leftpad.ko
In March of 2016, the removal of a package called left-pad from npm created a giant mess of failed builds all over the internet.
The package consisted only of a single function, leftPad()
, which pads strings to a given width.
This event demonstrates the disaster potential of failing to take left-padding seriously.
left-pad.io, created soon after the situation described above, was the first ever LPAAS (left-pad-as-a-service) product. LPAAS is a step in the right direction, but I believe we can do better.
To that end, here is an implementation of leftPad()
inside the kernel.
Parameters
width
: width to pad to (default 32)fill
: value of the character to fill with (e.g. 32 for ' ') (default 32)buffer_size
: size of the internal ring buffer (default 1024)
All parameters are mutable. The values at the time the device is opened determine the behavior of that instance.
IOCTL
800d3900
: set width800d3901
: set fill
Changes apply only to a specific instance.
Example Usage
$ make
$ insmod leftpad.ko width=10
$ mknod /dev/leftpad c 1337 0 -m 666
$ exec 8<>/dev/leftpad
$ echo foobar >&8
$ head -n 1 <&8
foobar
$ echo 20 > /sys/module/leftpad/parameters/width
$ echo 43 > /sys/module/leftpad/parameters/fill
$ exec 9<>/dev/leftpad
$ echo bazqux >&9
$ head -n 1 <&9
++++++++++++++bazqux
$ exec 10<>/dev/leftpad
$ python -c 'import fcntl; fcntl.ioctl(10, 0x800d3900, 12); fcntl.ioctl(10, 0x800d3901, ord("_"))'
$ echo zyzzy >&10
$ head -n 1 <&10
_______xyzzy
$ #
$ # \(@_@)/
$ #
Implementation Details
Each time /dev/leftpad
is opened, a ring buffer of size buffer_size
is associated with the open file.
Writing fails if there is not enough space in the buffer.
Reads happen by line, and block until there is a newline in the buffer.