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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Abstract Syntax Tree for C programs
Date: Tue, 04 Jun 2019 22:11:00 -0700
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David Brown <david.brown@hesbynett.no> writes:
[...]
> Is there code that is valid in C90 and C99, but has a different meaning? 
>   I can't think of any off-hand, but I would not rule it out.

#include <stdio.h>
int main(void) {
    int is_c90 = -1 //*
    /**/ +1;
    printf("This %s C90\n", is_c90 ? "is" : "is not");
    return 0;
}

In C90, the first "/" is a division operator, which is immediately
followed by a comment that ends just before the "+1", so the
initializer is `-1 / +1`.

In C99 or later, the "//" introduces an end-of-line comment, and
the next line consists of a comment followed by the second half of
the initializer, so the initializer is `-1 + 1`.  (The "+", which
is a unary operator in the C90 version, becomes a binary operator
in the C99/C11 version.)

Or there's the brute force approach:

#include <stdio.h>
int main(void) {
#if defined __STDC__
    #if !defined __STDC_VERSION__
        puts("This is C90");
    #elif __STDC_VERSION__ == 199901L
        puts("This is C99");
    #elif __STDC_VERSION__ == 201112L
        puts("This is C11");
    #else
        puts("__STDC_VERSION__ is not recognized");
    #endif
#else
    puts("__STDC__ is not defined");
#endif
    return 0;
}

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */