Is it possible to convert a C string literal to uppercase using the preprocessor (macros)?

It is not possible in portable C99 to have a macro which converts a constant string to all uppercase letters (in particular because the notion of letter is related to character encoding. An UTF8 letter is not the same as an ASCII one).

However, you might consider some other solutions.

• customize your editor to do that. For example, you could write some emacs code which would update each C source file as you require.

• use some preprocessor on your C source code (perhaps a simple C code generator script which would emit a bunch of #define in some #include-d file).

• use GCC extensions to have perhaps

  #define TO_UPPERCASE_COUNTED(Str,Cnt)
  #define TO_UPPERCASE(Str) TO_UPPERCASE_COUNTED(Str,__COUNT__) {( \
     static char buf_##Cnt[sizeof(Str)+4]; \
     char *str_##Cnt = Str; \
     int ix_##Cnt = 0; \
     for (; *str_##Cnt; str_##Cnt++, ix_##Cnt++) \
       if (ix_##Cnt < sizeof(buf_##Cnt)-1) \
           buf_##Cnt[ix_##Cnt] = toupper(*str_##Cnt); \
     buf_##Cnt; )}
  

• customize GCC, perhaps using MELT (a domain specific language to extend GCC), to provide your __builtin_capitalize_constant to do the job (edit: MELT is now an inactive project). Or code in C++ your own GCC plugin doing that (caveat, it will work with only one given GCC version).