🚀 go-pugleaf

In article <693@jenny.cl.cam.ac.uk> am@cl.cam.ac.uk (Alan Mycroft) writes:
>   #define has_nullbyte_(x) ((x - 0x01010101) & ~x & 0x80808080)
>Then if e is an expression without side effects (e.g. variable)
>   has_nullbyte_(e)
>is nonzero iff the value of e has a null byte.

If one of the bytes contains 0x80, then has_nullbyte() will
say 'yes'. This can be circumvented by a more thorough test
after this one to see if there really is a null there.

Jack Jensen's subroutine does this apparently by accident;
there is a 'while( *tc++ = *fc++ );' after the test finds a 'null'
so the only effect of having a 0x80 byte will be to revert to
the normal strcpy for the rest of the string.

Someone posted a similar but usually better test on comp.arch ( I think )
a little while ago:

#define has_nullbyte(e)  ((e+0x7efefeff)^e)&0x81010100) != 0x81010100

This one is only 'fooled' by an 0x80 in the most significant byte.
which makes the following test much simpler ( a sign test ).
In either case ( especially in this one with two identical
constants ) it helps a lot if the constants are in registers while
the loop is running. When using C, you can do this explicitly:

register k1=0x7efefeff, k2=0x81010100;

-- 
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Greg Smith     University of Toronto      UUCP: ..utzoo!utcsri!greg
Have vAX, will hack...