在 Julia 中将变量转换为 void 指针

Question

在C/C++中我们可以这样做：

double my_var = 4.32;
void* my_var_ptr = &my_var;

这导致 my_var_ptr 成为指向存储值 4.32 的内存的空指针。

我正尝试在 Julia 中做同样的事情，但我遇到了几个错误。天真地，我试过这个：

my_var=Cdouble(4.32)
my_var_ptr=Ptr{Cvoid}(pointer(my_var))

因错误而失败： ERROR: MethodError: no method matching pointer(::Float64) 失败（我假设）是因为我试图从 Cdouble 对象创建一个 Cvoid 指针。

有什么解决办法吗？

提前致谢！

Answer 1

TL;博士

julia> my_var = Cdouble(4.32)
julia> my_var_ptr = Ptr{Cvoid}(pointer_from_objref(Ref(my_var)))
Ptr{Nothing} @0x00007f2a9148c6c0

pointer 仅适用于 array-like 个元素

julia> methods(pointer)
# 18 methods for generic function "pointer":
[1] pointer(a::Random.UnsafeView) in Random at /usr/share/julia/stdlib/v1.7/Random/src/RNGs.jl:516
[2] pointer(x::SuiteSparse.CHOLMOD.Sparse{Tv}) where Tv in SuiteSparse.CHOLMOD at /usr/share/julia/stdlib/v1.7/SuiteSparse/src/cholmod.jl:359
[3] pointer(x::SuiteSparse.CHOLMOD.Dense{Tv}) where Tv in SuiteSparse.CHOLMOD at /usr/share/julia/stdlib/v1.7/SuiteSparse/src/cholmod.jl:358
[4] pointer(x::SuiteSparse.CHOLMOD.Factor{Tv}) where Tv in SuiteSparse.CHOLMOD at /usr/share/julia/stdlib/v1.7/SuiteSparse/src/cholmod.jl:360
[5] pointer(V::SubArray{T, N, P, I, true} where {T, N, P, I<:Union{Tuple{Vararg{Real}}, Tuple{AbstractUnitRange, Vararg{Any}}}}, i::Int64) in Base at subarray.jl:431
[6] pointer(V::SubArray{T, N, P, I, true} where {T, N, P, I}, i::Int64) in Base at subarray.jl:430
[7] pointer(V::SubArray{<:Any, <:Any, <:Array, <:Tuple{Vararg{Union{Int64, AbstractRange{Int64}}}}}, is::Base.AbstractCartesianIndex{N}) where N in Base at subarray.jl:433
[8] pointer(V::SubArray{<:Any, <:Any, <:Array, <:Tuple{Vararg{Union{Int64, AbstractRange{Int64}}}}}, is::Tuple) in Base at deprecated.jl:212
[9] pointer(A::PermutedDimsArray, i::Integer) in Base.PermutedDimsArrays at permuteddimsarray.jl:61
[10] pointer(x::AbstractArray{T}) where T in Base at abstractarray.jl:1164
[11] pointer(x::AbstractArray{T}, i::Integer) where T in Base at abstractarray.jl:1165
[12] pointer(p::Cstring) in Base at c.jl:186
[13] pointer(buffer::Base64.Buffer) in Base64 at /usr/share/julia/stdlib/v1.7/Base64/src/buffer.jl:20
[14] pointer(x::SubString{String}) in Base at strings/substring.jl:122
[15] pointer(x::SubString{String}, i::Integer) in Base at strings/substring.jl:123
[16] pointer(p::Cwstring) in Base at c.jl:187
[17] pointer(s::String) in Base at strings/string.jl:95
[18] pointer(s::String, i::Integer) in Base at strings/string.jl:96

但是，您应该检查pointer_from_objref，例如：

julia> pointer_from_objref(Ref(1))
Ptr{Nothing} @0x00007f2a914feb60

但是来自 wiki...

Get the memory address of a Julia object as a Ptr. The existence of the resulting Ptr will not protect the object from garbage collection, so you must ensure that the object remains referenced for the whole time that the Ptr will be used.

This function may not be called on immutable objects, since they do not have stable memory addresses.

See also unsafe_pointer_from_objref.

在 Julia 中将变量转换为 void 指针

Cast a variable to void pointer in Julia

c

c++

void-pointers

julia

TL;博士