typedef struct usb_common_descriptor_tag
{
u8_t  bLength;
u8_t  bDescriptorType;
} usb_common_descriptor_t;

usb_common_descriptor_t *ptr;
...
(u8_t*)ptr += ptr-> bLength;

usb_common_descriptor_t *ptr;
u8_t *ptr8;
...
ptr8 = (u8_t*)ptr;
ptr8 +=  ptr-> bLength;
ptr = (usb_common_descriptor_t*)ptr8;

ptr += ptr-> bLength;

    ptr = (usb_common_descriptor_t*)&USB_ConfigDescriptor;
    while(ptr->bLength)
    {
      switch (ptr->bDescriptorType)
      {
        case USB_CONFIGURATION_DESCRIPTOR_TYPE:
             ...
          break;
        case USB_INTERFACE_DESCRIPTOR_TYPE:    
             ...
          break;
        case USB_ENDPOINT_DESCRIPTOR_TYPE:      
             ...
          break;
       case USB_VENDOR_STATUS_TYPE:      
             ...
          break;
      }
      (u8_t*)ptr += ptr->bLength;
    };

ptr = (usb_common_descriptor_t *)((uint8_t *)ptr + ptr->bLength);

  ptr = (usb_common_descriptor_t *)((u8_t*)ptr + ptr-> bLength);