Radix sort slower than Quick sort?
qsort is not Quicksort. qsort is whatever the implementor of the standard library decided. 
Sorting 50 million identical values is highly unrepresentative. There are qsort implementations that will sort 50 million identical, or 50 million sorted or reverse sorted elements in linear time. 
Radix sort of 50 million numbers with six digits is obviously nonsense. Six digits means you expect only 64 different values and therefore would use counting sort. With less than 26 digits it doesn't make sense. 
Your radix sort does an awful lot of copying, and worse, uncached copying. 
Your radix sort doesn't produce a correctly sorted result if digits is an odd number. 
Your qsort call doesn't produce a correctly sorted result if the array contains very large and very small numbers.