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| <title>Debugging with GDB: Dynamic Printf</title> |
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| <a name="Dynamic-Printf-1"></a> |
| <h4 class="subsection">5.1.8 Dynamic Printf</h4> |
| |
| <a name="index-dynamic-printf"></a> |
| <a name="index-dprintf"></a> |
| <p>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> |
| <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> |
| <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 <small>GDB</small>. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-dprintf-1"></a> |
| </dd> |
| <dt><code>dprintf <var>location</var>,<var>template</var>,<var>expression</var>[,<var>expression</var>…]</code></dt> |
| <dd><p>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. |
| </p> |
| </dd> |
| <dt><code>set dprintf-style <var>style</var></code></dt> |
| <dd><p>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.) |
| </p> |
| </dd> |
| <dt><code>gdb</code></dt> |
| <dd><a name="index-dprintf_002dstyle-gdb"></a> |
| <p>Handle the output using the <small>GDB</small> <code>printf</code> command. |
| </p> |
| </dd> |
| <dt><code>call</code></dt> |
| <dd><a name="index-dprintf_002dstyle-call"></a> |
| <p>Handle the output by calling a function in your program (normally |
| <code>printf</code>). |
| </p> |
| </dd> |
| <dt><code>agent</code></dt> |
| <dd><a name="index-dprintf_002dstyle-agent"></a> |
| <p>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. |
| </p> |
| </dd> |
| <dt><code>set dprintf-function <var>function</var></code></dt> |
| <dd><p>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 <small>GDB</small> can evaluate to a function, as per the <code>call</code> |
| command. |
| </p> |
| </dd> |
| <dt><code>set dprintf-channel <var>channel</var></code></dt> |
| <dd><p>Set a “channel” for dprintf. If set to a non-empty value, |
| <small>GDB</small> 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> |
| <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: |
| </p> |
| <div class="example"> |
| <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></div> |
| |
| <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. |
| </p> |
| </dd> |
| <dt><code>set disconnected-dprintf on</code></dt> |
| <dt><code>set disconnected-dprintf off</code></dt> |
| <dd><a name="index-set-disconnected_002ddprintf"></a> |
| <p>Choose whether <code>dprintf</code> commands should continue to run if |
| <small>GDB</small> has disconnected from the target. This only applies |
| if the <code>dprintf-style</code> is <code>agent</code>. |
| </p> |
| </dd> |
| <dt><code>show disconnected-dprintf off</code></dt> |
| <dd><a name="index-show-disconnected_002ddprintf"></a> |
| <p>Show the current choice for disconnected <code>dprintf</code>. |
| </p> |
| </dd> |
| </dl> |
| |
| <p><small>GDB</small> 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, <small>GDB</small> will report an error. |
| </p> |
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