[SOLVED] Feature Request RTT: SEGGER_RTT_GetAvailWriteSpace

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  • [SOLVED] Feature Request RTT: SEGGER_RTT_GetAvailWriteSpace


    We're using RTT to write out a binary packet log file (e.g. like .pcap). To avoid using a special outgoing packet buffer and extra memcpy calls, we first write the packet header and then the packet itself with SEGGER_RTT_Write. If the RTT buffer gets full after writing the header and the RTT mode is set to SEGGER_RTT_MODE_NO_BLOCK_SKIP, the packet can not get written and the log gets corrupted.

    Would it be possible to add a SEGGER_RTT_GetAvailWriteSpace(bufferIndex) or SEGGER_RTT_CanWriteBytes(bufferIndex, numBytes)?

    Here's a draft: github.com/bluekitchen/btstack…639c0a9cb764edb04e3d5e483

    I'm aware that this would only works without multi-threading or when using RTT from ISRs.

    Thanks for consideration
  • Hello Matthias,

    Thank you for your feedback.
    We discussed this internally and will add a API call that returns number of free space in the buffer.
    This should be available with the next J-Link release.

    But keep in mind that the buffer will be free in most cases after the callback _GetAvailWriteSpace has been executed in SEGGER_RTT_WRITE due to the speed of J-Link and asynchronous nature of RTT as the callback will only return the current buffer state while in the background J-Link continues grabbing the data.

    Another way would be to use RTT in TRIM mode and use the return value from SEGGER_RTT_Write(). Then try to resend the difference of requested bytes and written bytes again.

    Best regards,
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  • Thanks, that's Awesome!

    Yes, I think I'm aware of the limitations and the async nature. To me, it looks like RTT TRIM + retry is similar to NO_SKIP_BLOCK, if the retry happens right after the write (if there's another queue, the retry could happen later, but then, a larger RTT buffer could be used ).

    Best regards