What is the difference between "short int" and "int" in C?

In theory/by the C standard, they could be of any size as long as 16 bit <= short <= int.

In the real world, this is how the sizes are implemented.

CPU             short   int
8 bit           16      16
16 bit          16      16
32 bit          16      32
64 bit          16      32

Never rely on a datatype being a given size in C. Always check the bounds in limits.h if in doubt.

They may have the same size, but it is guaranteed that int is equal to or bigger than short int.