Can C macros be expanded in gdb when the program was compiled using clang?

Question

I have a macro that fetch's the J-th bit of an integer:

#define TWO_TO_THE(POWER) (1 << POWER)

#define JTH_BIT(BITS, J)  ((TWO_TO_THE((J-1)) & BITS) != 0)

However, I can't use it from gdb by issuing the command print JTH_BIT(i,j), can macros in C even be used while debugging?

This should be reopened, as this is quetion is about clang and the assumed duplicate is about gcc. — alk
– alk, Commented Feb 10, 2014 at 7:12

alk · Accepted Answer · 2019-12-04 08:01:22Z

7

Macros are handled by the pre-processor. The compiler doesn't even know about them.

However, if lucky, GCC's option -g3 would do the job and generate code allowing gdb to expand a macro.

From the gdb documentation (emphasis by me):

-glevel

[...] Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.

edited Dec 4, 2019 at 8:01

answered Feb 9, 2014 at 21:37

alk

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6 Comments

Tim Over a year ago

But this question is about the debugger, not the compiler. The debugger has access to both the binary and the source (typically), so anything is possible.

tlehman Over a year ago

I changed my makefile to compile with -g3, but when I try to print the result of the macro, I get "GDB has no preprocessor macro information for that code", do I need to load the header files in gdb?

alk Over a year ago

@TobiLehman: What does info macro JTH_BIT tell you? And: Did you do a make clean?

tlehman Over a year ago

@alk I did a make clean and then a make, and the -g3 arg used when compiling. Then when in the debugger at a line where the macro is in scope, info macro JTH_BIT returns GDB has no preprocessor macro information for that code.

alk Over a year ago

@TobiLehman: Which gcc are you using though, on which platform? You should perhaps test this with a simple (2 liner) example, if the version of gcc you use does support macro expansion if having used -g3.

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Tim · Accepted Answer · 2014-02-09 21:51:44Z

3

It should work, if you compile your program with the right options. In gcc, you need to say "-g3" when you compile

See here and here.

answered Feb 9, 2014 at 21:51

Tim

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1 Comment

Alexander Oh Over a year ago

I doubt that's helpful when the op is apparently using clang.

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