Systems Programming 

operate and control the computer hardware and to provide a platform for running application software http://en.wikipedia.org/wiki/System_software “ 

application software “ 

web software “ 

Building Blocks The Kernel System Calls Sockets File Descriptors HTTP 

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The Kernel 

Your Code The Kernel Hardware 

Your Code 

Your Code User Mode 

Your Code Math User Mode 

Your Code Math Access Memory User Mode 

The Kernel 

The Kernel Anything 

The Kernel Anything Everything 

System Calls 

User Mode Kernel Mode 

User Mode Kernel Mode Your Program System Calls 

$ strace -p 

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120,602 Calls One Request 

linux$ wc -l syscall_table.S 326 

mov! eax, 5 mov! ebx, path mov! ecx, flags int! 80h open(path, flags) 

linux$ cat syscall_table.S | grep -c sys_ni_syscal 65 Not Implemented 

File Descriptors 

Everything is a File 

Things that are Files (Non Exhaustive) 

Things that are Files (Non Exhaustive) Files 

Things that are Files (Non Exhaustive) Links DVDs Hard Drives USB Keys Sockets* Files Directories Sound Cards Graphics Cards Printers Pipes Terminals UNIX Sockets Shared Memory 

read() write() close() lseek()* File API 

0 -> /dev/null 1 -> unicorn.log 2 -> unicorn.log 3 -> pipe:[3753224456] 4 -> pipe:[3753224456] 5 -> socket:[19730] 6 -> pipe:[3753224457] 7 -> socket:[19731] 8 -> socket:[19732] 9 -> pipe:[3753224457] 10 -> production.log 11 -> socket:[3766066290] 12 -> socket:[3753228795] 13 -> socket:[3753229266] 14 -> socket:[3753229276] 15 -> socket:[3753229285] 16 -> socket:[3753229307] 17 -> socket:[3753235718] 18 -> socket:[3753235910] 19 -> socket:[3753242775] 20 -> socket:[3753242776] 21 -> socket:[3753242778] 22 -> socket:[3753239097] 23 -> socket:[3753242942] 24 -> socket:[3753242944] 25 -> socket:[3753239202] 26 -> socket:[3753239203] 27 -> socket:[3753239204] 28 -> socket:[3753242981] 29 -> socket:[3753242982] 30 -> socket:[3753243929] 31 -> socket:[3753240046] 32 -> socket:[3753245881] 33 -> socket:[3753248970] 34 -> socket:[3753253476] 35 -> pygments-ruby.log 36 -> /dev/null 37 -> pipe:[3753260503] 38 -> pipe:[3753260504] 39 -> socket:[3753344667] 40 -> pipe:[3753260505] 41 -> socket:[3765810413] 42 -> socket:[3765810440] 43 -> socket:[3765848228] $ ls -l /proc/32660/fd 

BSD Sockets 

1980 1970 1960 1990 Multics Unix (Unics) BSD BSD 4.2 TCP/IP 

socket() bind() listen() accept() read() and write() close() 

HTTP 

HTTP Request GET /index.html HTTP/1.1 User-Agent: curl/7.24.0 Host: www.google.com Accept: */* 

HTTP Response HTTP/1.1 200 OK Date: Tue, 05 Mar 2013 23:03:52 GMT Cache-Control: private, max-age=0 Content-Type: text/html Server: gws Transfer-Encoding: chunked 

