Remove unneeded cglobal in cglobal(dlsym(...))

[topolarity]

The cglobal is not needed here.

We'd like to consider deprecating / removing this cglobal(::Ptr) support in Julia 1.14 (JuliaLang/julia#61707).

[mkitti]

ERROR: LoadError: TypeError: in cglobal: first argument not a pointer or valid constant expression, expected Ptr, got a value of type Tuple{Ptr{Nothing}, Symbol}

This does not seem to work...

[mkitti]

Do we need a macro here instead?

[topolarity]

Ah right this is a dynamic usage, sorry about that.

The error is a bit obtuse, but it means that the argument is not known statically. The new form with Ptr{T}(dlsym(...)) should work (and be equivalent to what was there before).

[mkitti]

Kind of. We actually use all of these statically...

HDF5.jl/src/api/types.jl

Lines 385 to 401 in 66ee074

	const H5P_OBJECT_CREATE = _read_const(:H5P_CLS_OBJECT_CREATE_ID_g)
	const H5P_FILE_CREATE = _read_const(:H5P_CLS_FILE_CREATE_ID_g)
	const H5P_FILE_ACCESS = _read_const(:H5P_CLS_FILE_ACCESS_ID_g)
	const H5P_DATASET_CREATE = _read_const(:H5P_CLS_DATASET_CREATE_ID_g)
	const H5P_DATASET_ACCESS = _read_const(:H5P_CLS_DATASET_ACCESS_ID_g)
	const H5P_DATASET_XFER = _read_const(:H5P_CLS_DATASET_XFER_ID_g)
	const H5P_FILE_MOUNT = _read_const(:H5P_CLS_FILE_MOUNT_ID_g)
	const H5P_GROUP_CREATE = _read_const(:H5P_CLS_GROUP_CREATE_ID_g)
	const H5P_GROUP_ACCESS = _read_const(:H5P_CLS_GROUP_ACCESS_ID_g)
	const H5P_DATATYPE_CREATE = _read_const(:H5P_CLS_DATATYPE_CREATE_ID_g)
	const H5P_DATATYPE_ACCESS = _read_const(:H5P_CLS_DATATYPE_ACCESS_ID_g)
	const H5P_STRING_CREATE = _read_const(:H5P_CLS_STRING_CREATE_ID_g)
	const H5P_ATTRIBUTE_ACCESS = _read_const(:H5P_CLS_ATTRIBUTE_ACCESS_ID_g)
	const H5P_ATTRIBUTE_CREATE = _read_const(:H5P_CLS_ATTRIBUTE_CREATE_ID_g)
	const H5P_OBJECT_COPY = _read_const(:H5P_CLS_OBJECT_COPY_ID_g)
	const H5P_LINK_CREATE = _read_const(:H5P_CLS_LINK_CREATE_ID_g)
	const H5P_LINK_ACCESS = _read_const(:H5P_CLS_LINK_ACCESS_ID_g)

[mkitti]

Would there be any advantage or disadvantage to doing:

julia> macro hdf5_global(ex)
           quote
               unsafe_load(Ptr{UInt64}(cglobal(($ex, HDF5_jll.libhdf5_path))))
           end
       end
@hdf5_global (macro with 1 method)

julia> @hdf5_global :H5T_STD_I16BE_g
0x030000000000004e

[topolarity]

Yes, that will opt in to Julia's built-in caching, rather than calling dlsym on every load

The macro would be a performance improvement over the status quo, in that case

[mkitti]

At the moment, we assign everything to a top-level const, so I guess this results in another layer of caching that gets stored in the precompile cache and may be a bit fragile.

 const H5P_DATASET_ACCESS   = _read_const(:H5P_CLS_DATASET_ACCESS_ID_g) 

If someone switched out the HDF5 library in such a way that does not HDF5.jl to recompile, these constants might be invalid. I will merge this for now and consider refactoring later.