bill@twwells.com (T. William Wells) (01/07/90)
From: bill@twwells.com (T. William Wells)
In article <498@longway.TIC.COM> uunet!stealth.acf.nyu.edu!brnstnd (Dan Bernstein) writes:
: From: brnstnd@stealth.acf.nyu.edu
:
: The question is self-explanatory. This is a practical question as well
: as a theoretical one: I'd like a solution that is both conformant and
: portable in the real world. Does (int) (unsigned int) ch do the trick?
: What about (int) (unsigned char)?
Excuse, but this is purely a C question, so I've directed
followups to comp.lang.c.
[ I'm not sure I agree, but I don't see comp.std.unix people clamoring
to answer this question, so let's send it to comp.lang.c to see if there
is more interest there. -mod ]
Anyway, I don't think that either is
guaranteed. One that is, assuming that the character is not in a
register, is: *(unsigned char *)&ch.
(NB: a char might be converted to a number larger than 255 if
characters are larger than eight bits.)
---
Bill { uunet | novavax | ankh | sunvice } !twwells!bill
bill@twwells.com
Volume-Number: Volume 18, Number 15brnstnd@longway.tic.com (Dan Bernstein) (01/10/90)
Apparently (int) (unsigned char) ch is guaranteed to be nonnegative and at most UCHAR_MAX, as long as sizeof(int) > sizeof(char). (If sizeof(int) == sizeof(char), there's obviously no way to fit all the characters into just the positive integers.) This answer is reasonably portable, though pre-ANSI machines must define UCHAR_MAX explicitly. I really did mean this as a UNIX C question, not just a C question; but POSIX doesn't seem to say anything about casts. Some other answers: (int) (unsigned int) ch will convert negative characters into negative integers, so it's wrong if char runs from, say, -128 to 127. ((int) ch) & 0xff works and answers my original question, but it won't handle machines with more than 256 characters. There's no compile-time way to find the right bit pattern---UCHAR_MAX + 1 may not be a power of two. ((int) ch) + UCHAR_MAX + 1) % (UCHAR_MAX + 1) produces the same result as (int) (unsigned char) ch (that is, if sizeof(int) > sizeof(char); otherwise it fails miserably) but is slow on most machines. This is a wonderful opportunity for me to make fun of the mathematical naivete that allows a negative value for x % y in some cases. *(unsigned char *)&ch seems an awfully complicated way to cast ch to an unsigned char. (Technically, it doesn't answer my question, because the result isn't an int.) Yes, Bill, I do have ch in a register, so forget it. ---Dan Volume-Number: Volume 18, Number 16
martin@mwtech.uucp (Martin Weitzel) (01/17/90)
From: martin@mwtech.uucp (Martin Weitzel) [ Please send all further articles on this subject to comp.lang.c. -mod ] In article <7544@cs.utexas.edu> Dan Bernstein <stealth.acf.nyu.edu!brnstnd@longway.tic.com> writes: [some lines deleted] >handle machines with more than 256 characters. There's no compile-time >way to find the right bit pattern---UCHAR_MAX + 1 may not be a power of >two. Look at my recent posting about a portable 'offsetof()'-Makro. The general principle outlined there, is almost allways usable in any situation, where you have a way to solve a problem at run time, but you need the answer at compile time. -- Martin Weitzel, email: martin@mwtech.UUCP, voice: 49-(0)6151-6 56 83 Volume-Number: Volume 18, Number 21