
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<p><a name="index-frameless-execution-512"></a>Some compilers provide a way to compile functions so that they operate
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without stack frames. (For example, the <span class="sc">gcc</span> option
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<pre class="smallexample"> ‘<samp><span class="samp">-fomit-frame-pointer</span></samp>’
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</pre>
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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. <span class="sc">gdb</span> 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, <span class="sc">gdb</span> 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, <span class="sc">gdb</span> has
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no provision for frameless functions elsewhere in the stack.
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