Not precompiling on Windows

[BeastyBlacksmith]

trying to install BaseDirs in a temporary environment yields

Failed to precompile BaseDirs [18cc8868-cbac-4acf-b575-c8ff214dc66f] to "~\\.julia\\compiled\\v1.11\\BaseDirs\\jl_11F7.tmp".
ERROR: LoadError: ReadOnlyMemoryError()
Stacktrace:
 [1] knownfolder(id::Symbol)
   @ BaseDirs ~\.julia\packages\BaseDirs\mpcEg\src\nt.jl:169
 [2] knownfolder
   @ ~\.julia\packages\BaseDirs\mpcEg\src\nt.jl:187 [inlined]
 [3] relevantfolders()
   @ BaseDirs ~\.julia\packages\BaseDirs\mpcEg\src\nt.jl:210
 [4] reload()
   @ BaseDirs ~\.julia\packages\BaseDirs\mpcEg\src\nt.jl:241
 [5] top-level scope
   @ ~\.julia\packages\BaseDirs\mpcEg\src\BaseDirs.jl:94
 [6] include
   @ .\Base.jl:562 [inlined]
 [7] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
   @ Base .\loading.jl:2881
 [8] top-level scope
   @ stdin:6
in expression starting at ~\.julia\packages\BaseDirs\mpcEg\src\BaseDirs.jl:1
in expression starting at stdin:

julia> versioninfo()
Julia Version 1.11.6
Commit 9615af0f26 (2025-07-09 12:58 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Windows (i686-w64-mingw32)
  CPU: 4 × Intel(R) Core(TM) i5-7300U CPU @ 2.60GHz
  WORD_SIZE: 32
  LLVM: libLLVM-16.0.6 (ORCJIT, skylake)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

[tecosaur]

Interesting, this is the first time I think I've ever seen ReadOnlyMemoryError().

From the backtrace, if nt.jl:169 is accurate, the error is thrown here:

BaseDirs.jl/src/nt.jl

Lines 169 to 172 in 1a49678

	result =
	ccall((:SHGetKnownFolderPath, "shell32"), stdcall, UInt32,
	(StupidWindowsGUID, Cuint, Ptr{Cvoid}, Ptr{Ptr{UInt16}}),
	guid, 0, C_NULL, ptr)

Looking at the error, I've also found:

https://github.com/JuliaLang/julia/blob/99663f5828517f31f7b50ab17ca1e2d59ae4f2e3/base/precompilation.jl#L527-L528

Which makes me wonder if this might be an problem upstream of BaseDirs.jl.

It seems to me the resolution here will either be not executing that code in precompilation, or handling the potential error somehow. I'm not particularly familiar with Windows, to that end:

• Could you elaborate on what you mean by temporary environment? I assume it's just pkg --temp but I'd like to make sure
• Can I interest you in copying the first ~170 lines of nt.jl into a file to load, and check whether knownfolder(:System) works in temp/non-temp environments outside of precompilation?

[BioTurboNick]

I don't know how much it matters, but should the output type be Ref{Ptr{UInt16}}? Because the container is Julia-managed, whereas the docs say to use Ptr when it is C-managed. That seems to be what libc.jl does.

[BeastyBlacksmith]

Sorry for the delay, I don't have access to that system often

• indeed I activated a tempory environment using pkg> activate --temp
• I can reproduce the error running include("nt.jl"); knownfolder(:System) without any precompilation involved

[tecosaur]

Thanks for the update Simon. Sounds like we might be able to rule out https://github.com/JuliaLang/julia/blob/99663f5828517f31f7b50ab17ca1e2d59ae4f2e3/base/precompilation.jl#L527-L528.

When you say you see the error with include("nt.jl"); knownfolder(:System), is that in both temp and non-temp environments?

[BeastyBlacksmith]

yes

I also think include("nt.jl"); knownfolder(:System) should not care in what environment it is executed.

[tecosaur]

Thanks for checking. I wouldn't have expected it to make a difference myself, but I also wouldn't expect an error to happen here, so I'm interested in verifying my expectations actually hold true.

That you see an error regardless makes me wonder how the Windows CI is passing at all 🤔 (the current CI failures are just about a change in warnings with Julia nightly, the relevant windows code is entirely covered by tests).

[BeastyBlacksmith]

Do note, that this is a 32bit machine (an ~6 year old windows surface)

[tecosaur]

Hmmm, that does seem potentially significant.

I'm not that familiar with "the Windows way" of doing anything, so I've consulted an LLM. It suggests I need to also pass the guid by reference instead of value, and add stdcall to the memory free call. Would you be able to locally replace knownfolder in nt.jl with

function knownfolder(id::Symbol)
    guid = KNOWN_FOLDER_IDS[id]
    ptr = Ref(Ptr{UInt16}())
    herr =
        ccall((:SHGetKnownFolderPath, "shell32"), stdcall, UInt32,
              (Ref{StupidWindowsGUID}, Cuint, Ptr{Cvoid}, Ptr{Ptr{UInt16}}),
              Ref(guid), 0, C_NULL, ptr)
    if iszero(herr)
        folder = unsafe_utf16string(ptr[])
        ccall((:CoTaskMemFree, "ole32.dll"), stdcall, Cvoid, (Ptr{Cvoid},), ptr[])
        folder
    end
end

and try again?

[jaakkor2]

I see the ERROR: LoadError: ReadOnlyMemoryError() with Julia v1.11.7, and I do not see it with v1.11.6.

The above replacement for function knownfolder(id::Symbol) in the previous message fixes it for me.

[tecosaur]

Thanks for checking. Are you also using 32-bit Julia on Windows?

[jaakkor2]

Huh, seem like juliaup installed 32bit version from the release channel! My 1.11.6 is 64-bit.

julia> versioninfo()
Julia Version 1.11.7
Commit f2b3dbda30 (2025-09-08 12:10 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Windows (i686-w64-mingw32)
  CPU: 20 × 13th Gen Intel(R) Core(TM) i7-1370P
  WORD_SIZE: 32
  LLVM: libLLVM-16.0.6 (ORCJIT, goldmont)
Threads: 1 default, 0 interactive, 1 GC (on 20 virtual cores)

[tecosaur]

Curious. We'll you've helped confirm that:

1. This issue is specific to 32-bit Windows builds of Julia
2. The alternative version of knownfolder I shared above works as intended

If you could check that the alternative version also works on 64-bit windows, I think that will give me the confidence I need to put that version in the next release.

@BeastyBlacksmith if you get a chance, I would also be reassured if you're able to test it out 🙂

[jaakkor2]

Whoops, both vanilla and alternative version fails ]test BaseDirs.

It is OS specific: Test Failed at C:\Users\mfi00jru\.julia\dev\BaseDirs\test\runtests.jl:238 trying to compare NTuple of paths, and fails comparing

"C:\\WINDOWS\\Fonts" == "C:\\Windows\\Fonts"

[tecosaur]

Oh that's actually fine, just me not fully accounting for Windows case insensitivity.

Funny that the Windows VMs that are used for GitHub CI produce differently-cased paths than your Windows though.