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. When using this style, it is possible to use the ‘%V’ format specifier (see %V Format Specifier).

call

Handle the output by calling a function in your program (normally printf). When using this style the supported format specifiers depend entirely on the function being called.

Most of GDB’s format specifiers align with those supported by the printf function, however, GDB’s ‘%V’ format specifier extension is not supported by printf. When using ‘call’ style dprintf, care should be taken to ensure that only format specifiers supported by the output function are used, otherwise the results will be undefined.

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. This style does not support the ‘%V’ format specifier.

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.