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Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!fletcher
From: stealth.acf.nyu.edu!brnstnd@longway.tic.com (Dan Bernstein)
Newsgroups: comp.std.unix,comp.lang.c,comp.std.c
Subject: Re: How to convert a char into an int from 0 through 255?
Message-ID: <7544@cs.utexas.edu>
Date: 10 Jan 90 01:27:10 GMT
Sender: fletcher@cs.utexas.edu
Reply-To: Dan Bernstein <stealth.acf.nyu.edu!brnstnd@longway.tic.com>
Followup-To: comp.std.unix
Organization: IR
Lines: 33
Approved: fletcher@cs.utexas.edu (Guest Moderator, Fletcher Mattox)

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