Undefined operation on a pointer variable

Problem: I received the following warning: operation on ‘vq’ may be undefined [-Wsequence-point]

Solution: The GNU Compiler Collection (GCC) 4.6.3 issues a warning when the sequence order of operations is ambiguous.

In the following example, the vq pointer variable is both set and compared on the same statement. It might not be immediately clear whether the vq pointer variable is updated first and then used in the comparison, or used in the comparison and then updated. Because of the precedence and associativity of operators, == has a higher order of precedence than =, so the compiler does the comparison first and then updates the vq pointer variable. This might or might not be what was intended. The undefined operation [-Wsequence-point] warning is enabled by specifying the -Wall compiler option for C/C++ language.

 if ((vq = *vp->v_hashchain) == vq->v_specnext)
 {
    … 
 }

Rewriting the code eliminates the compiler warning. In the following example, rewriting the code also has the advantage of removing the ambiguity.

 vq = *vp->v_hashchain;                           
 if (vq == vq->v_specnext)                        
 {     
    …    
 }

Note: The -Wsequence-point warning can be valuable; therefore, do not suppress the warning by using the -Wno-sequence-point compiler option unless you are sure that your code is correct.