Although I am a C++ programmer, I am interested in issues of the C language.

The ISO C99 standard added a new keyword restrict, and add it in the declaration of many standard functions.
In these functions, there is printf/fprintf/sprintf & co.

printf is now declared as:

int printf(const char * restrict format, ...);

If I understood correctly the standard, the restrict keyword means that during the execution of printf, the format string (at least bytes that are parsed, that is all bytes) must be accessed exclusively via expressions based on the format pointer, thus I think that:

const char*s="%s";
printf(s,s);

Has undefined behaviour (except if there is some special code which checks whether this case happends, what is improbable). :(

But behaviour of that piece of code was well defined (I guess) in a C89 program.

I think that it would be wrong for the ISO standard, to break compatibility with C89 (even if it would not break much code) only for a stupid optimization on a small-sized argument (format strings are usually very short) of a function which is generally not CPU-bound.

I hope that someone could contradict me, and show me where I am wrong.

Please, say that I am wrong.