An (abridged) Ruby Guide to
*nix Plumbing
The fun of a little knowledge applied unwisely
http://slides.games-with-brains.net

unix favours agility
only build what you need
reuse what you already have
change your tools as [des | requ]ired
use scripts and text files where possible

processes
Kernel.system
Kernel.spawn
Kernel.fork
Kernel.exec
IO.popen

files
IO.for_fd
IO.sys[open | read | write | seek]
Kernel.select
Fcntl

wrapping a file descriptor
require 'fcntl'
filemode = Fcntl::O_CREAT | Fcntl::O_RDWR | Fcntl::O_APPEND
descriptor = IO.sysopen “test.dat”, filemode
file = IO.for_fd descriptor
file.syswrite “hello”
file.sysseek 0
file.sysread 10
produces:
hello

interprocess communication
IO.pipe
sockets
Signal

accessing the *nix kernel
Kernal.syscall
ruby/dl

a fragile approach
the native syscall is a very primitive interface
error codes
file descriptors & pointer addresses
modifiable buffers
ruby syscall doesn’t support modifiable buffers
but it does wrap errors as Errno exceptions

IO.sysx the hard way
On MacOS X
file = syscall 5, “test.dat”, 1 # open O_WRONLY (fcntl.h)
syscall 4, file, “some text\\n”, 10 # write
syscall 3, file, “”, 5 # read
syscall 6, file # close
produces:
Errno::EFAULT: Bad address should be Errno::EBADF

posix semaphores
process 1 process 2
require ‘fcntl’ Open, Wait, TryWait, Post = 268, 271, 272, 273
Open, Wait, Post, Close = 268, 271, 273, 269 s = syscall Open, “/tmp/s”
s = syscall Open, “/tmp/s”, Fcntl::O_CREAT, 1911 begin
syscall Wait, s t = Time.now
puts “locked at #{Time.now}” syscall TryWait, s
sleep 50 puts “locked at #{t}”
puts “posted at #{Time.now}” rescue Exception => e
syscall Post, s puts “busy at #{t}”
syscall Close, s syscall Wait, s
puts “waited #{Time.now - t} seconds”
end
produces: produces:
locked at Thu May 28 01:03:23 +0100 2009 busy at Thu May 28 01:03:36 +0100 2009
posted at Thu May 28 01:04:13 +0100 2009 waited 47.056508 seconds

ruby/dl
part of the standard library since ruby 1.8
access to dynamically loaded libraries
.dll on windows
.so and .dylib on unix
supports C-style memory access
and callback functions written in Ruby

memory management
memory pointers encapsulated by DL::PtrData
garbage collected
uses free() or a custom deallocator
handles realloc() automatically
plays nicely with ruby strings and arrays
[String | Array].to_ptr
PtrData.struct! and PtrData.union!

require ‘dl’ file = open “test.dat”, 0x0209
CRT = DL.dlopen ‘libc.dylib’ write file, “text\\n”
F = ‘syscall’ close file
def open file, mode file = open “test.dat”, 0x0000
CRT[F, ‘IISI’].call(5, file, mode)[0] text = read file, 10
end close file
def write fd, string, bytes = string.length
CRT[F, ‘IIISI’].call(4, fd, string, bytes)[0]
end
def read fd, bytes = 1
buffer = DL.malloc(bytes)
CRT[F, ‘IIIsI’].call(3, fd, buffer, bytes)[1][2]
end
def close fd
CRT[F, ‘III’].call(6, fd)[0]
end

further reading
http://slides.games-with-brains.net
http://www.jbrowse.com/text/rdl_en.html
http://www.kegel.com/c10k.html
http://www.ecst.csuchico.edu/~beej/guide/ipc/
http://beej.us/guide/bgnet/
http://wiki.netbsd.se/kqueue_tutorial