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unixdrivers

Are pseudo-devices implemented as device drivers within the kernel?

I kinda wonder if pseduo-devices like /dev/null and /dev/zero are implemented as device drivers within the kernel. I'm sure they are, but not exactly 100% sure. Both /dev/null and /dev/zero device files have same major number but different minor numbers; they use the same driver which doesn't really drive a physical hardware and the minor numbers to specify different functionality within the driver.

Is my hypothesis right?

Solution

  • At least classically, all devices — pseudo or not — were implemented as device drivers in the kernel. Frequently, the implementations of the support functions were minimal for the pseudo-devices. The write code for /dev/null does nothing successfully (and the same is likely true of /dev/zero); the read code for /dev/null reports 'no data'; the read code for /dev/zero zeroes the buffer that it is given. And so on. Things may have changed a little over the last 20 years, but that's more or less how it used to be and how I'd expect it still to be (but I live to be surprised).

    On the Linux system I'm using, and maybe on other Linux systems, /dev/null, /dev/zero and /dev/random all have the same major number though.

    That needn't be a problem. They have different minor numbers, and as a result can and do do different things from each other, just as different disk drives may share a major number but the minor numbers distinguish different subsections of the main device, or even different drives altogether.

    On a Mac (running macOS Sierra 10.12.5), /dev/null and /dev/zero share the same major number, and /dev/random and /dev/urandom share the same major number that's different from the other two.

    crw-rw-rw-  1 root      wheel            3,   2 Jul 10 20:10 /dev/null
crw-rw-rw-  1 root      wheel           14,   0 Jun 13 13:34 /dev/random
crw-rw-rw-  1 root      wheel           14,   1 Jun 13 13:34 /dev/urandom
crw-rw-rw-  1 root      wheel            3,   3 Jun 13 13:34 /dev/zero

    More intriguing is that there are a number of devices with the same major and minor numbers:

    crw-------  1 jleffler  staff            0,   0 Jul  8 01:54 /dev/console
crw-rw-rw-  1 root      wheel            0,   0 Jun 13 13:34 /dev/fbt
crw-rw-rw-  1 root      wheel            0,   0 Jun 13 13:34 /dev/lockstat
crw-rw-rw-  1 root      wheel            0,   0 Jun 13 13:34 /dev/machtrace
crw-rw-rw-  1 root      wheel            0,   0 Jun 13 13:34 /dev/profile
crw-rw-rw-  1 root      wheel            0,   0 Jun 13 13:34 /dev/sdt
crw-rw-rw-  1 root      wheel            0,   0 Jun 13 13:34 /dev/systrace

    I'm not sure how those distinguish what they're supposed to do.