Question

Trying to compare voids for a sorting algorithm. I have this so far but it kinda defeats the purpose IMO if you cast them to ints. Is there a way to compare voids? My professor ran out of time and how were stuck scowering the web. Any help is appreciated. Thanks

int fcmp(const void *one, const void *two)
{
    if (*(int*)one > *(int*)two) return 1;
    if (*(int*)one < *(int*)two) return -1;
    return 0;
}

Answer 1

This looks to be a standard comparison function used in several of the standard library functions like qsort() in which you call the function with some kind of array of data items along with the comparison function that indicates whether two elements are equal to each or or not and if not what is their collating order.

So what the void pointer points to is kind of up to the programmer. That is the purpose of using the void pointers in the comparison function interface because the function, such as qsort(), which is calling the comparison function just wants to know what order two array elements are to go in. It does not know what the array elements are or how to do the comparison, it just knows the starting address of the array and the size of each element and how many elements are there.

Another function from the standard library is the bsearch() function.

So to use this you might have code like the following:
see qsort() man page.

typedef struct {
   int iValue;
   char  sName[10];
} DataValue;

// compare two elements of the array and indicate which one is higher
// or lower in collating sequence or if they are equal.
int dataComp (void *one, void *two)
{
    return ((DataValue *)one)->iValue - ((DataValue *)two)->iValue;
}

int main (int argc, char *argv[])
{
    DataValue myData[25];
   //.. put some data stuff in the array.
   // call qsort with the array.  specify number of elements and size of each one
   qsort (myData, sizeof(myData)/sizeof(myData[0]), sizeof(myData[0]), dataComp);
}