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这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

Serial communications program for PostScript printers.

Runs as a single read/write process (by default). Use the -R2 option
or set splitme to TRUE (file postio.c) to get separate read and write
processes. Although not the default, we recommend using separate read
and write processes.

Sends occasional status queries (control Ts) while transmitting files.
Use the -q option or set quiet (file postio.c) to TRUE to disable status
queries.

Datakit connections are supported on System V and Ninth Edition systems.
The syntax (for connecting to a Datakit destination) varies. Check the
SYSV and V9 versions of setupline() in file ifdef.c.

Set DKHOST and DKSTREAMS to TRUE in postio.mk for streams based DKHOST
support. When DKSTREAMS is TRUE postio.mk uses "dknetty" as the stream
module. Settings like DKSTREAMS=dkty select a different stream module
and may be required for full Datakit support on some systems.