Книга: Real-Time Concepts for Embedded Systems

12.3.1 Standard I/O Functions

12.3.1 Standard I/O Functions

The I/O subsystem presented in the example in this section defines a set of functions as the standard I/O function set. Table 12.1 lists those functions that are considered part of the set in the general approach to uniform I/O. Again, remember that the example approach is used for illustration purposes in describing and discussing the I/O subsystem in general. The number of functions in the standard I/O API set, function names, and functionality of each is dependent on the embedded system and implementation. The next few sections put these functions into perspective.

Table 12.1: I/O functions.

Note that all these functions operate on a so-called 'virtual instance' of the I/O device. In other words, these functions do not act directly on the I/O device, but rather on the driver, which passes the operations to the I/O device. When the open, read, write, and close operations are described, these operations should be understood as acting indirectly on an I/O device through the agency of a virtual instance.

The create function creates a virtual instance of an I/O device in the I/O subsystem, making the device available for subsequent operations, such as open, read, write, and ioctl. This function gives the driver an opportunity to prepare the device for use. Preparations might include mapping the device into the system memory space, allocating an available interrupt request line (IRQ) for the device, installing an ISR for the IRQ, and initializing the device into a known state. The driver allocates memory to store instance-specific information for subsequent operations. A reference to the newly created device instance is returned to the caller.

The destroy function deletes a virtual instance of an I/O device from the I/O subsystem. No more operations are allowed on the device after this function completes. This function gives the driver an opportunity to perform cleanup operations, such as un-mapping the device from the system memory space, de-allocating the IRQ, and removing the ISR from the system. The driver frees the memory that was used to store instance-specific information.

The open function prepares an I/O device for subsequent operations, such as read and write. The device might have been in a disabled state when the create function was called. Therefore, one of the operations that the open function might perform is enabling the device. Typically, the open operation can also specify modes of use; for example, a device might be opened for read-only operations or write-only operations or for receiving control commands. The reference to the newly opened I/O device is returned to the caller. In some implementations, the I/O subsystem might supply only one of the two functions, create and open, which implements most of the functionalities of both create and open due to functional overlaps between the two operations.

The close function informs a previously opened I/O device that its services are no longer required. This process typically initiates device-specific cleanup operations. For example, closing a device might cause it to go to a standby state in which it consumes little power. Commonly, the I/O subsystem supplies only one of the two functions, destroy and close, which implements most of the functionalities of both destroy and close, in the case where one function implements both the create and open operations.

The read function retrieves data from a previously opened I/O device. The caller specifies the amount of data to retrieve from the device and the location in memory where the data is to be stored. The caller is completely isolated from the device details and is not concerned with the I/O restrictions imposed by the device.

The write function transfers data from the application to a previously opened I/O device. The caller specifies the amount of data to transfer and the location in memory holding the data to be transferred. Again, the caller is isolated from the device I/O details.

The Ioctl function is used to manipulate the device and driver operating parameters at runtime.

An application is concerned with only two things in the context of uniform I/O: the device on which it wishes to perform I/O operations and the functions presented in this section. The I/O subsystem exports this API set for application use.

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