Marshaling C array of structs into c#

Question

I am trying to (for the past 10 hours.... grr) to make this thing work but so far whatever i tried - it refuses :)

Basically i am doing a favor for someone - my strength is not windows / .NET coding for sure and i am trying to patch in some code from something i already have around.

What is the problem?

I am trying to call a C DLL library method returning a 2d array of structs to c#.

But it seems i am doing something wrong regarding in how i read the data from c#.

I've developed a simple C console app and i am calling the DLL from there - all fine - no issues at all. Only the c# fails!

Here is the C implementation of the method:

int get_available_devices(idevice_info_t **devices, uint32_t *count) {
char **dev_list = NULL;
char *dev_name = NULL;
int i, total_devices;

if (idevice_get_device_list(&dev_list, &total_devices) < 0) {
    fprintf(stderr, "ERROR: Unable to retrieve device list!\n");
    return -1;
}

idevice_info_t *tmpArr = (idevice_info_t*)calloc(total_devices, sizeof(idevice_info));

int ii = 0;
int res_name = 0;

idevice_info_t dtmp = NULL;

for (i = 0; i <= total_devices - 1; i++) {
    res_name = idevice_get_device_name(dev_list[i], &dev_name);
    dev_name = (res_name == 0 ? dev_name : "");

    printf("%s: %s\n", dev_name, dev_list[i]);

    dtmp = (idevice_info_t)malloc(sizeof(struct idevice_info));
    strncpy_s(dtmp->udid, sizeof dtmp->udid - 1, dev_list[i], sizeof dtmp->udid - 1);
    strncpy_s(dtmp->name, sizeof dtmp->name - 1, dev_name, sizeof dtmp->name - 1);
    tmpArr[i] = dtmp;
}
idevice_device_list_free(dev_list);

*devices = tmpArr;
*count = total_devices;

return 0;}

Here is what i am doing in c# side:

[DllImport(LIBNAME, CallingConvention = CallingConvention.Cdecl)]
    static public extern short get_available_devices(out IntPtr devices, out uint count);

public static Dictionary<string, string> getAvailableDevices()
    {
        IntPtr p = IntPtr.Zero;
        Dictionary<string, string> ret = null;

        uint totalDevices = 0;

        int res = External.get_available_devices(out p, out totalDevices);

        if (res != 0 || totalDevices < 1)
        {
            return null;
        }

        ret = new Dictionary<string, string>();

        External.idevice_info ppStruct;
        int sSize = Marshal.SizeOf(typeof(External.idevice_info));

        for (int i = 0; i <= totalDevices - 1; i++)
        {
            p = (IntPtr)Marshal.PtrToStructure(p, typeof(IntPtr));
            ppStruct = (External.idevice_info)Marshal.PtrToStructure(p, typeof(External.idevice_info));

            ret.Add(ppStruct.udid, ppStruct.name);

            p = new IntPtr(p.ToInt64() + sSize);
        }

        return ret;
    }

What is the actual problem?

As soon as i reach the second iteration of the for cycle() i get an access violation:

An unhandled exception of type 'System.AccessViolationException' occurred in mscorlib.dll

Unhandled exception: Access Violation

I guess i am not calculating the pointer properly but... i really tried a lot of different scenarios and nothing works.

HELP! :)

leppie · Accepted Answer · 2015-01-16 11:14:53Z

1

You are dereferencing p with p = (IntPtr)Marshal.PtrToStructure(p, typeof(IntPtr)); and then at the end when trying to increment, all hell breaks loose.

Use a fresh local so that the original pointer is not continuously dereferenced.

Eg:

    for (int i = 0; i <= totalDevices - 1; i++)
    {
        IntPtr pp = (IntPtr)Marshal.PtrToStructure(p, typeof(IntPtr));
        ppStruct = (External.idevice_info)Marshal.PtrToStructure(pp, 
                      typeof(External.idevice_info));

        ret.Add(ppStruct.udid, ppStruct.name);

        p += sSize; // easier, does the same :)
    }

edited Jan 16, 2015 at 11:14

answered Jan 16, 2015 at 11:02

leppie

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8 Comments

Martin Kovachev Over a year ago

I seeee :) But it still goes bad :( Issue 1 is that (p += sSize) does not work - i am on 3.5 sdk - i guess that is the reason - so i've replaced it again with: p = new IntPtr(p.ToInt64() + sSize); The exception now is different: NullReferenceException - again on the 'ppstruct = ...' line. Before it is invoked 'p' has became zero.

Martin Kovachev Over a year ago

Aaand i just solved it :))) I should be doing: p = new IntPtr(p.ToInt64() + IntPtr.Size); instead of: p = new IntPtr(p.ToInt64() + sSize); All perfect now! Thank you BIG!!!!!!

leppie Over a year ago

@MartinKovachev: Glad I could help :D p += IntPtr.Size; should work too then

Martin Kovachev Over a year ago

It doesn't recognize the syntax. That's why i reworked it like this in the first place :) the + operation.

Martin Kovachev Over a year ago

I know for sure it works in 4.0 - as i tested it - i guess it's a SDK addon.

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