Passing variable type as function parameter

You could make an enum for all different types possible, and use a switch to make the dereferencing:

typedef enum {
    CHAR,
    INT,
    FLOAT,
    DOUBLE
} TYPE;

void foo(TYPE t, void* x){
    switch(t){
        case CHAR:
            (char*)x;
            break;
        case INT:
            (int*)x;
            break;
         ...
    }
}