MINGW compile error: void value not ignored as it ought to be

Question

While building the below code in eclipse I am getting void value not ignored as it ought to be, generally this error is encounter when we are expecting a value from a function returning void. But here I am not using any function, I am using a void pointer. Code is below:

typedef struct MessageName_T 
{
MPM_ParameterName_t  parameterName;
uint32_t minRange;
uint32_t maxRange;
bool_t isInRange;
DataType_t dataType;
bool_t isValueNa;
void* const data;
}MessageName_t ;


MessageName_t messagefr[3] =
/* Parameter,                 Minimum,       Maximum,      isInRange    dataType     isValueNa    data*/
{
  { Parameter,                Minimum,       Maximum,      isInRange,    dataType,     isValueNa,  &stuctureA.data1},
  { Parameter1,               Minimum1,      Maximum1,     isInRange1,   dataType1,   isValueNa1,  &stuctureA.data2},
  { Parameter2,               Minimum2,      Maximum2,     isInRange2,   dataType2,   isValueNa2,  &stuctureA.data3}
}

void rangecheck(int index)
{
   if(messagefr[index].isValueNa == FALSE)
   {
     if(*(messagefr[index].data) >= (messagefr[index].minRange) &&
    *(messagefr[index].data) <= (messagefr[index].maxRange))
 {printf("N");
     messagefr[index].messagefr=TRUE;
 }
 else
 {printf("Y");
     messagefr[index].isInRange =FALSE;
 }
}
else
{
  NOP();
}
}

In line *(messagefr[index].data) >= (messagefr[index].minRange) the error "void value not ignored as it ought to be" is coming. data is void pointer and I am assigning it the value of &stuctureA.data1 . Now while accessing this information of data ie *(messagefr[index].data) I am getting the error, if I use (messagefr[index].data) I am getting address assigned to data.

Answer 1

In your code, the data type of messagefr[index].data is void * , and you cannot dereference it.

You need to cast it to some other complete type before you can derefence it.

Considering you are comparing the value with minRange , something like

 *(uint32_t *)(messagefr[index].data) 

will work.

---

To add some elaboration regarding the why part, first let's see about complete and incomplete types. Quoting

...] an object type may be incomplete (lacking sufficient information to determine the size of objects of that type) or complete (having sufficient information).

Now, coming to chapter §6.5.3.2, unary * operator,

The unary * operator denotes indirection. If the operand points to a function, the result is a function designator; if it points to an object, the result is an lvalue designating the object. If the operand has type ''pointer to type'' , the result has type ''type'' . [...]

However, from chapter §6.2.5/19, we know

The void type comprises an empty set of values; it is an incomplete object type that cannot be completed.

Thus, a pointer to void cannot be dereferenced because, the resulting type ( void ) is incomplete and the size required for the resulting object cannot be determined. so, we need to cast the pointer to a complete type before attempt to dereference.