The following information is implementation-dependent.
In the open routine of ptem(7M) a STREAMS message block is allocated for a zero-length message for delivering a hangup message. This allocation of a buffer is done before it is needed to ensure that a buffer is available. An M_SETOPTS message is sent upstream to set the read-side stream head queues, to assign high-water and low-water marks (1024 and 256 respectively), and to establish a controlling terminal.
The same default values as for the line-discipline module are assigned to cflags, and INUSE to the state field.
These default values are currently being examined and may change in the future.
The open routine fails if:
No free entries are found when the ptem(7M) structure is searched
sflag is not set to MODOPEN
A zero-length message cannot be allocated (no buffer is available)
A stroptions(9S) structure cannot be allocated