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28 lines
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1 year ago
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# MAVLink Log Format
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_QGroundControl_ allows you to generate plain MAVLink packet logs that can be
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replayed (with QGroundControl) to watch a mission again for analysis.
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The format is binary:
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- Byte 1-8: Timestamp in microseconds since Unix epoch as unsigned 64 bit integer
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- Byte 9-271: MAVLink packet (263 bytes maximum packet length, not all bytes have to be actual data, the packet might be shorter. Includes packet start sign)
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## Debugging
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To check your data, open your written file in a hex editor. You should see after 8 bytes **0x55**. The first 8 bytes should also convert to a valid timestamp, so something either close to zero or around the number **1294571828792000** (which is the current Unix epoch timestamp in microseconds).
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## C++ Sketch for logging MAVLink
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The code fragment below shows how to implement logging using [C++ streams](http://www.cplusplus.com/reference/iostream/istream/) from the C++ standard library.
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```cpp
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//write into mavlink logfile
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const int len = MAVLINK_MAX_PACKET_LEN+sizeof(uint64_t);
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uint8_t buf[len];
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uint64_t time = getSystemTimeUsecs();
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memcpy(buf, (void*)&time, sizeof(uint64_t));
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mavlink_msg_to_send_buffer(buf+sizeof(uint64_t), msg);
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mavlinkFile << buf << flush;
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```
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