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160 lines
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<html lang="en">
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<head>
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<title>Calling - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Altering.html#Altering" title="Altering">
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<link rel="prev" href="Returning.html#Returning" title="Returning">
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Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Free Software'' and ``Free Software Needs
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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and with the Back-Cover Texts as in (a) below.
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</head>
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<body>
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<div class="node">
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<a name="Calling"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Patching.html#Patching">Patching</a>,
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Previous: <a rel="previous" accesskey="p" href="Returning.html#Returning">Returning</a>,
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Up: <a rel="up" accesskey="u" href="Altering.html#Altering">Altering</a>
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<hr>
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</div>
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<h3 class="section">17.5 Calling Program Functions</h3>
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<a name="index-calling-functions-1175"></a>
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<a name="index-inferior-functions_002c-calling-1176"></a>
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<dl><dt><code>print </code><var>expr</var><dd>Evaluate the expression <var>expr</var> and display the resulting value.
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The expression may include calls to functions in the program being
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debugged.
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<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>
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returned values.
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<p>You can use this variant of the <code>print</code> command if you want to
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execute a function from your program that does not return anything
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(a.k.a. <dfn>a void function</dfn>), but without cluttering the output
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with <code>void</code> returned values that <span class="sc">gdb</span> will otherwise
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print. If the result is not void, it is printed and saved in the
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value history.
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</dl>
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<p>It is possible for the function you call via the <code>print</code> or
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<code>call</code> command to generate a signal (e.g., if there's a bug in
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the function, or if you passed it incorrect arguments). What happens
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in that case is controlled by the <code>set unwindonsignal</code> command.
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<p>Similarly, with a C<tt>++</tt> program it is possible for the function you
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call via the <code>print</code> or <code>call</code> command to generate an
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exception that is not handled due to the constraints of the dummy
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frame. In this case, any exception that is raised in the frame, but has
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an out-of-frame exception handler will not be found. GDB builds a
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dummy-frame for the inferior function call, and the unwinder cannot
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seek for exception handlers outside of this dummy-frame. What happens
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in that case is controlled by the
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<code>set unwind-on-terminating-exception</code> command.
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<dl>
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<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
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that <span class="sc">gdb</span> called in the program being debugged. If set to on,
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<span class="sc">gdb</span> unwinds the stack it created for the call and restores
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the context to what it was before the call. If set to off (the
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default), <span class="sc">gdb</span> stops in the frame where the signal was
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received.
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<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
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<span class="sc">gdb</span>.
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<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
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unhandled while in a function that <span class="sc">gdb</span> called in the program being
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debugged. If set to on (the default), <span class="sc">gdb</span> unwinds the stack
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it created for the call and restores the context to what it was before
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the call. If set to off, <span class="sc">gdb</span> the exception is delivered to
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the default C<tt>++</tt> exception handler and the inferior terminated.
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<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
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<span class="sc">gdb</span>.
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</dl>
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<h4 class="subsection">17.5.1 Calling functions with no debug info</h4>
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<p><a name="index-no-debug-info-functions-1186"></a>Sometimes, a function you wish to call is missing debug information.
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In such case, <span class="sc">gdb</span> does not know the type of the function,
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including the types of the function's parameters. To avoid calling
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the inferior function incorrectly, which could result in the called
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function functioning erroneously and even crash, <span class="sc">gdb</span> refuses
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to call the function unless you tell it the type of the function.
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<p>For prototyped (i.e. ANSI/ISO style) functions, there are two ways
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to do that. The simplest is to cast the call to the function's
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declared return type. For example:
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<pre class="smallexample"> (gdb) p getenv ("PATH")
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'getenv' has unknown return type; cast the call to its declared return type
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(gdb) p (char *) getenv ("PATH")
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$1 = 0x7fffffffe7ba "/usr/local/bin:/"...
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</pre>
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<p>Casting the return type of a no-debug function is equivalent to
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casting the function to a pointer to a prototyped function that has a
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prototype that matches the types of the passed-in arguments, and
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calling that. I.e., the call above is equivalent to:
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<pre class="smallexample"> (gdb) p ((char * (*) (const char *)) getenv) ("PATH")
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</pre>
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<p class="noindent">and given this prototyped C or C++ function with float parameters:
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<pre class="smallexample"> float multiply (float v1, float v2) { return v1 * v2; }
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</pre>
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<p class="noindent">these calls are equivalent:
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<pre class="smallexample"> (gdb) p (float) multiply (2.0f, 3.0f)
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(gdb) p ((float (*) (float, float)) multiply) (2.0f, 3.0f)
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</pre>
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<p>If the function you wish to call is declared as unprototyped (i.e.
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old K&R style), you must use the cast-to-function-pointer syntax, so
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that <span class="sc">gdb</span> knows that it needs to apply default argument
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promotions (promote float arguments to double). See <a href="ABI.html#ABI">float promotion</a>. For example, given this unprototyped C function with
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float parameters, and no debug info:
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<pre class="smallexample"> float
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multiply_noproto (v1, v2)
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float v1, v2;
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{
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return v1 * v2;
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}
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</pre>
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<p class="noindent">you call it like this:
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<pre class="smallexample"> (gdb) p ((float (*) ()) multiply_noproto) (2.0f, 3.0f)
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</pre>
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</body></html>
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