Convenient (but unsafe) pointer access
Author cjdoris
1 Star
Updated Last
1 Year Ago
Started In
July 2020


Convenient (but unsafe) pointer accesses.

# 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.

Useful for example for accessing or modifying data exposed by C interfaces through pointers.


pkg> add UnsafePointers



A pointer to the contents of r which may be a Ptr, Ref, Array, String or anything with a pointer method. T specifies the element type.

It has convenient (but unsafe) semantics:

  • p[] dereferences the element, and can be assigned to.
  • p[i] dereferences the ith element, assuming the pointer points to an array.
  • p.name is an UnsafePtr to the name field of p[]. For tuples, p._n refers to the nth field.
  • p+i is an UnsafePtr to the ith next element. (p+i-1)[] and p[i] are equivalent.
  • p-q is the number of elements between p and q, so that p === q+(p-q).
  • Iteration yields p[1], p[2], ... forever.
  • Array(p, dims...) is an array view of contiguous data pointed to by p (equivalent to unsafe_wrap(Array, pointer(p), dims)).
  • p[idxs]/view(p, idxs) is an array/view of the ith element for each i ∈ idxs.
  • String(p, [length]) converts p to a string (equivalent to unsafe_string(pointer(p), length)).

The first four operations have these C equivalents: *p, p[i-1], &(p->name) and p+i.

If the result of dereferencing is pointer-like then an UnsafePtr is returned instead (see doautowrap). Use p[!,i] or unsafe_load(p,i) to get the original value.


It is the caller's responsibility to ensure that the pointer remains valid, e.g. by ensuring that r is not garbage collected.

You will likely crash Julia if you assign to a non-bitstype value.


Here we access and modify the individual fields of a (mutable) reference to a (immutable) named tuple.

r = Ref((a=1, b=(2, 3)))
@show r[]            # (a = 1, b = (2, 3))
p = UnsafePtr(r)
p.a[] = 99
p.b._2[] *= 10
@show r[]            # (a = 99, b = (2, 30))
@show Array(p.a, 3)  # [99, 2, 30]

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