Q: Non-atomic message sent over Unix Domain socket with file desriptor. Is FD sent twice? I am developing a client server application where the client application sends different types of messages to server. One type message consists of file descriptor that is to be passed between processes.
Generally on Posix API pages, not much information is found about sendmsg and recvmsg. My question is if the sent message is too big that cannot be sent atomically, will the attached file descriptor be sent for each pieces of the message, or just first one?
Why this confuses me is that on connected sockets, if messages are sent too quickly, kernel is merging messages to each other, then file descriptors (integer number) must be merged aligned with messages as well.

A: UNIX domain sockets support passing file descriptors or process credentials to ... The send(2) MSG_MORE flag is not supported by UNIX domain sockets. ... For historical reasons the ancillary message types listed below are specified with ... To pass file descriptors or credentials over a SOCK_STREAM
