Print function macro definition in gdb

Question

Here is the source file fragment:

#define TEST 34
#define PRINT_CONCAT(a, b) \
    printf("%d\n", a##b)

Compiling with GCC and linking this source file into a binary with flags -ggdb3 -O3 and running the app with gdb shows up the following behavior:

(gdb) p TEST                                                                                                                                                                                                       
$3 = 34                                                                                                                                                                                                            
(gdb) p PRINT_CONCAT                                                                                                                                                                                               
No symbol "PRINT_CONCAT" in current context.

Is there a way to make gdb expand function macros in any way?

To see the effect of the macro, run just the preprocessor and examine the locations where the macro has been used... — user17592432
– user17592432, Commented Jul 31, 2022 at 23:51
@Fe2O3 It might be an approach, but when a project is huge enough it might not be a practical way... — Some Name
– Some Name, Commented Jul 31, 2022 at 23:52
What do you hope p PRINT_CONCAT would actually print, given that PRINT_CONCAT is a function-like macro? — John Bollinger
– John Bollinger, Commented Aug 1, 2022 at 0:03
Macro expansion is something that happens at compile time, so a debugging session is a poor context in which to explore it. Gdb may nevertheless be able to help you out with that a bit, but what you really ought to do is run the preprocessor (only) on some example code, and examine the result. Since you are using gcc, you can use the command gcc -E to do the preprocessing. — John Bollinger
– John Bollinger, Commented Aug 1, 2022 at 0:16
@JohnBollinger I agree with that, but the output of gcc -E might be very noisy and it's usually required to extract compile_command fragment for the specific file to run. — Some Name
– Some Name, Commented Aug 1, 2022 at 0:27

Some Name · Accepted Answer · 2022-08-01 00:07:57Z

3

It turned to be as easy as macro expand

(gdb) macro expand PRINT_CONCAT(2, 4)                                                                                                                                                                              
expands to: printf("%d\n", 24)

-ggdb3 is not even required, -g3 is enough. -g2 does not seem to include the desired information.

answered Aug 1, 2022 at 0:07

Some Name

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2 Comments

Bill Morgan Over a year ago

Really nice job on figuring out how to do this given other commenters telling you to use the preprocessor because they didn't know the answer.

chtz Over a year ago

You may want to mention info macro PRINT_CONCAT as well (the title of your question asks for the macro definition, while the body asks for the expansion).

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