<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs
Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
and with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom." -->
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
<head>
<title>Non-debug DLL Symbols (Debugging with GDB)</title>
<meta name="description" content="Non-debug DLL Symbols (Debugging with GDB)">
<meta name="keywords" content="Non-debug DLL Symbols (Debugging with GDB)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Cygwin-Native.html#Cygwin-Native" rel="up" title="Cygwin Native">
<link href="Hurd-Native.html#Hurd-Native" rel="next" title="Hurd Native">
<link href="Cygwin-Native.html#Cygwin-Native" rel="prev" title="Cygwin Native">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>
</head>
<body lang="en">
<a name="Non_002ddebug-DLL-Symbols"></a>
<div class="header">
<p>
Up: <a href="Cygwin-Native.html#Cygwin-Native" accesskey="u" rel="up">Cygwin Native</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>
</div>
<hr>
<a name="Support-for-DLLs-without-Debugging-Symbols"></a>
<h4 class="subsubsection">21.1.4.1 Support for DLLs without Debugging Symbols</h4>
<a name="index-DLLs-with-no-debugging-symbols"></a>
<a name="index-Minimal-symbols-and-DLLs"></a>
<p>Very often on windows, some of the DLLs that your program relies on do
not include symbolic debugging information (for example,
<samp>kernel32.dll</samp>). When <small>GDB</small> doesn’t recognize any debugging
symbols in a DLL, it relies on the minimal amount of symbolic
information contained in the DLL’s export table. This section
describes working with such symbols, known internally to <small>GDB</small> as
“minimal symbols”.
</p>
<p>Note that before the debugged program has started execution, no DLLs
will have been loaded. The easiest way around this problem is simply to
start the program — either by setting a breakpoint or letting the
program run once to completion.
</p>
<a name="DLL-Name-Prefixes"></a>
<h4 class="subsubsection">21.1.4.2 DLL Name Prefixes</h4>
<p>In keeping with the naming conventions used by the Microsoft debugging
tools, DLL export symbols are made available with a prefix based on the
DLL name, for instance <code>KERNEL32!CreateFileA</code>. The plain name is
also entered into the symbol table, so <code>CreateFileA</code> is often
sufficient. In some cases there will be name clashes within a program
(particularly if the executable itself includes full debugging symbols)
necessitating the use of the fully qualified name when referring to the
contents of the DLL. Use single-quotes around the name to avoid the
exclamation mark (“!”) being interpreted as a language operator.
</p>
<p>Note that the internal name of the DLL may be all upper-case, even
though the file name of the DLL is lower-case, or vice-versa. Since
symbols within <small>GDB</small> are <em>case-sensitive</em> this may cause
some confusion. If in doubt, try the <code>info functions</code> and
<code>info variables</code> commands or even <code>maint print msymbols</code>
(see <a href="Symbols.html#Symbols">Symbols</a>). Here’s an example:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) info function CreateFileA
All functions matching regular expression "CreateFileA":
Non-debugging symbols:
0x77e885f4 CreateFileA
0x77e885f4 KERNEL32!CreateFileA
</pre></div>
<div class="smallexample">
<pre class="smallexample">(gdb) info function !
All functions matching regular expression "!":
Non-debugging symbols:
0x6100114c cygwin1!__assert
0x61004034 cygwin1!_dll_crt0@0
0x61004240 cygwin1!dll_crt0(per_process *)
[etc...]
</pre></div>
<a name="Working-with-Minimal-Symbols"></a>
<h4 class="subsubsection">21.1.4.3 Working with Minimal Symbols</h4>
<p>Symbols extracted from a DLL’s export table do not contain very much
type information. All that <small>GDB</small> can do is guess whether a symbol
refers to a function or variable depending on the linker section that
contains the symbol. Also note that the actual contents of the memory
contained in a DLL are not available unless the program is running. This
means that you cannot examine the contents of a variable or disassemble
a function within a DLL without a running program.
</p>
<p>Variables are generally treated as pointers and dereferenced
automatically. For this reason, it is often necessary to prefix a
variable name with the address-of operator (“&”) and provide explicit
type information in the command. Here’s an example of the type of
problem:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) print 'cygwin1!__argv'
'cygwin1!__argv' has unknown type; cast it to its declared type
</pre></div>
<div class="smallexample">
<pre class="smallexample">(gdb) x 'cygwin1!__argv'
'cygwin1!__argv' has unknown type; cast it to its declared type
</pre></div>
<p>And two possible solutions:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) print ((char **)'cygwin1!__argv')[0]
$2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
</pre></div>
<div class="smallexample">
<pre class="smallexample">(gdb) x/2x &'cygwin1!__argv'
0x610c0aa8 <cygwin1!__argv>: 0x10021608 0x00000000
(gdb) x/x 0x10021608
0x10021608: 0x0022fd98
(gdb) x/s 0x0022fd98
0x22fd98: "/cygdrive/c/mydirectory/myprogram"
</pre></div>
<p>Setting a break point within a DLL is possible even before the program
starts execution. However, under these circumstances, <small>GDB</small> can’t
examine the initial instructions of the function in order to skip the
function’s frame set-up code. You can work around this by using “*&”
to set the breakpoint at a raw memory address:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) break *&'python22!PyOS_Readline'
Breakpoint 1 at 0x1e04eff0
</pre></div>
<p>The author of these extensions is not entirely convinced that setting a
break point within a shared DLL like <samp>kernel32.dll</samp> is completely
safe.
</p>
<hr>
<div class="header">
<p>
Up: <a href="Cygwin-Native.html#Cygwin-Native" accesskey="u" rel="up">Cygwin Native</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>
</div>
</body>
</html>