Question

I am writing a program that will call a function from an external library, given a list of bitfield flags provided by the end user of my program. I was initially planning on maintaining a lookup table of all the possible flags, but the list of flags varies by platform and I don’t have complete documentation for the library. I had planned to use the c preprocessor to include only the defined flags for the current platform in the table, but this is turning out to be hard to maintain because I have to rely on users reporting missing flags for platforms to which I don’t have access.

I then thought I could allow the users to pass in the flag names as strings. But I can’t find a way to test if a macro is defined, given only the string name of the macro. So if a user wanted to set the flag FLAG_ABC, they’d pass in “FLAG_ABC” as an argument and I’d have to do some magic to check if &FLAG_ABC != NULL. Is this possible?

Answer 1

All #define‘s are lost after preprocessing. If you want to have the information of preprocessor-symbols available at run-time you have to have the names (and values of course) of these macros as strings in your executable OR as a separate file which you then fread at run-time. I could imagine it would be anoying to have two lists of flags, one with the #define-symbols and one with the strings and then manually caring that if you update the one you also update the other without error.

I would go with the separate file you load at run-time but instead of having the updating-trouble just have that file be a header-file you also #include. Specifically what i suggest is: move all your flag-definition #define's into a separate header-file, include it in your sources so you can use it as you are used to, create a mini-parser reading in the file at run-time, i.e. parsing each line #define asdf 0xF1A6 into a string and an int. If this works in practice and you then want to prevent the user from accidentally changing that header-file you could either hash it or binary-encode it.

The alternative with which you can have the #define-symbols as strings actually in your binary would be to generate a .rc-file from that header file (or both the header-file and the .rc from a .csv-file) and then using the resource-compiler to inline it in your binary’s .rsrc-section, you have to write the helper-program (which transforms the file) though and integrate a call to that transformer-program into your make-script or build-system to always have the .rc up-to-date.