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| <h3 class="section">17.5 Calling Program Functions</h3> |
| |
| |
| <a name="index-calling-functions-1175"></a> |
| <a name="index-inferior-functions_002c-calling-1176"></a> |
| <dl><dt><code>print </code><var>expr</var><dd>Evaluate the expression <var>expr</var> and display the resulting value. |
| The expression may include calls to functions in the program being |
| debugged. |
| |
| <p><a name="index-call-1177"></a><br><dt><code>call </code><var>expr</var><dd>Evaluate the expression <var>expr</var> without displaying <code>void</code> |
| returned values. |
| |
| <p>You can use this variant of the <code>print</code> command if you want to |
| execute a function from your program that does not return anything |
| (a.k.a. <dfn>a void function</dfn>), but without cluttering the output |
| with <code>void</code> returned values that <span class="sc">gdb</span> will otherwise |
| print. If the result is not void, it is printed and saved in the |
| value history. |
| </dl> |
| |
| <p>It is possible for the function you call via the <code>print</code> or |
| <code>call</code> command to generate a signal (e.g., if there's a bug in |
| the function, or if you passed it incorrect arguments). What happens |
| in that case is controlled by the <code>set unwindonsignal</code> command. |
| |
| <p>Similarly, with a C<tt>++</tt> program it is possible for the function you |
| call via the <code>print</code> or <code>call</code> command to generate an |
| exception that is not handled due to the constraints of the dummy |
| frame. In this case, any exception that is raised in the frame, but has |
| an out-of-frame exception handler will not be found. GDB builds a |
| dummy-frame for the inferior function call, and the unwinder cannot |
| seek for exception handlers outside of this dummy-frame. What happens |
| in that case is controlled by the |
| <code>set unwind-on-terminating-exception</code> command. |
| |
| <dl> |
| <dt><code>set unwindonsignal</code><dd><a name="index-set-unwindonsignal-1178"></a><a name="index-unwind-stack-in-called-functions-1179"></a><a name="index-call-dummy-stack-unwinding-1180"></a>Set unwinding of the stack if a signal is received while in a function |
| that <span class="sc">gdb</span> called in the program being debugged. If set to on, |
| <span class="sc">gdb</span> unwinds the stack it created for the call and restores |
| the context to what it was before the call. If set to off (the |
| default), <span class="sc">gdb</span> stops in the frame where the signal was |
| received. |
| |
| <br><dt><code>show unwindonsignal</code><dd><a name="index-show-unwindonsignal-1181"></a>Show the current setting of stack unwinding in the functions called by |
| <span class="sc">gdb</span>. |
| |
| <br><dt><code>set unwind-on-terminating-exception</code><dd><a name="index-set-unwind_002don_002dterminating_002dexception-1182"></a><a name="index-unwind-stack-in-called-functions-with-unhandled-exceptions-1183"></a><a name="index-call-dummy-stack-unwinding-on-unhandled-exception_002e-1184"></a>Set unwinding of the stack if a C<tt>++</tt> exception is raised, but left |
| unhandled while in a function that <span class="sc">gdb</span> called in the program being |
| debugged. If set to on (the default), <span class="sc">gdb</span> unwinds the stack |
| it created for the call and restores the context to what it was before |
| the call. If set to off, <span class="sc">gdb</span> the exception is delivered to |
| the default C<tt>++</tt> exception handler and the inferior terminated. |
| |
| <br><dt><code>show unwind-on-terminating-exception</code><dd><a name="index-show-unwind_002don_002dterminating_002dexception-1185"></a>Show the current setting of stack unwinding in the functions called by |
| <span class="sc">gdb</span>. |
| |
| </dl> |
| |
| <h4 class="subsection">17.5.1 Calling functions with no debug info</h4> |
| |
| <p><a name="index-no-debug-info-functions-1186"></a>Sometimes, a function you wish to call is missing debug information. |
| In such case, <span class="sc">gdb</span> does not know the type of the function, |
| including the types of the function's parameters. To avoid calling |
| the inferior function incorrectly, which could result in the called |
| function functioning erroneously and even crash, <span class="sc">gdb</span> refuses |
| to call the function unless you tell it the type of the function. |
| |
| <p>For prototyped (i.e. ANSI/ISO style) functions, there are two ways |
| to do that. The simplest is to cast the call to the function's |
| declared return type. For example: |
| |
| <pre class="smallexample"> (gdb) p getenv ("PATH") |
| 'getenv' has unknown return type; cast the call to its declared return type |
| (gdb) p (char *) getenv ("PATH") |
| $1 = 0x7fffffffe7ba "/usr/local/bin:/"... |
| </pre> |
| <p>Casting the return type of a no-debug function is equivalent to |
| casting the function to a pointer to a prototyped function that has a |
| prototype that matches the types of the passed-in arguments, and |
| calling that. I.e., the call above is equivalent to: |
| |
| <pre class="smallexample"> (gdb) p ((char * (*) (const char *)) getenv) ("PATH") |
| </pre> |
| <p class="noindent">and given this prototyped C or C++ function with float parameters: |
| |
| <pre class="smallexample"> float multiply (float v1, float v2) { return v1 * v2; } |
| </pre> |
| <p class="noindent">these calls are equivalent: |
| |
| <pre class="smallexample"> (gdb) p (float) multiply (2.0f, 3.0f) |
| (gdb) p ((float (*) (float, float)) multiply) (2.0f, 3.0f) |
| </pre> |
| <p>If the function you wish to call is declared as unprototyped (i.e. |
| old K&R style), you must use the cast-to-function-pointer syntax, so |
| that <span class="sc">gdb</span> knows that it needs to apply default argument |
| promotions (promote float arguments to double). See <a href="ABI.html#ABI">float promotion</a>. For example, given this unprototyped C function with |
| float parameters, and no debug info: |
| |
| <pre class="smallexample"> float |
| multiply_noproto (v1, v2) |
| float v1, v2; |
| { |
| return v1 * v2; |
| } |
| </pre> |
| <p class="noindent">you call it like this: |
| |
| <pre class="smallexample"> (gdb) p ((float (*) ()) multiply_noproto) (2.0f, 3.0f) |
| </pre> |
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