[julia-users] Unable to de-reference a Ptr{Uint8} to a string - unsafe load causes an EXCEPTION_ACCESS_VIOLATION in Julia v0.3.9

[Joel Hobson]

Hi all,

I'm trying to use some C functions, but I'm running into trouble using the 
results.

This is the function I'm trying to 
call: https://msdn.microsoft.com/en-us/library/dd798469(v=vs.85).aspx
And this is the struct that it's expecting a pointer 
to: https://msdn.microsoft.com/en-us/library/dd798467(v=vs.85).aspx

Here's my struct definition in Julia:

type MidiOutCaps
    wMid::Uint16
    wPid::Uint16
    vDriverVersion::Uint32 
    szPname::Ptr{Uint8}
    wTechnology::Uint16
    wVoices::Uint16
    wNotes::Uint16
    wChannelMask::Uint16
    dwSupport::Uint32

    MidiOutCaps() = new(0, 0, 0, Array(Uint8, 32), 0, 0, 0, 0, 0)
end

And the function for loading it:

function getoutputdeviceswindows()
    numberofdevices = ccall( (:midiOutGetNumDevs, :Winmm), Int32, ())

    for i in [0:numberofdevices-1]
        output_struct = MidiOutCaps()

        err = ccall( (:midiOutGetDevCapsA, :Winmm), Uint32, (Ptr{Uint32}, 
Ptr{MidiOutCaps}, Uint32), i, &output_struct, sizeof(output_struct))

        println("Err is $(err)")
        println("Result is $(output_struct)")

        println("Result name is $(unsafe_load(output_struct.szPname, 0))")
    end
end

Both ccalls execute without error, and it appears to populate the struct 
correctly, but when I try to do an unsafe load on the szPname, Julia 
crashes and I get this:

Please submit a bug report with steps to reproduce this fault, and any 
error messages that follow (in their entirety). Thanks.
Exception: EXCEPTION_ACCESS_VIOLATION at 0xdbc6d34 -- unsafe_load at 
pointer.jl:45
unsafe_load at pointer.jl:45
jlcall_unsafe_load_1323 at  (unknown line)
jl_apply_generic at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
jl_interpret_toplevel_expr at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
jl_interpret_toplevel_thunk_with at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
jl_eval_with_compiler_p at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
jl_f_top_eval at C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll 
(unknown line)
eval_user_input at REPL.jl:53
jlcall_eval_user_input_1054 at  (unknown line)
jl_apply_generic at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
anonymous at task.jl:95
jl_handle_stack_switch at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
julia_trampoline at 
C:\Users\Joel\AppData\Local\Julia-0.3.9\bin\libjulia.dll (unknown line)
unknown function (ip: 4202784)
unknown function (ip: 4199370)
unknown function (ip: 4199672)
BaseThreadInitThunk at C:\WINDOWS\system32\KERNEL32.DLL (unknown line)
RtlUserThreadStart at C:\WINDOWS\SYSTEM32\ntdll.dll (unknown line)
RtlUserThreadStart at C:\WINDOWS\SYSTEM32\ntdll.dll (unknown line)

I suspect that it's a problem with my code and not Julia, which is why I 
haven't submitted a bug report yet. 

I've tried just setting szPname to 0 instead of an array, and I've tried 
different index values for unsafe_load, but no luck. Any ideas where I'm 
going wrong? The only other issue I can see is that MSDN defines szPname to 
be an array of TCHARs, but according to the MSDN, TCHAR is just a 
char. https://msdn.microsoft.com/en-us/library/office/cc842072.aspx

Thanks!