Question

Calling all C macro gurus…

Is there any way to write a C macro that will replace something like this:

my_var = 5; 

with this:

setVar(&my_var, 5); 

In other words, can I write a C macro that will override assignments for a specific variable (in the above example, my_var) and instead pass it to a function whose job it is to set that variable? If possible, I’d like to be able to hook into assignments of a specific variable.

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EDIT: After thinking about this some more, I’m not sure it could be done. Even if you can come up with a macro to do it, setVar wouldn’t necessarily know the type of the variable its setting, so what would be the type of its second argument?

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EDIT: The reason I’d like to hook assignments of specific variables is for use in a primitive debugger for some specialized embedded C code. It would be nice to be able to have a ‘watch list’, essentially like you have in an IDE. My first instinct was to try to hook variable assignments with a C macro so you could just drop the macro into your code and have that variable ‘watched’, but then again I’ve never really written a debugger before so maybe I’m going about that all wrong.

Answer 1

Not with the standard preprocessor. It cannot change the parsing of the file, only replace proper names with a piece of code (and ‘=’ isn’t valid in a name).

If you’re feeling adventurous, you can try to replace the executable ‘cpp’ with a small script which pre-processes the source code. But that might wreck havoc with the debugging information (file name and, if you’re replacing one line of code with several, with line number information, too). The script would call ‘sed’`:

sed -e 's/my_var\s*=\s*([^;]+);/MY_VAR(my_var, $1);/' file.c > file_tmp.c 

But your best bet is probably to put this into a script and simply run it on all your sources. This will change the code and you’ll see what is happening in your debugger.