There Will be Code 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end Require the socket lib 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end Some constants for later 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end socket = Socket.new(:INET, :STREAM) 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end socket.listen(5) 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end while client_socket = socket.accept[0] 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end path = client_socket.readline.split[1] 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end path = client_socket.readline.split[1] GET /index.html HTTP/1.1 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end filename = File.expand_path("../#{path}", __FILE__) 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end if File.file?(filename) 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") OK = "HTTP/1.1 200 OK\n" 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end client_socket.write(NOT_FOUND) 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end client_socket.write(NOT_FOUND) NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end client_socket.close 

require 'socket' NOT_FOUND = "HTTP/1.1 404 Not Found\nContent-Length: 9\n\nNot Found" OK = "HTTP/1.1 200 OK\n" socket = Socket.new(:INET, :STREAM) sockaddr = Socket.sockaddr_in(11080, '127.0.0.1') socket.bind(sockaddr) socket.listen(5) while client_socket = socket.accept[0] path = client_socket.readline.split[1] filename = File.expand_path("../#{path}", __FILE__) if File.file?(filename) contents = File.read(filename) client_socket.write(OK) client_socket.write("Content-Length: #{contents.size}\n\n") client_socket.write("#{contents}") else client_socket.write(NOT_FOUND) end client_socket.close end 

def new_ipv6_server(addr, port, opt) opt.key?(:ipv6only) or return Kgio::TCPServer.new(addr, port) defined?(IPV6_V6ONLY) or abort "Socket::IPV6_V6ONLY not defined, upgrade Ruby and/or your OS" sock = Socket.new(AF_INET6, SOCK_STREAM, 0) sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, opt[:ipv6only] ? 1 : 0) sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1) sock.bind(Socket.pack_sockaddr_in(port, addr)) IO_PURGATORY << sock Kgio::TCPServer.for_fd(sock.fileno) end ret = IO.select(l, nil, SELF_PIPE, @timeout) and ready = ret[0] Unicorn 

def new_ipv6_server(addr, port, opt) opt.key?(:ipv6only) or return Kgio::TCPServer.new(addr, port) defined?(IPV6_V6ONLY) or abort "Socket::IPV6_V6ONLY not defined, upgrade Ruby and/or your OS" sock = Socket.new(AF_INET6, SOCK_STREAM, 0) sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, opt[:ipv6only] ? 1 : 0) sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1) sock.bind(Socket.pack_sockaddr_in(port, addr)) IO_PURGATORY << sock Kgio::TCPServer.for_fd(sock.fileno) end ret = IO.select(l, nil, SELF_PIPE, @timeout) and ready = ret[0] Unicorn sock = Socket.new(AF_INET6, SOCK_STREAM, 0) 

def new_ipv6_server(addr, port, opt) opt.key?(:ipv6only) or return Kgio::TCPServer.new(addr, port) defined?(IPV6_V6ONLY) or abort "Socket::IPV6_V6ONLY not defined, upgrade Ruby and/or your OS" sock = Socket.new(AF_INET6, SOCK_STREAM, 0) sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, opt[:ipv6only] ? 1 : 0) sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1) sock.bind(Socket.pack_sockaddr_in(port, addr)) IO_PURGATORY << sock Kgio::TCPServer.for_fd(sock.fileno) end ret = IO.select(l, nil, SELF_PIPE, @timeout) and ready = ret[0] Unicorn sock.bind( Socket.pack_sockaddr_in(port, addr)) 

def new_ipv6_server(addr, port, opt) opt.key?(:ipv6only) or return Kgio::TCPServer.new(addr, port) defined?(IPV6_V6ONLY) or abort "Socket::IPV6_V6ONLY not defined, upgrade Ruby and/or your OS" sock = Socket.new(AF_INET6, SOCK_STREAM, 0) sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, opt[:ipv6only] ? 1 : 0) sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1) sock.bind(Socket.pack_sockaddr_in(port, addr)) IO_PURGATORY << sock Kgio::TCPServer.for_fd(sock.fileno) end ret = IO.select(l, nil, SELF_PIPE, @timeout) and ready = ret[0] Unicorn IO.select(l, nil, SELF_PIPE, @timeout) 

The Kernel System Calls Sockets File Descriptors HTTP 

Further Reading Linux System Programming Robert Love 

Further Reading Unix Network Programming Richard Stevens 

Further Reading Advanced Programming in the UNIX Environment Richard Stevens 

Andy Delcambre adelcambre @ 

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Thanks! adelcambre @