Converting Received Data to CanardFrame

kyle-pawlowski · May 7, 2020, 8:49pm

I’m trying to make a service request/response setup with libcanard. I’m a little confused about how data is taken in with the canardRxAccept function. I see the function takes the received data as an arguement in the format of a CanardFrame. How would I pack my raw CAN stream into a CanardFrame? Can I just set it to be the payload of a CanardFrame? Or are all of the attributes of the CanardFrame contained in the data packet? Or am I completely lost?

pavel.kirienko · May 8, 2020, 6:22am

CanardFrame models one CAN frame. Just assign the fields using the values from your CAN frame as described in the docs:

github.com

UAVCAN/libcanard/blob/bec890304a2888bc516416e4ebf252f761558b92/libcanard/canard.h#L179-L200


typedef struct
{
    /// For RX frames: reception timestamp.
    /// For TX frames: transmission deadline.
    /// The time system may be arbitrary as long as the clock is monotonic (steady).
    CanardMicrosecond timestamp_usec;


    /// 29-bit extended ID. The bits above 29-th shall be zero.
    uint32_t extended_can_id;


    /// The useful data in the frame. The length value is not to be confused with DLC!
    /// If the payload is empty (payload_size = 0), the payload pointer may be NULL.
    /// For RX frames: the library does not expect the lifetime of the pointee to extend beyond the point of return
    /// from the API function. That is, the pointee can be invalidated immediately after the frame has been processed.
    /// For TX frames: the frame and the payload are allocated within the same dynamic memory fragment, so their
    /// lifetimes are identical; when the frame is freed, the payload is invalidated.
    /// A more detailed overview of the dataflow and related resource management issues is provided in the API docs.
    size_t      payload_size;
    const void* payload;
} CanardFrame;

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If you don’t have receive timestamps, just use zero.

Then feed the result into canardRxAccept.