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103 lines
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<title>Inline Functions - Debugging with GDB</title>
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<div class="node">
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<a name="Inline-Functions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Tail-Call-Frames.html#Tail-Call-Frames">Tail Call Frames</a>,
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Up: <a rel="up" accesskey="u" href="Optimized-Code.html#Optimized-Code">Optimized Code</a>
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</div>
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<h3 class="section">11.1 Inline Functions</h3>
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<p><a name="index-inline-functions_002c-debugging-836"></a>
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<dfn>Inlining</dfn> is an optimization that inserts a copy of the function
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body directly at each call site, instead of jumping to a shared
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routine. <span class="sc">gdb</span> displays inlined functions just like
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non-inlined functions. They appear in backtraces. You can view their
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arguments and local variables, step into them with <code>step</code>, skip
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them with <code>next</code>, and escape from them with <code>finish</code>.
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You can check whether a function was inlined by using the
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<code>info frame</code> command.
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<p>For <span class="sc">gdb</span> to support inlined functions, the compiler must
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record information about inlining in the debug information —
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<span class="sc">gcc</span> using the <span class="sc">dwarf 2</span> format does this, and several
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other compilers do also. <span class="sc">gdb</span> only supports inlined functions
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when using <span class="sc">dwarf 2</span>. Versions of <span class="sc">gcc</span> before 4.1
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do not emit two required attributes (‘<samp><span class="samp">DW_AT_call_file</span></samp>’ and
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‘<samp><span class="samp">DW_AT_call_line</span></samp>’); <span class="sc">gdb</span> does not display inlined
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function calls with earlier versions of <span class="sc">gcc</span>. It instead
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displays the arguments and local variables of inlined functions as
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local variables in the caller.
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<p>The body of an inlined function is directly included at its call site;
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unlike a non-inlined function, there are no instructions devoted to
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the call. <span class="sc">gdb</span> still pretends that the call site and the
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start of the inlined function are different instructions. Stepping to
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the call site shows the call site, and then stepping again shows
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the first line of the inlined function, even though no additional
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instructions are executed.
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<p>This makes source-level debugging much clearer; you can see both the
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context of the call and then the effect of the call. Only stepping by
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a single instruction using <code>stepi</code> or <code>nexti</code> does not do
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this; single instruction steps always show the inlined body.
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<p>There are some ways that <span class="sc">gdb</span> does not pretend that inlined
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function calls are the same as normal calls:
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<ul>
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<li>Setting breakpoints at the call site of an inlined function may not
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work, because the call site does not contain any code. <span class="sc">gdb</span>
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may incorrectly move the breakpoint to the next line of the enclosing
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function, after the call. This limitation will be removed in a future
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version of <span class="sc">gdb</span>; until then, set a breakpoint on an earlier line
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or inside the inlined function instead.
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<li><span class="sc">gdb</span> cannot locate the return value of inlined calls after
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using the <code>finish</code> command. This is a limitation of compiler-generated
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debugging information; after <code>finish</code>, you can step to the next line
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and print a variable where your program stored the return value.
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</ul>
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</body></html>
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