GCC dump preprocessor defines

Question

Is there a way for gcc g   to dump its preprocessor defines from the command line  I mean things like   GNUC      STDC    and so on

User · Answer

While working in a big project which has complex build system and where it is hard to get  or modify  the gcc g   command directly there is another way to see the result of macro expansion  Simply redefine the macro  and you will get output similiar to following   file h  note  this is the location of the previous definition  define MACRO current value

User · Answer

A portable approach that works equally well on Linux or Windows  where there is no  dev null    echo   gcc -dM -E -   For c   you may use  replace c  11 with whatever version you use    echo   gcc -x c   -std c  11 -dM -E -   It works by telling gcc to preprocess stdin  which is produced by echo  and print all preprocessor defines  search for -dletters   If you want to know what defines are added when you include a header file you can use -dD option which is similar to -dM but does not include predefined macros   echo   include  lt stdlib h gt     gcc -x c   -std c  11 -dD -E -   Note  however  that empty input still produces lots of defines with -dD option

User · Answer

I usually do it this way     gcc -dM -E -  lt   dev null   Note that some preprocessor defines are dependent on command line options - you can test these by adding the relevant options to the above command line  For example  to see which SSE3 SSE4 options are enabled by default     gcc -dM -E -  lt   dev null   grep SSE 34   define   SSE3   1  define   SSSE3   1   and then compare this when -msse4 is specified     gcc -dM -E -msse4 -  lt   dev null   grep SSE 34   define   SSE3   1  define   SSE4 1   1  define   SSE4 2   1  define   SSSE3   1   Similarly you can see which options differ between two different sets of command line options  e g  compare preprocessor defines for optimisation levels -O0  none  and -O3  full      gcc -dM -E -O0 -  lt   dev null  gt   tmp O0 txt   gcc -dM -E -O3 -  lt   dev null  gt   tmp O3 txt   sdiff -s  tmp O0 txt  tmp O3 txt   define   NO INLINE   1         lt                                  gt   define   OPTIMIZE   1

User · Answer

The simple approach  gcc -dM -E -  lt   dev null  works fine for gcc but fails for g    Recently I required a test for a C  11 C  14 feature  Recommendations for their corresponding macro names are published at https   isocpp org std standing-documents sd-6-sg10-feature-test-recommendations  But   g   -dM -E -  lt   dev null   fgrep   cpp alias templates   always fails  because it silently invokes the C-drivers  as if invoked by gcc   You can see this by comparing its output against that of gcc or by adding a g  -specific command line option like  -std c  11  which emits the error message cc1  warning  command line option    -std c  11    is valid for C   ObjC   but not for C   Because  the non C    gcc will never support  Templates Aliases   see http   www open-std org jtc1 sc22 wg21 docs papers 2007 n2258 pdf  you must add the -x c   option to force the invocation of the C   compiler  Credits for using the -x c   options instead of an empty dummy file go to yuyichao  see below    g   -dM -E -x c    dev null   fgrep   cpp alias templates   There will be no output because g    revision 4 9 1  defaults to -std gnu  98  does not enable C  11-features by default  To do so  use  g   -dM -E -x c   -std c  11  dev null   fgrep   cpp alias templates   which finally yields   define   cpp alias templates 200704   noting that g   4 9 1 does support  Templates Aliases  when invoked with -std c  11

User · Answer

Yes  use -E -dM options instead of -c  Example  outputs them to stdout     gcc -dM -E -  lt   dev null   For C     g   -dM -E -x c   -  lt   dev null   From the gcc manual      Instead of the normal output  generate   a list of   define  directives for all   the macros defined during the   execution of the preprocessor    including predefined macros  This   gives you a way of finding out what is   predefined in your version of the   preprocessor  Assuming you have no   file foo h  the command  touch foo h  cpp -dM foo h       will show all the predefined macros       If you use -dM without the -E option    -dM is interpreted as a synonym for -fdump-rtl-mach

User · Answer

Late answer - I found the other answers useful - and wanted to add a bit extra     How do I dump preprocessor macros coming from a particular header file   echo   include  lt sys socket h gt     gcc -E -dM -   or  thanks to  mymedia for the suggestion    gcc -E -dM -include sys socket h -  lt   dev null   In particular  I wanted to see what SOMAXCONN was defined to on my system   I know I could just open up the standard header file  but sometimes I have to search around a bit to find the header file locations   Instead I can just use this one-liner     gcc -E -dM -include sys socket h -  lt   dev null   grep SOMAXCONN  define SOMAXCONN 128

[gcc] GCC dump preprocessor defines

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