Debugging with GDB: Dynamic Printf

Next: Save Breakpoints, Previous: Break Commands, Up: Breakpoints [Contents][Index]

#### 5.1.8 Dynamic Printf

The dynamic printf command `dprintf` combines a breakpoint with formatted printing of your program’s data to give you the effect of inserting `printf` calls into your program on-the-fly, without having to recompile it.

In its most basic form, the output goes to the GDB console. However, you can set the variable `dprintf-style` for alternate handling. For instance, you can ask to format the output by calling your program’s `printf` 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.

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 GDB.

`dprintf locspec,template,expression[,expression…]`

  Whenever execution reaches a code location that results from resolving locspec, print the values of one or more expressions under the control of the string template. To print several values, separate them with commas.

`set dprintf-style style`

  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.)

  `gdb`

    Handle the output using the GDB `printf` command.

  `call`

    Handle the output by calling a function in your program (normally `printf`).

  `agent`

    Have the remote debugging agent (such as `gdbserver`) handle the output itself. This style is only available for agents that support running commands on the target.

`set dprintf-function function`

  Set the function to call if the dprintf style is `call`. By default its value is `printf`. You may set it to any expression. that GDB can evaluate to a function, as per the `call` command.

`set dprintf-channel channel`

  Set a “channel” for dprintf. If set to a non-empty value, GDB will evaluate it as an expression and pass the result as a first argument to the `dprintf-function`, in the manner of `fprintf` and similar functions. Otherwise, the dprintf format string will be the first argument, in the manner of `printf`.

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

      (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)

  Note that the `info break` displays the dynamic printf commands as normal breakpoint commands; you can thus easily see the effect of the variable settings.

`set disconnected-dprintf on`
`set disconnected-dprintf off`

  Choose whether `dprintf` commands should continue to run if GDB has disconnected from the target. This only applies if the `dprintf-style` is `agent`.

`show disconnected-dprintf off`

  Show the current choice for disconnected `dprintf`.

GDB 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, GDB will report an error.

Next: Save Breakpoints, Previous: Break Commands, Up: Breakpoints [Contents][Index]