C6237

warning C6237: (<zero> && <expression>) is always zero. <expression> is never evaluated and may have side effects

This warning indicates that a constant value of zero was detected on the left side of a logical-and operation that occurs in a test context. The resulting expression always evaluates to false. Therefore, the right side of the logical-AND operation is not evaluated. This is referred to as "short-circuit evaluation."

You should examine the right side of the expression carefully to ensure that any side effects such as assignment, function call, increment, and decrement operations needed for proper functionality are not affected by the short-circuit evaluation.

The expression (0 && n) produces no side effects and is commonly used to selectively choose code paths.

Example

The following code shows various code samples that generate this warning:

#include <stdio.h>  
#define INPUT_TYPE 0  
  
int test();  
  
// side effect: n not incremented  
void f1( int n )  
{  
   if(INPUT_TYPE && n++) //warning: 6237  
   {  
      puts("code path disabled");  
   }  
   else  
   {  
      printf_s("%d - n was not incremented",n);  
   }  
}  
  
// side effect: test() not called  
void f2( )  
{  
   if(INPUT_TYPE && test()) //warning: 6237  
   {  
      puts("code path disabled");  
   }  
   else  
   {  
      puts("test() was not called");  
   }  
}  
  
//side effect: assignment and function call did not occur  
void f3( int n )  
{  
   if(INPUT_TYPE && ( n=test() )) //warning: 6237  
   {  
      puts("code path disabled");  
   }  
   else  
   {  
      printf_s("%d -- n unchanged. test() was not called", n);  
   }  
}  

To correct this warning, use the following code:

#include <stdio.h>  
#define INPUT_TYPE 0  
int test();  
  
void f1( int n )  
{  
if(INPUT_TYPE)  
{  
  if(n++)  
    {  
      puts("code path disabled");  
    }  
}  
else  
  {  
    puts("n was not incremented");  
  }  
}  
  
void f2( )  
{  
   if(INPUT_TYPE)  
   {  
      if( test() )  
      {  
         puts("code path disabled");  
      }  
   }  
   else  
   {  
      puts("test() was not called");  
   }  
}  
  
void f3( int n )  
{  
   if(INPUT_TYPE)  
   {  
     n = test();  
      if( n )  
      {  
         puts("code path disabled");  
      }  
   }  
   else  
   {  
      puts("test() was not called");  
   }  
}  
  

See Also

C Logical Operators