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| <title>Debugging with GDB: Inline Functions</title> |
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| <a name="Inline-Functions-1"></a> |
| <h3 class="section">11.1 Inline Functions</h3> |
| <a name="index-inline-functions_002c-debugging"></a> |
| |
| <p><em>Inlining</em> is an optimization that inserts a copy of the function |
| body directly at each call site, instead of jumping to a shared |
| routine. <small>GDB</small> 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> |
| <p>For <small>GDB</small> to support inlined functions, the compiler must |
| record information about inlining in the debug information — |
| <small>GCC</small> using the <small>DWARF 2</small> format does this, and several |
| other compilers do also. <small>GDB</small> only supports inlined functions |
| when using <small>DWARF 2</small>. Versions of <small>GCC</small> before 4.1 |
| do not emit two required attributes (‘<samp>DW_AT_call_file</samp>’ and |
| ‘<samp>DW_AT_call_line</samp>’); <small>GDB</small> does not display inlined |
| function calls with earlier versions of <small>GCC</small>. It instead |
| displays the arguments and local variables of inlined functions as |
| local variables in the caller. |
| </p> |
| <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. <small>GDB</small> 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> |
| <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> |
| <p>There are some ways that <small>GDB</small> does not pretend that inlined |
| function calls are the same as normal calls: |
| </p> |
| <ul> |
| <li> Setting breakpoints at the call site of an inlined function may not |
| work, because the call site does not contain any code. <small>GDB</small> |
| 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 <small>GDB</small>; until then, set a breakpoint on an earlier line |
| or inside the inlined function instead. |
| |
| </li><li> <small>GDB</small> 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. |
| |
| </li></ul> |
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