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From: ajr@inesc.UUCP (Julio Raposo)
Newsgroups: comp.lang.c
Subject: Re: Portable use of unions
Summary: an explanation
Keywords: unions
Message-ID: <154@inesc.UUCP>
Date: 5 Dec 89 09:46:08 GMT
References: <203@sixhub.UUCP>
Organization: INESC - Inst. Eng. Sistemas e Computadores, LISBOA. PORTUGAL.
Lines: 49

In article <203@sixhub.UUCP>, davidsen@sixhub.UUCP (Wm E. Davidsen Jr) writes:
> 
> ================ start code
> 
>   /* test of code generated for unions */
>   
>   char buf[30];		/* simple char buffer		*/
>   
>   union {
>     int t_sscr;		/* subscript of location	*/
>     char *t_adrs;		/* actual location		*/
>   } demo;
>   
>   main() {
>     demo.t_sscr = 4;
>     demo.t_adrs = &buf[demo.t_sscr];
>   }
> 
> ================ end code
> 

	This code is non-portable because in the C language the evaluation
	order is undefined. The compiler may evaluate first demo.t_adrs and
	when it tries to evaluate demo.t_sscr complains that the address is
	already beeing used.

	For that example I would write: (coments erased)

 
 ================ start code
 
   char buf[30];
   
   union {
     int t_sscr;
     char *t_adrs;
   } demo;
   
   main() {
     char *aux;
     demo.t_sscr = 4;
     aux = &buf[demo.t_sscr];
     demo.t_adrs = aux;
   }
 
 ================ end code
 

			Antonio Julio Raposo (ajr@inesc, LISBOA, PORTUGAL)