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141 lines
7.3 KiB
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<title>Dynamic Printf - Debugging with GDB</title>
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<div class="node">
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<a name="Dynamic-Printf"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Save-Breakpoints.html#Save-Breakpoints">Save Breakpoints</a>,
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Previous: <a rel="previous" accesskey="p" href="Break-Commands.html#Break-Commands">Break Commands</a>,
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Up: <a rel="up" accesskey="u" href="Breakpoints.html#Breakpoints">Breakpoints</a>
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<hr>
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</div>
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<h4 class="subsection">5.1.8 Dynamic Printf</h4>
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<p><a name="index-dynamic-printf-330"></a><a name="index-dprintf-331"></a>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>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>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 <span class="sc">gdb</span>.
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<a name="index-dprintf-332"></a>
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<dl><dt><code>dprintf </code><var>location</var><code>,</code><var>template</var><code>,</code><var>expression</var><code>[,</code><var>expression</var><code>...]</code><dd>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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<br><dt><code>set dprintf-style </code><var>style</var><dd>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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<dl>
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<dt><code>gdb</code><dd><a name="index-dprintf_002dstyle-gdb-333"></a>Handle the output using the <span class="sc">gdb</span> <code>printf</code> command.
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<br><dt><code>call</code><dd><a name="index-dprintf_002dstyle-call-334"></a>Handle the output by calling a function in your program (normally
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<code>printf</code>).
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<br><dt><code>agent</code><dd><a name="index-dprintf_002dstyle-agent-335"></a>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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</dl>
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<br><dt><code>set dprintf-function </code><var>function</var><dd>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 <span class="sc">gdb</span> can evaluate to a function, as per the <code>call</code>
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command.
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<br><dt><code>set dprintf-channel </code><var>channel</var><dd>Set a “channel” for dprintf. If set to a non-empty value,
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<span class="sc">gdb</span> 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>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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<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>
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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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<br><dt><code>set disconnected-dprintf on</code><dt><code>set disconnected-dprintf off</code><dd><a name="index-set-disconnected_002ddprintf-336"></a>Choose whether <code>dprintf</code> commands should continue to run if
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<span class="sc">gdb</span> 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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<br><dt><code>show disconnected-dprintf off</code><dd><a name="index-show-disconnected_002ddprintf-337"></a>Show the current choice for disconnected <code>dprintf</code>.
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</dl>
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<p><span class="sc">gdb</span> 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, <span class="sc">gdb</span> will report an error.
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</body></html>
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