Q68265: Signed Is Converted to unsigned in Comparison with unsigned

Article: Q68265
Product(s): See article
Version(s): 6.00 6.00a | 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 1-FEB-1991

The additional adherence to the ANSI standard beginning with C
versions 6.00 and 6.00a and QuickC versions 2.50 and 2.51 will cause a
negative signed integer to be converted to an unsigned integer in a
comparison. This is the correct and intended behavior, although it may
produce results that are unexpected. The sample code below
demonstrates this behavior.

This is new in C 6.00 and is documented in "Microsoft C Advanced
Programming Techniques" on page 422, section B.1.2. The following
program will produce warning C4018 "signed/unsigned mismatch" at
warning level 3 or 4. Casting of the variables will preserve their
relationship in the expression.

Sample Code
-----------

#include <stdio.h>

void main( void)
{
   int a = -1;
   unsigned b = 1;

   if ( b < a )
      printf( "Signed was converted to unsigned\n");
   else
      printf( "Sign was preserved\n");
}

The output from the program above will be:

   Signed was converted to unsigned

Either of the following methods may be used to achieve the expected
behavior:

1. If you are sure that b is less than 32768, change the if statement
   to:

      if ( (signed int) b <  a)

   Note: This will produce the most efficient code.

2. Change the if statement to:

      if ( (long) b < (long) a)

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.