libpointer.Value => The datatype and size of the value must be defined before the value can be retrieved.

Hi everyone,

I've been trying to communicate with an FTDI cable that uses 'libMPSSE.dll'. I have read a couple of thing about libpointers here that were very helpfull. I'm now able to open the library and get some structs containing pointers.

loadlibrary('libMPSSE.dll','libMPSSE.DLL'); calllib('libMPSSE','Init_libMPSSE');

DeviceInfo = libpointer;

>> [x DeviceInfo] = calllib('libMPSSE','SPI_GetChannelInfo', 0 ,DeviceInfo);

>> [x DeviceInfo] = calllib('libMPSSE','SPI_GetChannelInfo', 0 ,DeviceInfo); %I have to do this twice for some reason?

>> DeviceInfo

DeviceInfo =

           Flags: [1x1 lib.pointer]
            Type: [1x1 lib.pointer]
              ID: [1x1 lib.pointer]
           LocId: 129
    SerialNumber: [1x1 lib.pointer]
     Description: [1x1 lib.pointer]
        ftHandle: []

now when I try to get the value from Description for example I do:

>> DeviceInfo.Description.Value

??? The datatype and size of the value must be defined before the value can be retrieved.

>> DeviceInfo.Description.DataType

ans =

int8Ptr

Now when I look in the documentation of the .dll I get redirected to another document that tells me how the deviceInfo looks:

typedef struct _ft_device_list_info_node { DWORD Flags; DWORD Type; DWORD ID; DWORD LocId; char SerialNumber[16]; char Description[64]; FT_HANDLE ftHandle; } FT_DEVICE_LIST_INFO_NODE;

Does anyone know how I can get the value from this pointer? Thanks in advance!

Hans

Answer 1

[Friedrich]

Hi,

I guess you need to use setdatatype (take a look at the doc to get more information about that function). But I guess you need to use it like this:

setdatatype(DeviceInfo.Description,'int8Ptr',1,64)
get(DeviceInfo.Description)

Hope that helps

[Hans]

This indeed does something, now I get an array of 64 ints. I'll get back to you!

Thanks for the fast reply!!

[Hans]

Ok, I looked into this a bit more but this doesn't seem to do it for me.. If I use setdatatype() I can get an array of 64 ints as output by typing 'DeviceInfo.Description.Value' But each time I type this, I get different numbers..

Any other ideas?

Thanks again!

Answer 2

[Philip Borghesani]

What version of MATLAB are you useing? The DeviceInfo structure will not work in versions prior to R2007a.

The two calls to

   >> [x DeviceInfo] = calllib('libMPSSE','SPI_GetChannelInfo', 0 ,DeviceInfo);

are a sign of problems. Only one should be needed. It is best to not use the same variable on both sides of the operation. When the input is a pointer or libstruct type then the output should not be needed.

I will guess that the first call is changing DeviceInfo from a libpointer to a MATLAB structure and then the second call is retuning an updated structure.

It looks to me that SPI_GetChannelInfo expects an allocated structure as input not a null pointer the correct code to call it should be something like:

DeviceInfo=libstruct('ft_device_list_info_node',struct('Flags',0));
x=calllib('libMPSSE','SPI_GetChannelInfo', 0 ,DeviceInfo);
get(DeviceInfo)

The data you show for DeviceInfo suggest that the header was not properly processed by loadlibrary. I suggest you examine any warnings you get from loadlibrary and fix your header files or generated prototype file so MATLAB can work with the correct data types.