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<h4 class="subsection">5.1.8 Dynamic Printf</h4>

<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
formatted printing of your program's data to give you the effect of
inserting <code>printf</code> calls into your program on-the-fly, without
having to recompile it.

   <p>In its most basic form, the output goes to the GDB console.  However,
you can set the variable <code>dprintf-style</code> for alternate handling. 
For instance, you can ask to format the output by calling your
program's <code>printf</code> function.  This has the advantage that the
characters go to the program's output device, so they can recorded in
redirects to files and so forth.

   <p>If you are doing remote debugging with a stub or agent, you can also
ask to have the printf handled by the remote agent.  In addition to
ensuring that the output goes to the remote program's device along
with any other output the program might produce, you can also ask that
the dprintf remain active even after disconnecting from the remote
target.  Using the stub/agent is also more efficient, as it can do
everything without needing to communicate with <span class="sc">gdb</span>.

     
<a name="index-dprintf-332"></a>
<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
more <var>expressions</var> under the control of the string <var>template</var>. 
To print several values, separate them with commas.

     <br><dt><code>set dprintf-style </code><var>style</var><dd>Set the dprintf output to be handled in one of several different
styles enumerated below.  A change of style affects all existing
dynamic printfs immediately.  (If you need individual control over the
print commands, simply define normal breakpoints with
explicitly-supplied command lists.)

          <dl>
<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.

          <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
<code>printf</code>).

          <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
the output itself.  This style is only available for agents that
support running commands on the target. 
</dl>

     <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
default its value is <code>printf</code>.  You may set it to any expression. 
that <span class="sc">gdb</span> can evaluate to a function, as per the <code>call</code>
command.

     <br><dt><code>set dprintf-channel </code><var>channel</var><dd>Set a &ldquo;channel&rdquo; for dprintf.  If set to a non-empty value,
<span class="sc">gdb</span> will evaluate it as an expression and pass the result as
a first argument to the <code>dprintf-function</code>, in the manner of
<code>fprintf</code> and similar functions.  Otherwise, the dprintf format
string will be the first argument, in the manner of <code>printf</code>.

     <p>As an example, if you wanted <code>dprintf</code> output to go to a logfile
that is a standard I/O stream assigned to the variable <code>mylog</code>,
you could do the following:

     <pre class="example">          (gdb) set dprintf-style call
          (gdb) set dprintf-function fprintf
          (gdb) set dprintf-channel mylog
          (gdb) dprintf 25,"at line 25, glob=%d\n",glob
          Dprintf 1 at 0x123456: file main.c, line 25.
          (gdb) info break
          1       dprintf        keep y   0x00123456 in main at main.c:25
                  call (void) fprintf (mylog,"at line 25, glob=%d\n",glob)
                  continue
          (gdb)
</pre>
     <p>Note that the <code>info break</code> displays the dynamic printf commands
as normal breakpoint commands; you can thus easily see the effect of
the variable settings.

     <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
<span class="sc">gdb</span> has disconnected from the target.  This only applies
if the <code>dprintf-style</code> is <code>agent</code>.

     <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>.

   </dl>

   <p><span class="sc">gdb</span> does not check the validity of function and channel,
relying on you to supply values that are meaningful for the contexts
in which they are being used.  For instance, the function and channel
may be the values of local variables, but if that is the case, then
all enabled dynamic prints must be at locations within the scope of
those locals.  If evaluation fails, <span class="sc">gdb</span> will report an error.

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