libctf: can I reuse Bruno's code from appendix B of Uli's DSO howto?

[Bruno Haible]

Hi Nick,

> Bruno wrote a nice little
> automated fix for this general problem: it's included in Uli's DSO howto
> at <https://www.akkadia.org/drepper/dsohowto.pdf>: rather than
> re-engineering it, I thought I'd add a buncha comments and reuse the
> code in libctf.
> 
> But it's probably long enough and surely tricksy enough to be
> copyright-significant in the FSF's eyes

The code I originally sent to Ulrich, and which I hereby put in the public
domain:

       #include <stddef.h>

       #define MSGSTRFIELD(line) MSGSTRFIELD1(line)
       #define MSGSTRFIELD1(line) str##line
       static struct msgstr_t {
       #define _S(s) char MSGSTRFIELD(__LINE__) [sizeof(s)];
       #include "stringtab.h"
       #undef _S
       } msgstr = {
       #define _S(s) s,
       #include "stringtab.h"
       #undef _S
       };
       static int msgidx[] = {
       #define _S(s) offsetof(struct msgstr_t, MSGSTRFIELD(__LINE__)),
       #include "stringtab.h"
       #undef _S
       };

       const char *errstr (int nr) {
         return (char*)&msgstr + msgidx[nr];
       }

       And stringtab.h contains:

         _S("message for err1")
         _S("message for err2")
         _S("message for err3")

The changes Ulrich did were:
  - Add a parameter 'n' to the _S macro, to avoid mismatches between index
    and message.
  - Turn msgstr_t into a union with an unnamed struct as first element,
    to get rid of the cast (char*)&msgstr that may trigger a GCC warning.

Bruno