SOCK_SEQPACKE

The SOCK_SEQPACKET socket type is similar to the SOCK_STREAM type, and is also connection-oriented. The only difference between these types is that record boundaries are maintained using the SOCK_SEQPACKET type. A record can be sent using one or more output operations and received using one or more input operations, but a single operation never transfers parts of more than one record. Record boundaries are visible to the receiver via the MSG_EOR flag in the received message flags returned by the recvmsg() function. It is protocol-specific whether a maximum record size is imposed

 

It seems there's atleast 3 different local/unix socket types (PF_UNIX) , SOCK_STREAM, SOCK_DGRAM and SOCK_SEQPACKET.

While I know that a SOCK_STREAM gives you a bi-directional byte stream, like TCP or a bidirectional pipe, and the other two gives you a messge/packet API, what's the difference between a unix socket of SOCK_DGRAM and SOCK_SEQPACKET ?

As these are local only, I can't think of a good reason someone would implement SOCK_DGRAM in a manner it could reorder packets.

Also, does SOCK_DGRAM/SOCK_SEQPACKET employ flow control, or can messages be dropped in case of slow readers ?