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| <a name="Returning-from-a-Function"></a> |
| <h3 class="section">17.4 Returning from a Function</h3> |
| |
| <dl compact="compact"> |
| <dd><a name="index-returning-from-a-function"></a> |
| <a name="index-return"></a> |
| </dd> |
| <dt><code>return</code></dt> |
| <dt><code>return <var>expression</var></code></dt> |
| <dd><p>You can cancel execution of a function call with the <code>return</code> |
| command. If you give an |
| <var>expression</var> argument, its value is used as the function’s return |
| value. |
| </p></dd> |
| </dl> |
| |
| <p>When you use <code>return</code>, <small>GDB</small> discards the selected stack frame |
| (and all frames within it). You can think of this as making the |
| discarded frame return prematurely. If you wish to specify a value to |
| be returned, give that value as the argument to <code>return</code>. |
| </p> |
| <p>This pops the selected stack frame (see <a href="Selection.html#Selection">Selecting a |
| Frame</a>), and any other frames inside of it, leaving its caller as the |
| innermost remaining frame. That frame becomes selected. The |
| specified value is stored in the registers used for returning values |
| of functions. |
| </p> |
| <p>The <code>return</code> command does not resume execution; it leaves the |
| program stopped in the state that would exist if the function had just |
| returned. In contrast, the <code>finish</code> command (see <a href="Continuing-and-Stepping.html#Continuing-and-Stepping">Continuing and Stepping</a>) resumes execution until the |
| selected stack frame returns naturally. |
| </p> |
| <p><small>GDB</small> needs to know how the <var>expression</var> argument should be set for |
| the inferior. The concrete registers assignment depends on the OS ABI and the |
| type being returned by the selected stack frame. For example it is common for |
| OS ABI to return floating point values in FPU registers while integer values in |
| CPU registers. Still some ABIs return even floating point values in CPU |
| registers. Larger integer widths (such as <code>long long int</code>) also have |
| specific placement rules. <small>GDB</small> already knows the OS ABI from its |
| current target so it needs to find out also the type being returned to make the |
| assignment into the right register(s). |
| </p> |
| <p>Normally, the selected stack frame has debug info. <small>GDB</small> will always |
| use the debug info instead of the implicit type of <var>expression</var> when the |
| debug info is available. For example, if you type <kbd>return -1</kbd>, and the |
| function in the current stack frame is declared to return a <code>long long |
| int</code>, <small>GDB</small> transparently converts the implicit <code>int</code> value of -1 |
| into a <code>long long int</code>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">Breakpoint 1, func () at gdb.base/return-nodebug.c:29 |
| 29 return 31; |
| (gdb) return -1 |
| Make func return now? (y or n) y |
| #0 0x004004f6 in main () at gdb.base/return-nodebug.c:43 |
| 43 printf ("result=%lld\n", func ()); |
| (gdb) |
| </pre></div> |
| |
| <p>However, if the selected stack frame does not have a debug info, e.g., if the |
| function was compiled without debug info, <small>GDB</small> has to find out the type |
| to return from user. Specifying a different type by mistake may set the value |
| in different inferior registers than the caller code expects. For example, |
| typing <kbd>return -1</kbd> with its implicit type <code>int</code> would set only a part |
| of a <code>long long int</code> result for a debug info less function (on 32-bit |
| architectures). Therefore the user is required to specify the return type by |
| an appropriate cast explicitly: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">Breakpoint 2, 0x0040050b in func () |
| (gdb) return -1 |
| Return value type not available for selected stack frame. |
| Please use an explicit cast of the value to return. |
| (gdb) return (long long int) -1 |
| Make selected stack frame return now? (y or n) y |
| #0 0x00400526 in main () |
| (gdb) |
| </pre></div> |
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