Q32889: Toggling the Sign Bit on a Float or Double

Article: Q32889
Product(s): See article
Version(s): 3.00 4.00 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 20-JUL-1988

Problem:
   I want to toggle the sign bit on a float or double by either
and'ing it with 0x7fff (make it positive) or or'ing it with 0x8000
(make it negative). However, the compiler will not accept the
following syntax:

   fl &= 0x7fff;
   fl |= 0x8000;

Response:
   The bitwise operators only works correctly with integral types; you
need to cast the float to be an integral type.
   The following are two macros that will allow you to toggle the sign
bit on a float or a double (note that the same thing can be
accomplished by multiplying the value by -1, but the macros are much
faster because they do not make any calls to the floating-point
library):

/* Macro to make either a float or a double Negative by setting sign bit */
#define NEG(arg) ((unsigned char *)&arg)[sizeof(arg)-1] |= \
                   (unsigned char)0x8000

/* Macro to make either a float or a double Positive by clearing sign bit */
#define POS(arg) ((unsigned char *)&arg)[sizeof(arg)-1] &= \
                   (unsigned char)0x7fff

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