[igorsk]

Is this better?

struct Request {
 DWORD reqNo;
 DWORD reserved[2];
 DWORD extralen;
};
struct Answer {
 DWORD reserved[3];
 DWORD dataLen;
 char  data[1]; //variable length
};
struct SendBuffer {
 DWORD type; //0x10005 for WriteFile
 DWORD reserved[2];
 DWORD dataLen;
 char  data[1]; //variable length
};

int UsbReceiveProc(Request* request, size_t answersize, Answer* answer);
void UsbBuffFree(Answer* answer);
int UsbSendProc(Request* request, SendBuffer* buf, size_t sendsize, DWORD* bytesSent);

Every request and answer further defines the variable data part, e.g.:

struct FileOpenRequest: Request {
 DWORD nPathLen;
 char path[1]; //variable length
};
struct FileOpenAnswer: Asnswer {
 DWORD hFile;
};