Category : C Source Code
Archive   : C_STRING.ZIP
Filename : BCMP.C

/*f File : bcmp.c
Author : Richard A. O'Keefe.
Updated: 23 April 1984
Defines: bcmp()

bcmp(s1, s2, len) returns 0 if the "len" bytes starting at "s1" are
identical to the "len" bytes starting at "s2", non-zero if they are
different. The 4.2bsd manual page doesn't say what non-zero value
is returned, though the BUGS note says that it takes its parameters
backwards from strcmp. This suggests that it is something like
for (; --len >= 0; s1++, s2++)
if (*s1 != *s2) return *s2-*s1;
return 0;
There, I've told you how to do it. As the manual page doesn't come
out and *say* that this is the result, I tried to figure out what a
useful result might be. (I'd forgotten than strncmp stops when it
hits a NUL, which the above does not do.) What I came up with was:
the result is the number of bytes in the differing tails. That is,
after you've skipped the equal parts, how many characters are left?
To put it another way, N-bcmp(s1,s2,N) is the number of equal bytes
(the size of the common prefix). After deciding on this definition
I discovered that the CMPC3itrction does exactly what I wanted.
The code assumes that N is non-negative.

Note: the "b" routines are there to exploit certain VAX order codes,
but the CMPC3 instruction will only test 65535 characters. The asm
code is presented for your interest and amusement.
*/

#include "strings.h"

#if VaxAsm

int bcmp(s1, s2, len)
char *s1, *s2;
int len;
{
asm("cmpc3 12(ap),*4(ap),*8(ap)");
}

#else ~VaxAsm

int bcmp(s1, s2, len)
register char *s1, *s2;
register int len;
{
while (--len >= 0 && *s1++ == *s2++) ;
return len+1;
}

#endif VaxAsm