gdb: Dynamic Printf
5.1.8 Dynamic Printf
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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 LOCATION,TEMPLATE,EXPRESSION[,EXPRESSION...]'
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.)
'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.
Info Catalog
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