Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!bbn!cc5.bbn.com!keesan
From: keesan@cc5.bbn.com.UUCP
Newsgroups: comp.lang.c
Subject: Re: Copying a constant number of bytes
Message-ID: <1790@cc5.bbn.com.BBN.COM>
Date: Mon, 15-Jun-87 17:21:10 EDT
Article-I.D.: cc5.1790
Posted: Mon Jun 15 17:21:10 1987
Date-Received: Thu, 18-Jun-87 01:44:34 EDT
References: <900@bloom-beacon.MIT.EDU>
Reply-To: keesan@bbn.com (Morris M. Keesan)
Organization: Bolt Beranek and Newman, Cambridge, MA
Lines: 27
Summary: structure assignment may be LESS efficient

In article <900@bloom-beacon.MIT.EDU> newman@athena.mit.edu (Ron Newman) writes:
>I want to copy a constant number of bytes NBYTES from address "a"
>to address "b", where "a" and "b" are pointers of unspecified type.
. . .
>Method 1)
>      bcopy ((char *)a, (char *)b, NBYTES);
>
>Method 2)
>      struct nbytes {char s[NBYTES];};
>      *(struct nbytes *)b = *(struct nbytes *)a;

I will repeat the advice of others, which is to use bcopy.  Portability and
questionable casts aside, the structure assignment may be less efficient.  The
compiler I'm most familiar with does structure assignments by generating a
function call to the function "strasg".  What does strasg do?  It calls bcopy,
but because bcopy doesn't return a value and assignments do [the value of an
assignment expression is the value of the lhs after the assignment], it returns
its first argument.  So instead of getting a simple call to bcopy (which is
in MICROCODE on this machine), you end up taking the address of the two
structures and getting an extra subroutine call, not to mention the overhead
of pushing a return value on the stack and then dereferencing it to get the
structure-valued return.  Even if the optimizer undoes some of this, you've
still got the overhead of the extra function call.
-- 
Morris M. Keesan
keesan@bbn.com
{harvard,decvax,ihnp4,etc.}!bbn!keesan