Communications Technical Reference, Volume 2 putmsg System Call Purpose Sends a message on a stream. The message may contain either a data part, a control part or both. The data and control parts to be sent are distinguished by placement in separate buffers.
Parameters fd Specifies a file descriptor referencing an open stream.
Holds the control part of the message. Holds the data part of the message. The len field in the strbuf structure indicates the number of bytes to be sent, and the buf field points to the buffer where the control information or data resides. The maxlen field is not used in the putmsg system call. To send the data part of a message, the dataptr parameter must be nonnull and the len field of the dataptr parameter must have a value of 0 or greater.