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How Mac OS X Device I/O Works

Mac OS X provides many of the traditional UNIX mechanisms for device I/O. However, individual device driver designs determine whether or not to use these mechanisms.

In Mac OS X, disk devices, serial port devices, the random-number generator pseudo-device, and pseudo-tty devices (pttys) are traditionally accessed through standard UNIX-style block and character devices. Other pseudo-devices (devices without hardware backing) can also be implemented as BSD devices. You can use these device files in the same way as you would use them on any other UNIX-based or UNIX-like system.

Most actual hardware devices, however, such as USB and FireWire devices are not handled with block or character devices in Mac OS X. Instead, the primary mechanism for accessing hardware devices is through I/O Kit user clients, which are basically remote procedure call or system call interfaces that allow you to call functions within a kernel driver from an application.

To access a hardware device, you must use APIs to search the device tree for a device matching various parameters. You then call functions within the driver. The mechanism used to call these functions depends on the design of the device interface, as do the functions themselves. You can also get information about the device using a standardized mechanism provided by the I/O Kit to examine its properties in the device tree.

A device cannot be controlled from an application unless there is a driver in the kernel. In many cases, the driver may be a simple pass-through that presents a device interface that allows the real device driver to be part of the application itself. In other cases, it may be a full driver for the device that presents a device interface for configuration. The device interface may be provided by Mac OS X itself (such as the user space device interface for USB HID devices) or it may be provided by the individual hardware vendor (such as an audio card driver).

The implementation of a user client is beyond the scope of this document. It is described in detail in the book Accessing Hardware From Applications. Since the design of a user client is also heavily dependent on the design of the device interface to which it is connecting, you may also need to read documentation specific to the technology area in question. In the case of device interfaces created by hardware vendors, you may also need to contact the hardware vendor for additional programming information.

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