|Debugging with GDB: Dynamic Printf|
The dynamic printf command
dprintf combines a breakpoint with
formatted printing of your program’s data to give you the effect of
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
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.
Whenever execution reaches location, 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.)
Handle the output using the GDB
Handle the output by calling a function in your program (normally
Have the remote debugging agent (such as
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
default its value is
printf. You may set it to any expression.
that GDB can evaluate to a function, as per the
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
As an example, if you wanted
dprintf output to go to a logfile
that is a standard I/O stream assigned to the variable
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
dprintf commands should continue to run if
GDB has disconnected from the target. This only applies
show disconnected-dprintf off
Show the current choice for disconnected
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.