mmap_array fails for empty file (0-length array)

[stevengj]

mmap_array(Int, (0,), open("foo", "r")) on an empty file "foo" gives:

ERROR: SystemError: memory mapping failed: Invalid argument
 in systemerror at /Users/stevenj/Code/julia/usr/lib/julia/sys.dylib
 in mmap at mmap.jl:35
 in mmap_array at mmap.jl:108
 in mmap_array at mmap.jl:4

(See also JuliaIO/JSON.jl#98.)

[quinnj]

The current behavior is an explicit check that the file size is > 0 or an ArgumentError is thrown. I guess I can see the argument that we should return a 0-length array. Do people think that would generally be a better behavior?

[stevengj]

Since 0-length arrays are valid, I think it would be better to support any length >= 0 here. 0 length will come up as a corner case in a lot of algorithms, and it's better not to force the caller to check for that.

[kcajf]

Sorry to raise an old issue. I just encountered the following:

julia> using Mmap

julia> x = Mmap.mmap(open("test.dat", "w+"), Array{Float64,2}, (1, 10))
1×10 Matrix{Float64}:
 0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0
 
julia> x = Mmap.mmap(open("test.dat", "w+"), Array{Float64,2}, (0, 10))
0×0 Matrix{Float64}

I would expect the second call to return a 0x10 Matrix{Float64} instead of 0x0. I traced the cause to the line added in this issue/PR:

julia/stdlib/Mmap/src/Mmap.jl

Line 196 in 5faad5f

len == 0 && return Array{T}(undef, ntuple(x->0,Val(N)))

It looks like this could be an oversight, and that line could just be replaced with:

len == 0 && return Array{T}(undef, dims)

What do you think?

[stevengj]

Yes, that seems like an oversight in d208f8d, and would make a good PR.

On an unrelated note, it seems like the len >= 0 check in that function should really be all(>=(0), dims)