Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!mcsun!unido!infbs!ramz!ruediger
From: ruediger@ramz.UUCP (Ruediger Helsch)
Newsgroups: comp.lang.c
Subject: Re: memcpy versus assignment
Message-ID: <1175@ramz.UUCP>
Date: 3 Jan 90 17:12:52 GMT
References: <1657@uwm.edu> <1989Dec31.005904.1910@utzoo.uucp>
Reply-To: ruediger@ramz.UUCP (Ruediger Helsch)
Organization: TU Braunschweig Mechanikzentrum, Germany
Lines: 32

In article <1989Dec31.005904.1910@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>On the other hand, many such compilers will generate simple rather than
>optimal copy code for the assignment, while the memcpy() may be well
>optimized once you get past the startup overhead.  (In particular, there
>is a naive belief that hardware provisions for fast copy -- string/block
>instructions, "loop mode", etc. -- are always the fastest way to do such
>operations, which is often untrue. 

On the other side, you are not sure wether your computers memcpy() uses
the fast hardware instructions. Here follows the Ultrix (3.0) implementation
of memcpy(): 

/*
 * Copy s2 to s1, always copy n bytes.
 * Return s1
 */
char *
memcpy(s1, s2, n)
register char *s1, *s2;
register int n;
{
	register char *os1 = s1;

	while (--n >= 0)
		*s1++ = *s2++;
	return (os1);
}

VAXen do have fast copy commands, but even copying wordwise would surely 
be faster than byte after byte. 

P.S.: I hope i didn't break any copyrights!