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From: bzs@bu-cs.UUCP
Newsgroups: net.lang.c
Subject: SWAP macro (utter insanity+interesting question)
Message-ID: <844@bu-cs.UUCP>
Date: Wed, 25-Jun-86 01:47:13 EDT
Article-I.D.: bu-cs.844
Posted: Wed Jun 25 01:47:13 1986
Date-Received: Sat, 28-Jun-86 07:34:24 EDT
Organization: Boston Univ Comp. Sci.
Lines: 83


The attached bit of total brain-damage seems to do the job in YAW (yet
another way.) I don't actually proffer it as a solution that you might
use, no way, don't bother...BUT

Interesting question:

What is the semantics of:

		sizeof(*ip++)

?? Try it before you guess, I was shocked (late entry for the
obfuscated C contest??) The semantics I found are essential for
my solution.

P.S. It compiled and ran correctly on (code follows):

	Hardware	OS		Software	Notes
	--------	---		--------	---------------
	SUN3/180	UNIX4.2 R3.0	PCC
	DEC2060		TOPS-20 5.4	MIT/PCC		provided bcopy()
	ATT/3B5		UNIX SYSVR2	C		used memcpy()
	IBM3090/200	VPS/VM		PCC		provided bcopy()
	Encore/MultiMax	UNIX 4.2	PCC
	VAX/750		UNIX 4.2	PCC
	VAX/780		VMS4.3		VMS/C		provided bcopy()
	Celerity/1200	UNIX 4.2	PCC
	DG MV/10000	AOS/VS 6.03	MVUX 2.01/C	used memcpy()

I also tried it on the SUN, DEC2060, IBM3090, MV10000 and VAX750 with
double's instead of ints and it compiled and worked fine, I don't have
the energy right now to go through all the others, requests accepted.

It also passes lint.

YOW, am I portable yet?

	-Barry Shein, Boston University
____________
#include <stdio.h>

#define swap(x,y) \
     { \
       char t1[sizeof(x)], t2[sizeof(t1)]; \
       char *p, *q; \
 \
       p = (char *) &(x); \
       q = (char *) &(y); \
       bcopy(p,t1,sizeof(t1)); \
       bcopy(q,t2,sizeof(t1)); \
       bcopy(t2,p,sizeof(t1)); \
       bcopy(t1,q,sizeof(t1)); \
     }
	 
main(argc,argv) int argc; char **argv;
{
  int ai[2], *ip = ai;
  int bi[2], *ip2 = bi;

  ai[0] = 1;
  ai[1] = 2;
  bi[0] = 3;
  bi[1] = 4;
  swap(*ip++,*ip2++);
  printf("ai[0] = %d *ip = %d bi[0] = %d *ip2 = %d\n",
	 ai[0], *ip, bi[0], *ip2);
  exit(0);
}

Should print out something like:

ai[0] = 3 *ip = 2 bi[0] = 1 *ip2 = 4

If I said I used memcpy() I added:

#define bcopy(x,y,z) memcpy(y,x,z)

If I said I provided bcopy() I added:

bcopy(from,to,n) char *from, *to; int n;
{
	while(n--) *to++ = *from++;
}