Question

I need to be override certain macro definition by my header file. And I am not allowed to change source code. And I have to use gcc, but if anyone is aware of something similar on any other compiler then also it will help.

Here is what I exactly need:

Lets say I have code base with lot of .c files. These .c files include .h files. After all the .h files have been included for each file I want the compiler to behave as if I have another extra.h file which I want to specify when invoking the compiler. What I do in that .h file is #undef some macro and re-define the macro the way I want them to be.

Note: I am aware of --preinclude option in gcc, but using --preinclude over-rides my extra.h by the .h of the original source code. What I need is some kind of post include option.

Answer 1

Unless you uniformly have one specific header that is always included last in the source files, this is going to be tricky.

I think the way I’d approach it, if I had to, would be:

• Create a new directory, call it headers.
• Put in there suitable dummy headers with the same name as the regular headers, which would contain #include "extra.h" at the end (or possibly #include <extra.h>, but I would try to avoid that).
• The dummy headers would also include the original files by some mechanism, possibly even using #include "/usr/include/header.h" but preferably some other technique – such as #include "include/header.".
• The extra.h header would always redefine all its macros – it would not have the normal #ifndef EXTRA_H_INCLUDED / #define EXTRA_H_INCLUDED / #endif multiple inclusion guards, so that each time it is included, it would redefine the relevant macros.
• Consequently, extra.h cannot define any types. (Or, more precisely, if it does, those must be protected against multiple definition by multiple include guards; the key point is that the macros must be defined each time the file is included – a bit like <assert.h>.)
• Each redefined macro would be explicitly protected by #undef REDEFINED_MACRO and then #define REDEFINED_MACRO ....
• There is no point in testing whether the macro is defined before undefining it.
• The build process would be modified to look in the headers directory before looking anywhere else. The compiler option would be -I./headers or something similar, depending on exactly where you locate the headers directory.
• Depending on how you have decided to locate the normal versions of the headers, you might need another -I option (such as -I/usr if you’ve used #include "include/header.h" notation) to locate the standard headers again.

The upshot is that your private headers get used directly by the compiler, but they include the standard headers and then your extra.h header – thus achieving what you wanted without modifying the C source or the normal headers.

But there is something misguided about the whole attempt…you would be better off not trying this.