Sometimes you need to interface with a C library. Sometimes that C library gives you pointers to structs containing pointers to arrays of pointers to…
# In C you do this:
p->second_field[3] = 9
# In Julia you do this:
unsafe_store!(unsafe_load(unsafe_load(p) + fieldoffset(eltype(p), 2)) + 3*sizeof(fieldtype(eltype(p), 2)), 9)
# Now you can do this:
q = UnsafePtr(p)
q.second_field[][4] = 9
This package exports one type, UnsafePtr{T}, which behaves similarly to a regular Ptr{T} but has some convenient (but unsafe) pointer access semantics.
Also Array(q, dims...), view(q, start:stop), String(q) are various ways to reinterpret array data.
Under the hood we are calling the unsafe_* family of functions. This library is a way of declaring once that you know what you are doing, and thereafter getting nice syntax for unsafe operations.