ID Number: Q60734
6.00 6.00a 6.00ax | 6.00 6.00a
MS-DOS | OS/2
Summary:
SYMPTOMS
Microsoft C Compiler version 6.0 has a new compiler warning:
C4127: conditional expression is constant
Microsoft C/C++ version 7.0 has the following warning:
C4727: conditional expression is constant
This warning is designed to inform you that the controlling
expression of an if statement or while loop evaluates to a
constant, so the body of the loop is ALWAYS executed or NEVER
executed.
The warning may appear in certain expressions that don't seem to be
constants, but this is because the compiler will generate this
warning if ANY subexpression in a larger conditional expression
evaluates to a constant.
STATUS
The warning is strictly informational and does not necessarily
indicate any problems in the code.
More Information:
In the sample code below, warning C4127 is generated if the code is
compiled at warning level three or four (/W3 or /W4). The
following expression
( hours >= 0 && hours <= 24 )
is NOT a constant because hours could be EITHER in the range 0 (zero)
to 24, or out of that range. However, this expression generates
warning C4127 because the left subexpression
hours >= 0
ALWAYS evaluates to true since hours is unsigned and an unsigned int
is ALWAYS greater than or equal to zero. The compiler generates the
warning to inform you of this situation.
Note that this warning is generated only by the full optimizing
compiler because the quick compiler (/qc) does not check for this
situation.
Sample Code
-----------
#include <stdio.h>
void main(void)
{
unsigned hours;
scanf ( "%ud", &hours );
if ( hours >= 0 && hours <= 24 )
printf("Hours OK\n");
else
printf("Hours BAD\n");
}
Making a simple change, such as replacing the ">=" with a ">",
eliminates the warning because the left expression can now evaluate to
either true or false (for example, false if hours = 0; true
otherwise).
Additional reference words: 6.00 6.00a 6.00ax