LiS: Linux STREAMS

The input/output system in UNIX is far from simple and involves many different modules: networking involves different protocol stages arranged in protocol stacks; terminal I/O involves different “line disciplines” stacked over (perhaps network) devices. All those modules perform some processing on existing I/O data flows.

In Linux and most BSD systems, I/O modules live inside the kernel and the relations between them are more or less hard-wired into the code. As an example, the TCP/IP protocol stack is a carefully programmed set of modules with strong interrelations. It is designed to work well on typical configurations.

STREAMS is a flexible input/output system, initially designed to overcome the inflexibility found in previous UNIX systems (see Resources 1). It is an alternative to sockets and is used in most commercial UNIX versions. Some sort of STREAMS is needed if we ever want networking software from systems such as Solaris, Unixware, etc. to run off-the-shelf on Linux.

A STREAM (see Figure 1) is, in essence, a dynamically configurable stack of modules. Each module does some processing on a data flow as it goes from the device to the user or vice-versa. The user perceives a STREAM as a file. It is handled with the usual open, read, write, ioctl and close system calls. Data written by the user is packaged into messages, which are sent downstream. Data read by the user comes from messages sent upstream by an underlying device driver.

Figure 1. Components of a STREAM

A couple of additional system calls, putpmsg and getpmsg, allow the user to send and receive STREAMS messages directly. Yet another system call, poll, provides an alternate interface for select. Therefore, each STREAM is composed of these elements:

A nice property of STREAMS is that different modules (or drivers) can be decoupled quite easily. Hence, they could be developed independently by different people who don't know the actual protocol stack where they will be used, provided the interfaces between the various modules and drivers are well-defined. STREAMS includes standard interfaces for the transport, network and data link layers. In addition, modules can be dynamically “pushed” onto (and popped off) the STREAM, which is a very convenient feature.

Finally, special multiplexor drivers allow several STREAMS to be multiplexed into another one (or ones). The ip module in Figure 2 is a multiplexor. In this example, it multiplexes both TCP and IP messages using either an Ethernet driver or an IP-on-X.25 driver. A full STREAMS network can be built (see Figure 2), and many different protocol stacks can be set up dynamically for operation.

Figure 2. A STREAMS Network