Mastering Embedded Linux Programming

Character devices

These devices are identified in user space by a filename: if you want to read from a UART, you open the device node, for example, the first serial port on the ARM Versatile Express would be /dev/ttyAMA0. The driver is identified differently in the kernel, using the major number which, in the example given, is 204. Since the UART driver can handle more than one UART, there is a second number, called the minor number, which identifies a specific interface, 64, in this case:

# ls -l /dev/ttyAMA*

crw-rw----    1 root     root      204,  64 Jan  1  1970 /dev/ttyAMA0
crw-rw----    1 root     root      204,  65 Jan  1  1970 /dev/ttyAMA1
crw-rw----    1 root     root      204,  66 Jan  1  1970 /dev/ttyAMA2
crw-rw----    1 root     root      204,  67 Jan  1  1970 /dev/ttyAMA3

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