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| <h3 class="section">8.1 Stack Frames</h3> |
| |
| <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 |
| frames</dfn>, or <dfn>frames</dfn> 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-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 |
| function <code>main</code>. This is called the <dfn>initial</dfn> frame or the |
| <dfn>outermost</dfn> 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 <dfn>innermost</dfn> frame. This is the most |
| recently created of all the stack frames that still exist. |
| |
| <p><a name="index-frame-pointer-509"></a>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 <dfn>frame pointer register</dfn> |
| (see <a href="Registers.html#Registers">$fp</a>) while execution is going on in that frame. |
| |
| <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 |
| 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 <span class="sc">gdb</span> commands. The terms |
| <dfn>frame number</dfn> and <dfn>frame level</dfn> can be used interchangeably to |
| describe this number. |
| |
| <!-- The -fomit-frame-pointer below perennially causes hbox overflow --> |
| <!-- underflow problems. --> |
| <p><a name="index-frameless-execution-512"></a>Some compilers provide a way to compile functions so that they operate |
| without stack frames. (For example, the <span class="sc">gcc</span> option |
| <pre class="smallexample"> ‘<samp><span class="samp">-fomit-frame-pointer</span></samp>’ |
| </pre> |
| <p>generates functions without a frame.) |
| This is occasionally done with heavily used library functions to save |
| the frame setup time. <span class="sc">gdb</span> has limited facilities for dealing |
| with these function invocations. If the innermost function invocation |
| has no stack frame, <span class="sc">gdb</span> 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, <span class="sc">gdb</span> has |
| no provision for frameless functions elsewhere in the stack. |
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