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Devices/Libraries/Systems/can-utils/can-j1939.md

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+# CAN-J1939 on linux
+
+The [Kickstart guide is here](can-j1939-kickstart.md)
+
+## CAN on linux
+
+See [Wikipedia:socketcan](http://en.wikipedia.org/wiki/Socketcan)
+
+## J1939 networking in short
+
+* Add addressing on top of CAN (destination address & broadcast)
+
+* Any (max 1780) length packets.
+  Packets of 9 or more use **Transport Protocol** (fragmentation)
+  Such packets use different CANid for the same PGN.
+
+* only **29**bit, non-**RTR** CAN frames
+
+* CAN id is composed of
+  * 0..8: SA (source address)
+  * 9..26:
+    * PDU1: PGN+DA (destination address)
+    * PDU2: PGN
+  * 27..29: PRIO
+
+* SA / DA may be dynamically assigned via j1939-81
+  Fixed rules of precedence in Specification, no master necessary
+
+## J1939 on SocketCAN
+
+J1939 is *just another protocol* that fits
+in the Berkely sockets.
+
+	socket(AF_CAN, SOCK_DGRAM, CAN_J1939)
+
+## differences from CAN_RAW
+### addressing
+
+SA, DA & PGN are used, not CAN id.
+
+Berkeley socket API is used to communicate these to userspace:
+
+  * SA+PGN is put in sockname ([getsockname](http://man7.org/linux/man-pages/man2/getsockname.2.html))
+  * DA+PGN is put in peername ([getpeername](http://man7.org/linux/man-pages/man2/getpeername.2.html))
+    PGN is put in both structs
+
+PRIO is a datalink property, and irrelevant for interpretation
+Therefore, PRIO is not in *sockname* or *peername*.
+
+The *data* that is [recv][recvfrom] or [send][sendto] is the real payload.
+Unlike CAN_RAW, where addressing info is data.
+
+### Packet size
+
+J1939 handles packets of 8+ bytes with **Transport Protocol** fragmentation transparently.
+No fixed data size is necessary.
+
+	send(sock, data, 8, 0);
+
+will emit a single CAN frame.
+
+	send(sock, data, 9, 0);
+
+will use fragmentation, emitting 1+ CAN frames.
+
+# Using J1939
+
+## BSD socket implementation
+* socket
+* bind / connect
+* recvfrom / sendto
+* getsockname / getpeername
+
+## Modified *struct sockaddr_can*
+
+	struct sockaddr_can {
+		sa_family_t can_family;
+		int         can_ifindex;
+		union {
+			struct {
+				__u64 name;
+				__u32 pgn;
+				__u8 addr;
+			} j1939;
+		} can_addr;
+	}
+
+* *can_addr.j1939.pgn* is PGN
+
+* *can_addr.j1939.addr* & *can_addr.j1939.name*
+  determine the ECU
+
+  * receiving address information,
+    *addr* is always set,
+    *name* is set when available.
+
+  * When providing address information,
+    *name* != 0 indicates dynamic addressing