Examples of resource managers

Before we get carried away, let's take a look at a couple of examples and see how they “abstract” some “service.” We'll look at an actual piece of hardware (a serial port) and something much more abstract (a filesystem).

Serial port

On a typical system, there usually exists some way for a program to transmit output and receive input from a serial, RS-232-style hardware interface. This hardware interface consists of a bunch of hardware devices, including a UART (Universal Asynchronous Receiver Transmitter) chip which knows how to convert the CPU's parallel data stream into a serial data stream and vice versa.

In this case, the “service” being provided by the serial resource manager is the capability for a program to send and receive characters on a serial port.

We say that an “abstraction” occurs, because the client program (the one ultimately using the service) doesn't know (nor does it care about) the details of the UART chip and its implementation. All the client program knows is that to send some characters it should call the fprintf() function, and to receive some characters it should call the fgets() function. Notice that we used standard, POSIX function calls to interact with the serial port.

Filesystem

As another example of a resource manager, let's examine the filesystem. This consists of a number of cooperating modules: the filesystem itself, the block I/O driver, and the disk driver.

The “service” being offered here is the capability for a program to read and write characters on some medium. The “abstraction” that occurs is the same as with the serial port example above—the client program can still use the exact same function calls (e.g., the fprintf() and fgets() functions) to interact with a storage medium instead of a serial port. In fact, the client really doesn't know or need to know which resource manager it's interacting with.