Re: macro fun [Re: [9fans] plan or side effect]

[Ralph Corderoy <ralph@inputplus.demon.co.uk>]

Hi ozan,

> > > i'll let you figure out what t.c trivially contains to make a soup
> > > with the "optimized" macro, thanks to simple textual substitution.
> > 
> > Is t.c a strictly conforming program?  If not, you got what you asked
> > for.
> 
> according to gcc, it is strictly conforming. it compiles
> perfectly. here is more strict version:
> 
>       bent!oz| gcc -Wall -ansi -pedantic -o t t.c
>       bent!oz| ./t
>       hello boyd!
> 
> now, with -O
> 
>       bent!oz| gcc -O -Wall -ansi -pedantic -o t t.c
>       t.c:4: parse error before `__extension__'
> 
> this is absolute crap. here is t.c.

In this case, it is t.c that's crap.

>       #include <stdio.h>
>       #include <string.h>
>       
>       extern char *strcpy(char *, const char *);

You can't do that in ANSI C and expect not to get bitten.  The compiler
is allowed to implement a standard library function as a macro if they
wish, but must still implement it as a function so its address can be
taken.

If you wish to refer to the function, without knowing if a macro
exists, you are meant to do either

    #ifdef strcpy
    #undef strcpy
    #endif

or

    extern char *(strcpy)(char *, const char *);

See a book like Harbison and Steele's _C, A Reference Manual_ for more
info on writing compliant source.

IBM's AIX C compiler also #defines strcpy as an optimisation.

    $ cc -qlanglvl=ansi -E t.c | grep strcpy
            extern char     *strcpy(char *, const char *);
    extern char *__strcpy(char *,const char *);
            __strcpy(buf,"hello boyd!");

So that doesn't give the effect you intended either.

Cheers,


Ralph.