[Bug preprocessor/48865] New: It would be useful to have a way to check the value of a #define at preprocessing time

[Bug preprocessor/48865] New: It would be useful to have a way to check the value of a #define at preprocessing time

[bero at arklinux dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48865

           Summary: It would be useful to have a way to check the value of
                    a #define at preprocessing time
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: preprocessor
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bero@arklinux.org


When debugging compile failures in huge projects that heavily (ab)use
preprocessor macros, it would be useful to have a way to output the value of a
preprocessor token, e.g. something like

#define XYZ 42 // buried deep inside some header
#warning XYZ is ##XYZ

should say
XYZ is 42

or, better yet:
XYZ is 42 (defined in something.h:123, included from somethingelse.h:456,
included from stillsomethingelse.h:789)

[Bug preprocessor/48865] It would be useful to have a way to check the value of a #define at preprocessing time

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48865

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-04 09:29:48 UTC ---
gcc -E -dD doesn't help?

[Bug preprocessor/48865] It would be useful to have a way to check the value of a #define at preprocessing time

[bero at arklinux dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48865

--- Comment #2 from bero at arklinux dot org 2011-05-04 11:16:01 UTC ---
It does - if you know how to read it.

It's easy to tell a student who has never used a compiler outside of an IDE how
to add a #warning -- teaching him how to read the output of -E -dD is quite a
bit harder and might trigger the old "Linux is hard, give me my Vi$ual $tudio"
myth.

Even for someone who does know how to read -E -dD, I think sometimes the "easy"
way could be quicker sometimes (especially locating the relevant place in
pieces of code that are so full of macros that the output doesn't bear much
resemblance to the original).

[Bug preprocessor/48865] It would be useful to have a way to check the value of a #define at preprocessing time

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48865

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-05-04 12:11:04 UTC ---
Teach them to how to pipe the output of gcc -E -dD to grep, it'll do them good
in the long run.

[Bug preprocessor/48865] It would be useful to have a way to check the value of a #define at preprocessing time

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48865

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-05-04 17:10:49 UTC ---
Or, as I've just done, learn how to do it in your favourite editor. For mine
(vim) it's [i to display the value and [^i to jump to the definition, which
seems at least as good as the IDE you mention, and much quicker than changing
the code and compiling it.