error C2036: 'void *' : unknown size

Question

I am working with a C project using visual studio. I tried to compile the following code:

void shuffle(void *arr, size_t n, size_t size)
{
        ....
        memcpy(arr+(i*size), swp,  size);
        ....  
}

I get the following error with Visual studio Compiler:

error C2036: 'void *' : unknown size

The code compile well with GCC. How to solve this error?

However you cannot index pointer of type void. You can try to cast it to char so that each increment is of one byte — terence hill
Because the compiler does not know what size a void* pointer is, although there could be GCC extension that permits it. To do pointer arithmetic, you have to know the size of the data it points to. Note that adding 1 to a pointer to a 32-bit int will add 4 to the pointer itself. Just like with array indexing, where you access the second element with arr[1] not with arr[4]. — Weather Vane

dbush dbush · Accepted Answer · 2016-05-26T12:20:33

You can't perform pointer arithmetic on a void * because void doesn't have a defined size.

Cast the pointer to char * and it will work as expected.

memcpy((char *)arr+(i*size), swp,  size);