2.3. Using Devices

To use a device, we need to get a handle (i.e., a reference) on it that we will pass to every function applicable to it. See the code here.

This code reads a 64-bit word from the urandom device that returns random data and another from the zero device that returns bytes set to 0. Finally, we write a string into the null device that discards what is written into it. These three devices are virtual and are always available with Linux’s default configuration.

Device Specific Interfaces

In the previous code example we have used read and write methods as if the device handle had been a normal file handle. Indeed Linux device drivers define the operational semantics they want to give to each system call applicable to a file handle: read, write, fseek, mmap, close, etc. Some system calls may be invalid with some device handles (e.g., write with the urandom driver).

This gives a weak unified interface to device drivers: the system calls are the same but the operational semantics depends on the driver. Moreover there are a lot of corner cases, such as system call parameters or flags only valid for some drivers. Finally, as there aren’t enough “generic” system calls to cover the whole spectrum of device features, the ioctl system call is used to send device specific commands to drivers. In practice you really have to know which device driver you’re working with to ensure that you use appropriate system calls.

To catch up as many errors at compile time as possible, in haskus-system we provide device specific interfaces that hide all this complexity. If you use them, you minimise the risk of accidentally using an invalid system call. Some of these interfaces are presented in the next chapters. Nevertheless you will have to use the low-level interface presented in this chapter if you want to write your own high-level interface to a device class not supported by haskus-system or if you want to extend an existing one.