Precompilation problem setting const Ref{Ptr} in a Module

Hi everyone,

I’m working on a PR to extend MPI.jl, which is a Julia package ccalling into a C/Fortran library.

I need to store inside my module a constant Ref{Ptr{Cvoid}}. Since the precompiler actually clears the value of all pointers, I learned that I must do it in the __init__ method of my Module.

Unfortunately, this does not seem to work unless I use an eval call, which breaks incremental compilation, and as such I want to avoid it.

For example, consider the following package:

bash> mkdir -p MyModule/src && echo "
module MyModule
 const a = Ref{Ptr{Cvoid}}()
 const b = Ref{Ptr{Cvoid}}()

 function __init__()
     a[] = Ptr{Cvoid}(1)
     eval(:(b[] = 1))
end" >> MyModule/src/MyModule.jl && cd MyModule

then, if you dev it and using it (which is necessary to trigger a precompilation), you will see that a has been reset:

julia> ] dev .
julia> using MyModule
julia> MyModule.a
Base.RefValue{Ptr{Nothing}}(Ptr{Nothing} @0x0000000000000000)
julia> MyModule.b
Base.RefValue{Ptr{Nothing}}(Ptr{Nothing} @0x0000000000000001)

How can I set this variable without using an eval call?

Just don’t use constants and call them pointers—either use runtime values or call them integers. It’s probably quite bad that we’re rewriting even the source code constant 1 into a null pointer, but that’s usually not observed in practice