2.2.2.5. Your first GCC hack
OK, now time to hack GCC.
For convenience, let’s use the User mode simulation.
If we run the program userland/c/gcc_hack.c:
./build-userland --static ./run --static --userland userland/c/gcc_hack.c
it produces the normal boring output:
i = 2 j = 0
So how about we swap ++ and -- to make things more fun?
Open the file:
vim submodules/gcc/gcc/c/c-parser.c
and find the function c_parser_postfix_expression_after_primary.
In that function, swap case CPP_PLUS_PLUS and case CPP_MINUS_MINUS:
diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c index 101afb8e35f..89535d1759a 100644 --- a/gcc/c/c-parser.c +++ b/gcc/c/c-parser.c @@ -8529,7 +8529,7 @@ c_parser_postfix_expression_after_primary (c_parser *parser, expr.original_type = DECL_BIT_FIELD_TYPE (field); } break; - case CPP_PLUS_PLUS: + case CPP_MINUS_MINUS: /* Postincrement. */ start = expr.get_start (); finish = c_parser_peek_token (parser)->get_finish (); @@ -8548,7 +8548,7 @@ c_parser_postfix_expression_after_primary (c_parser *parser, expr.original_code = ERROR_MARK; expr.original_type = NULL; break; - case CPP_MINUS_MINUS: + case CPP_PLUS_PLUS: /* Postdecrement. */ start = expr.get_start (); finish = c_parser_peek_token (parser)->get_finish ();
Now rebuild GCC, the program and re-run it:
./build-buildroot -- host-gcc-final-rebuild ./build-userland --static ./run --static --userland userland/c/gcc_hack.c
and the new ouptut is now:
i = 2 j = 0
We need to use the ugly -final thing because GCC has to packages in Buildroot, -initial and -final: https://stackoverflow.com/questions/54992977/how-to-select-an-override-srcdir-source-for-gcc-when-building-buildroot No one is able to example precisely with a minimal example why this is required: