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199 lines
8.8 KiB
HTML
199 lines
8.8 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>Dynamic Printf (Debugging with GDB)</title>
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<meta name="description" content="Dynamic Printf (Debugging with GDB)">
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<meta name="keywords" content="Dynamic Printf (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="Breakpoints.html#Breakpoints" rel="up" title="Breakpoints">
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<link href="Save-Breakpoints.html#Save-Breakpoints" rel="next" title="Save Breakpoints">
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<link href="Break-Commands.html#Break-Commands" rel="prev" title="Break Commands">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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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="Dynamic-Printf"></a>
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<div class="header">
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<p>
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Next: <a href="Save-Breakpoints.html#Save-Breakpoints" accesskey="n" rel="next">Save Breakpoints</a>, Previous: <a href="Break-Commands.html#Break-Commands" accesskey="p" rel="prev">Break Commands</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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="Dynamic-Printf-1"></a>
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<h4 class="subsection">5.1.8 Dynamic Printf</h4>
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<a name="index-dynamic-printf"></a>
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<a name="index-dprintf"></a>
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<p>The dynamic printf command <code>dprintf</code> combines a breakpoint with
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formatted printing of your program’s data to give you the effect of
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inserting <code>printf</code> calls into your program on-the-fly, without
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having to recompile it.
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</p>
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<p>In its most basic form, the output goes to the GDB console. However,
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you can set the variable <code>dprintf-style</code> for alternate handling.
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For instance, you can ask to format the output by calling your
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program’s <code>printf</code> function. This has the advantage that the
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characters go to the program’s output device, so they can recorded in
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redirects to files and so forth.
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</p>
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<p>If you are doing remote debugging with a stub or agent, you can also
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ask to have the printf handled by the remote agent. In addition to
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ensuring that the output goes to the remote program’s device along
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with any other output the program might produce, you can also ask that
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the dprintf remain active even after disconnecting from the remote
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target. Using the stub/agent is also more efficient, as it can do
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everything without needing to communicate with <small>GDB</small>.
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</p>
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<dl compact="compact">
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<dd><a name="index-dprintf-1"></a>
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</dd>
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<dt><code>dprintf <var>location</var>,<var>template</var>,<var>expression</var>[,<var>expression</var>…]</code></dt>
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<dd><p>Whenever execution reaches <var>location</var>, print the values of one or
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more <var>expressions</var> under the control of the string <var>template</var>.
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To print several values, separate them with commas.
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</p>
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</dd>
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<dt><code>set dprintf-style <var>style</var></code></dt>
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<dd><p>Set the dprintf output to be handled in one of several different
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styles enumerated below. A change of style affects all existing
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dynamic printfs immediately. (If you need individual control over the
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print commands, simply define normal breakpoints with
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explicitly-supplied command lists.)
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</p>
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<dl compact="compact">
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<dt><code>gdb</code></dt>
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<dd><a name="index-dprintf_002dstyle-gdb"></a>
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<p>Handle the output using the <small>GDB</small> <code>printf</code> command.
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</p>
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</dd>
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<dt><code>call</code></dt>
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<dd><a name="index-dprintf_002dstyle-call"></a>
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<p>Handle the output by calling a function in your program (normally
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<code>printf</code>).
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</p>
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</dd>
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<dt><code>agent</code></dt>
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<dd><a name="index-dprintf_002dstyle-agent"></a>
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<p>Have the remote debugging agent (such as <code>gdbserver</code>) handle
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the output itself. This style is only available for agents that
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support running commands on the target.
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</p></dd>
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</dl>
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</dd>
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<dt><code>set dprintf-function <var>function</var></code></dt>
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<dd><p>Set the function to call if the dprintf style is <code>call</code>. By
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default its value is <code>printf</code>. You may set it to any expression.
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that <small>GDB</small> can evaluate to a function, as per the <code>call</code>
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command.
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</p>
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</dd>
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<dt><code>set dprintf-channel <var>channel</var></code></dt>
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<dd><p>Set a “channel” for dprintf. If set to a non-empty value,
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<small>GDB</small> will evaluate it as an expression and pass the result as
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a first argument to the <code>dprintf-function</code>, in the manner of
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<code>fprintf</code> and similar functions. Otherwise, the dprintf format
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string will be the first argument, in the manner of <code>printf</code>.
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</p>
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<p>As an example, if you wanted <code>dprintf</code> output to go to a logfile
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that is a standard I/O stream assigned to the variable <code>mylog</code>,
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you could do the following:
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</p>
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<div class="example">
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<pre class="example">(gdb) set dprintf-style call
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(gdb) set dprintf-function fprintf
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(gdb) set dprintf-channel mylog
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(gdb) dprintf 25,"at line 25, glob=%d\n",glob
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Dprintf 1 at 0x123456: file main.c, line 25.
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(gdb) info break
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1 dprintf keep y 0x00123456 in main at main.c:25
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call (void) fprintf (mylog,"at line 25, glob=%d\n",glob)
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continue
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(gdb)
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</pre></div>
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<p>Note that the <code>info break</code> displays the dynamic printf commands
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as normal breakpoint commands; you can thus easily see the effect of
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the variable settings.
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</p>
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</dd>
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<dt><code>set disconnected-dprintf on</code></dt>
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<dt><code>set disconnected-dprintf off</code></dt>
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<dd><a name="index-set-disconnected_002ddprintf"></a>
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<p>Choose whether <code>dprintf</code> commands should continue to run if
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<small>GDB</small> has disconnected from the target. This only applies
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if the <code>dprintf-style</code> is <code>agent</code>.
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</p>
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</dd>
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<dt><code>show disconnected-dprintf off</code></dt>
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<dd><a name="index-show-disconnected_002ddprintf"></a>
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<p>Show the current choice for disconnected <code>dprintf</code>.
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</p>
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</dd>
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</dl>
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<p><small>GDB</small> does not check the validity of function and channel,
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relying on you to supply values that are meaningful for the contexts
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in which they are being used. For instance, the function and channel
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may be the values of local variables, but if that is the case, then
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all enabled dynamic prints must be at locations within the scope of
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those locals. If evaluation fails, <small>GDB</small> will report an error.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Save-Breakpoints.html#Save-Breakpoints" accesskey="n" rel="next">Save Breakpoints</a>, Previous: <a href="Break-Commands.html#Break-Commands" accesskey="p" rel="prev">Break Commands</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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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