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Move characte devices next to pseudo filesystems
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Ciro Santilli
155
README.adoc
155
README.adoc
@@ -3610,6 +3610,84 @@ Bibliography:
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* http://www.makelinux.net/ldd3/chp-14-sect-1
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* https://www.win.tue.nl/~aeb/linux/lk/lk-13.html
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==== Character devices
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Character devices can have arbitrary <<file-operations>> associated to them:
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....
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/character_device.sh
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echo $?
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....
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Outcome: the test passes:
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....
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0
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....
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Sources:
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* link:rootfs_overlay/character_device.sh[]
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* link:rootfs_overlay/mknoddev.sh[]
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* link:kernel_module/character_device.c[]
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Unlike <<procfs>> entires, character device files are created with userland `mknod` or `mknodat` syscalls:
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....
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mknod </dev/path_to_dev> c <major> <minor>
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....
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Intuitively, for physical devices like keyboards, the major number maps to which driver, and the minor number maps to which device it is.
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A single driver can drive multiple compatible devices.
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The major and minor numbers can be observed with:
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....
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ls -l /dev/urandom
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....
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Output:
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....
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crw-rw-rw- 1 root root 1, 9 Jun 29 05:45 /dev/urandom
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....
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which means:
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* `c` (first letter): this is a character device. Would be `b` for a block device.
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* `1, 9`: the major number is `1`, and the minor `9`
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To avoid device number conflicts when registering the driver we:
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* ask the kernel to allocate a free major number for us with: `register_chrdev(0`
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* find ouf which number was assigned by grepping `/proc/devices` for the kernel module name
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Bibliography: https://unix.stackexchange.com/questions/37829/understanding-character-device-or-character-special-files/371758#371758
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===== Automatically create character device file on insmod
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And also destroy it on `rmmod`:
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....
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/character_device_create.sh
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echo $?
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....
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Outcome: the test passes:
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....
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0
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....
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Sources:
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* link:kernel_module/character_device_create.c[]
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* link:rootfs_overlay/character_device_create.sh[]
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Bibliography: https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/45531867#45531867
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=== Pseudo files
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==== File operations
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@@ -3815,83 +3893,6 @@ Bibliography:
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* https://github.com/simonjhall/dma
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* https://github.com/ikwzm/udmabuf
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==== Character devices
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Character devices can have arbitrary <<file-operations>> associated to them:
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....
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/character_device.sh
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echo $?
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....
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Outcome: the test passes:
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....
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0
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....
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Sources:
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* link:rootfs_overlay/character_device.sh[]
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* link:rootfs_overlay/mknoddev.sh[]
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* link:kernel_module/character_device.c[]
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Unlike <<procfs>> entires, character device files are created with userland `mknod` or `mknodat` syscalls:
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....
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mknod </dev/path_to_dev> c <major> <minor>
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....
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Intuitively, for physical devices like keyboards, the major number maps to which driver, and the minor number maps to which device it is.
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A single driver can drive multiple compatible devices.
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The major and minor numbers can be observed with:
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....
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ls -l /dev/urandom
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....
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Output:
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....
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crw-rw-rw- 1 root root 1, 9 Jun 29 05:45 /dev/urandom
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....
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which means:
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* `c` (first letter): this is a character device. Would be `b` for a block device.
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* `1, 9`: the major number is `1`, and the minor `9`
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To avoid device number conflicts when registering the driver we:
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* ask the kernel to allocate a free major number for us with: `register_chrdev(0`
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* find ouf which number was assigned by grepping `/proc/devices` for the kernel module name
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Bibliography: https://unix.stackexchange.com/questions/37829/understanding-character-device-or-character-special-files/371758#371758
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===== Automatically create character device file on insmod
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And also destroy it on `rmmod`:
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....
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/character_device_create.sh
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echo $?
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....
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Outcome: the test passes:
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....
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0
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....
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Sources:
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* link:kernel_module/character_device_create.c[]
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* link:rootfs_overlay/character_device_create.sh[]
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Bibliography: https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/45531867#45531867
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==== Anonymous inode
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Anonymous inodes allow getting multiple file descriptors from a single filesystem entry, which reduces namespace pollution compared to creating multiple device files:
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