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226 lines
9.6 KiB
HTML
226 lines
9.6 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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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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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Calling (Debugging with GDB)</title>
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<meta name="description" content="Calling (Debugging with GDB)">
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<meta name="keywords" content="Calling (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Altering.html#Altering" rel="up" title="Altering">
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<link href="Patching.html#Patching" rel="next" title="Patching">
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<link href="Returning.html#Returning" rel="prev" title="Returning">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Calling"></a>
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<div class="header">
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<p>
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Next: <a href="Patching.html#Patching" accesskey="n" rel="next">Patching</a>, Previous: <a href="Returning.html#Returning" accesskey="p" rel="prev">Returning</a>, Up: <a href="Altering.html#Altering" accesskey="u" rel="up">Altering</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Calling-Program-Functions"></a>
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<h3 class="section">17.5 Calling Program Functions</h3>
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<dl compact="compact">
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<dd><a name="index-calling-functions"></a>
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<a name="index-inferior-functions_002c-calling"></a>
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</dd>
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<dt><code>print <var>expr</var></code></dt>
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<dd><p>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>
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<a name="index-call"></a>
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</dd>
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<dt><code>call <var>expr</var></code></dt>
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<dd><p>Evaluate the expression <var>expr</var> without displaying <code>void</code>
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returned values.
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</p>
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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. <em>a void function</em>), but without cluttering the output
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with <code>void</code> returned values that <small>GDB</small> 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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</p></dd>
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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>
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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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</p>
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<dl compact="compact">
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<dt><code>set unwindonsignal</code></dt>
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<dd><a name="index-set-unwindonsignal"></a>
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<a name="index-unwind-stack-in-called-functions"></a>
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<a name="index-call-dummy-stack-unwinding"></a>
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<p>Set unwinding of the stack if a signal is received while in a function
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that <small>GDB</small> called in the program being debugged. If set to on,
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<small>GDB</small> 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), <small>GDB</small> stops in the frame where the signal was
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received.
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</p>
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</dd>
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<dt><code>show unwindonsignal</code></dt>
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<dd><a name="index-show-unwindonsignal"></a>
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<p>Show the current setting of stack unwinding in the functions called by
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<small>GDB</small>.
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</p>
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</dd>
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<dt><code>set unwind-on-terminating-exception</code></dt>
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<dd><a name="index-set-unwind_002don_002dterminating_002dexception"></a>
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<a name="index-unwind-stack-in-called-functions-with-unhandled-exceptions"></a>
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<a name="index-call-dummy-stack-unwinding-on-unhandled-exception_002e"></a>
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<p>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 <small>GDB</small> called in the program being
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debugged. If set to on (the default), <small>GDB</small> 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, <small>GDB</small> the exception is delivered to
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the default C<tt>++</tt> exception handler and the inferior terminated.
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</p>
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</dd>
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<dt><code>show unwind-on-terminating-exception</code></dt>
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<dd><a name="index-show-unwind_002don_002dterminating_002dexception"></a>
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<p>Show the current setting of stack unwinding in the functions called by
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<small>GDB</small>.
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</p>
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</dd>
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</dl>
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<a name="Calling-functions-with-no-debug-info"></a>
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<h4 class="subsection">17.5.1 Calling functions with no debug info</h4>
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<a name="index-no-debug-info-functions"></a>
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<p>Sometimes, a function you wish to call is missing debug information.
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In such case, <small>GDB</small> 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, <small>GDB</small> refuses
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to call the function unless you tell it the type of the function.
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</p>
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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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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) p ((char * (*) (const char *)) getenv) ("PATH")
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</pre></div>
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<p>and given this prototyped C or C++ function with float parameters:
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</p>
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<div class="smallexample">
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<pre class="smallexample">float multiply (float v1, float v2) { return v1 * v2; }
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</pre></div>
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<p>these calls are equivalent:
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</p>
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<div class="smallexample">
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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></div>
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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 <small>GDB</small> 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
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promotion</a>. For example, given this unprototyped C function with
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float parameters, and no debug info:
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</p>
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<div class="smallexample">
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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></div>
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<p>you call it like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> (gdb) p ((float (*) ()) multiply_noproto) (2.0f, 3.0f)
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Patching.html#Patching" accesskey="n" rel="next">Patching</a>, Previous: <a href="Returning.html#Returning" accesskey="p" rel="prev">Returning</a>, Up: <a href="Altering.html#Altering" accesskey="u" rel="up">Altering</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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