sending IOKit command with dynamic length

I'm using IOKit framework to communicate with my driver using IOConnectCallMethod from the user-space client and IOExternalMethodDispatch on the driver side.

So far I was able to send fixed length commands, and now I wish to send a varied size array of chars (i.e. fullpath).

However, it seems that the driver and the client sides command lengths are coupled, which means that checkStructureInputSize from IOExternalMethodDispatch in driver must be equal to inputStructCnt from IOConnectCallMethod in client side.

Here are the struct contents on both sides :

DRIVER :

struct IOExternalMethodDispatch
{
    IOExternalMethodAction function;
    uint32_t           checkScalarInputCount;
    uint32_t           checkStructureInputSize;
    uint32_t           checkScalarOutputCount;
    uint32_t           checkStructureOutputSize;
};

CLIENT:

kern_return_t IOConnectCallMethod(
    mach_port_t  connection,        // In
    uint32_t     selector,      // In
    const uint64_t  *input,         // In
    uint32_t     inputCnt,      // In
    const void      *inputStruct,       // In
    size_t       inputStructCnt,    // In
    uint64_t    *output,        // Out
    uint32_t    *outputCnt,     // In/Out
    void        *outputStruct,      // Out
    size_t      *outputStructCnt)   // In/Out

Here's my failed attempt to use a varied size command :

std::vector<char> rawData; //vector of chars

// filling the vector with filePath ...

kr = IOConnectCallMethod(_connection, kCommandIndex , 0, 0, rawData.data(), rawData.size(), 0, 0, 0, 0);

And from the driver command handler side, I'm calling IOUserClient::ExternalMethod with IOExternalMethodArguments *arguments and IOExternalMethodDispatch *dispatch but this requires the exact length of data I'm passing from the client which is dynamic.

this doesn't work unless I set the dispatch function with the exact length of data it should expect.

Any idea how to resolve this or perhaps there's a different API I should use in this case ?