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| <h3 class="section">11.1 Inline Functions</h3> |
| |
| <p><a name="index-inline-functions_002c-debugging-663"></a> |
| <dfn>Inlining</dfn> is an optimization that inserts a copy of the function |
| body directly at each call site, instead of jumping to a shared |
| routine. <span class="sc">gdb</span> displays inlined functions just like |
| non-inlined functions. They appear in backtraces. You can view their |
| arguments and local variables, step into them with <code>step</code>, skip |
| them with <code>next</code>, and escape from them with <code>finish</code>. |
| You can check whether a function was inlined by using the |
| <code>info frame</code> command. |
| |
| <p>For <span class="sc">gdb</span> to support inlined functions, the compiler must |
| record information about inlining in the debug information — |
| <span class="sc">gcc</span> using the <span class="sc">dwarf 2</span> format does this, and several |
| other compilers do also. <span class="sc">gdb</span> only supports inlined functions |
| when using <span class="sc">dwarf 2</span>. Versions of <span class="sc">gcc</span> before 4.1 |
| do not emit two required attributes (‘<samp><span class="samp">DW_AT_call_file</span></samp>’ and |
| ‘<samp><span class="samp">DW_AT_call_line</span></samp>’); <span class="sc">gdb</span> does not display inlined |
| function calls with earlier versions of <span class="sc">gcc</span>. It instead |
| displays the arguments and local variables of inlined functions as |
| local variables in the caller. |
| |
| <p>The body of an inlined function is directly included at its call site; |
| unlike a non-inlined function, there are no instructions devoted to |
| the call. <span class="sc">gdb</span> still pretends that the call site and the |
| start of the inlined function are different instructions. Stepping to |
| the call site shows the call site, and then stepping again shows |
| the first line of the inlined function, even though no additional |
| instructions are executed. |
| |
| <p>This makes source-level debugging much clearer; you can see both the |
| context of the call and then the effect of the call. Only stepping by |
| a single instruction using <code>stepi</code> or <code>nexti</code> does not do |
| this; single instruction steps always show the inlined body. |
| |
| <p>There are some ways that <span class="sc">gdb</span> does not pretend that inlined |
| function calls are the same as normal calls: |
| |
| <ul> |
| <li>You cannot set breakpoints on inlined functions. <span class="sc">gdb</span> |
| either reports that there is no symbol with that name, or else sets the |
| breakpoint only on non-inlined copies of the function. This limitation |
| will be removed in a future version of <span class="sc">gdb</span>; until then, |
| set a breakpoint by line number on the first line of the inlined |
| function instead. |
| |
| <li>Setting breakpoints at the call site of an inlined function may not |
| work, because the call site does not contain any code. <span class="sc">gdb</span> |
| may incorrectly move the breakpoint to the next line of the enclosing |
| function, after the call. This limitation will be removed in a future |
| version of <span class="sc">gdb</span>; until then, set a breakpoint on an earlier line |
| or inside the inlined function instead. |
| |
| <li><span class="sc">gdb</span> cannot locate the return value of inlined calls after |
| using the <code>finish</code> command. This is a limitation of compiler-generated |
| debugging information; after <code>finish</code>, you can step to the next line |
| and print a variable where your program stored the return value. |
| |
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