The System V IPC facilities are message-passing facilities based
on the System V shared memory facility we described earlier.
Programs that cooperate using System V IPC usually define
shared protocols based on exchanging short (up to 8K) binary messages.
The relevant manual pages are
msgctl(2)
and friends. As this style has been largely superseded by text
protocols passed between sockets, we do not give an example
here.
The System V IPC facilities are present in
Linux
and other modern Unixes. However, as they are a legacy feature, they
are not exercised very often. The Linux version is still known to have
bugs as of mid-2003. Nobody seems to care enough to fix them.
Streams networking was invented for Unix Version 8 (1985) by
Dennis Ritchie. A re-implementation called STREAMS (yes,
it is all-capitals in the documentation) first became available in the
3.0 release of System V Unix (1986). The STREAMS facility provided a
full-duplex interface (functionally not unlike a BSD
socket, and like
sockets, accessible through normal
read(2)
and
write(2)
operations after initial setup) between a user process and a specified
device driver in the kernel. The device driver might be hardware such
as a serial or network card, or it might be a software-only
pseudodevice set up to pass data between user processes.
An interesting feature of both streams and
STREAMS[76] is that it is possible to push
protocol-translation modules into the kernel's processing path, so
that the device the user process ‘sees’ through the
full-duplex channel is actually filtered. This capability could be
used, for example, to implement a line-editing protocol for a terminal
device. Or one could implement protocols such as IP or TCP without
wiring them directly into the kernel.
Streams originated as an attempt to clean up a messy feature of the
kernel called ‘line disciplines’ — alternative modes
of processing character streams coming from serial terminals and early
local-area networks. But as serial terminals faded from view,
Ethernet LANs became ubiquitous, and TCP/IP drove out other protocol
stacks and migrated into Unix kernels, the extra flexibility provided
by STREAMS had less and less utility. In 2003, System V
Unix still
supports STREAMS, as do some System V/BSD hybrids such as Digital Unix and
Sun Microsystems' Solaris.
Linux and other open-source Unixes have
effectively discarded STREAMS. Linux kernel modules and libraries are
available from the LiS project, but (as
of mid-2003) are not integrated into the stock Linux kernel. They
will not be supported under non-Unix operating systems.
