Is there an idiom in Julia for taking a chunk of memory and reinterpreting it as a given struct?
For example, suppose I have a parametric type:
struct Foo{T}
a::T
b::Int
end
foo = Foo(1.0, 2)
How can I "dump" this object into a memory chunk and then later reinterpret this memory chunk as a Foo{T}
?
Do you mean like this?
julia> buf = reinterpret(reshape, Float64, [foo])
2×1 reinterpret(reshape, Float64, ::Vector{Foo{Float64}}) with eltype Float64:
1.0
1.0e-323
julia> reinterpret(Foo{Float64}, buf)
1×1 reinterpret(Foo{Float64}, reinterpret(reshape, Float64, ::Vector{Foo{Float64}})):
Foo{Float64}(1.0, 2)
Probably @Chad Scherrer :smile: I wonder why we need the reshape and Float64 in the first reinterpret call, will read the docstring...
reshape
is nice so that you have 1 column per foo
, with rows corresponding to the data of foo
.
To be honest I find it a bit strange that reshape is an argument to the function. Maybe it would be more intuitive to introduce a keyword reshape=true or ascolumns=true in future releases? Can reinterpret accept other functions that are not reshape?
Not sure. Yeah, the reshape
argument is weird to me too
If you don't like the reshape you don't have to use it
You can just reinterpret it as bools or UInt8s or whatever you like
julia> buf = reinterpret(UInt8, [Foo(1.0, 2), Foo(3.0, 4)])
32-element reinterpret(UInt8, ::Vector{Foo{Float64}}):
0x00
0x00
0x00
0x00
0x00
0x00
0xf0
0x3f
0x02
0x00
0x00
0x00
0x00
⋮
0x00
0x00
0x00
0x08
0x40
0x04
0x00
0x00
0x00
0x00
0x00
0x00
0x00
julia> buf[1] = 0x01
0x01
julia> reinterpret(Foo{Float64}, buf)
2-element Vector{Foo{Float64}}:
Foo{Float64}(1.0000000000000002, 2)
Foo{Float64}(3.0, 4)
Another option is to load a pointer of the given type from the memory. This is more efficient than creating a reinterpreted buffer, but it's not exactly idiomatic.
Something like:
julia> struct Foo
x::UInt32
y::Char
z::Int64
end
julia> x = (55, 1.0)
(55, 1.0)
julia> unsafe_load(Ptr{Foo}(pointer_from_objref(Ref(x))))
Foo(0x00000037, '\0', 4607182418800017408)
If you use Ref
+ unsafe_load
, the allocation will optimize away and it will be a no-op.
Of course you need to be a bit careful with this:
GC.@preserve
judiciously to prevent the GC from messing up your pointersLast updated: Nov 06 2024 at 04:40 UTC