INFO: Signed Converted to Unsigned in Comparison with Unsigned

• Microsoft C compiler for MS-DOS, versions 6.0, 6.0a, 6.0ax
• Microsoft C compiler for OS/2, versions 6.0, 6.0a
• Microsoft C/C++ compiler for MS-DOS, version 7.0
• Microsoft C/C++ compiler for OS/2, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

SUMMARY

The additional adherence to the ANSI standard, beginning with C version 6.0 and QuickC versions 2.5 and 2.51, will cause a negative signed integer to be converted to an unsigned integer in a comparison with an unsigned integer. This is the correct and intended behavior, although it may produce results that are unexpected. The sample code below demonstrates this behavior.

MORE INFORMATION

This is new in C 6.0 and is documented in the Microsoft C "Advanced Programming Techniques" version 6.0 manual on page 422, Section B.1.2. The following program produces warning C4018 "signed/unsigned mismatch" at warning level 3 or 4. Casting of the variables preserves their relationship in the expression.

Sample Code

   /* Compile options needed: none
   */

   #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");
   }

   Signed was converted to unsigned

• 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.

• Change the if statement to:

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