Code Snippets

This page holds random code snippets that do cool things:

This snippet creates a subprocess with a bi-directional pipe between them for communication. It can be used for (for example) wrapping a ssh connection (if you call setsid ssh will use the command defined in SSH_ASKPASS to request the password), daemonizing a process (see also Daemonize) or many other things...

 require 'socket'

 # create a new stream socket pair in the unix domain
 input, output = Socket.pair(Socket::AF_UNIX, Socket::SOCK_STREAM, 0)

 if pid = fork # forktastic
       Process.detach(pid) # if we exit, make sure the child doesn't zombie
       input.puts 'ho'
       sleep 10
       input.puts 'bye'
       puts output.gets
 else
       Process.setsid # make the forked process run in a new session
       puts output.gets
       puts output.gets
       sleep 5
       input.puts 'farewell'
 end

This snippet connects to a UNIX socket on the machine. If the server isn't running; it spawns one in a subprocess and detaches it. This is useful for robustness.

 require 'socket'

 PATH=File.join(ENV['HOME'], '.testsock')

 begin
   sock = UNIXSocket.new(PATH)
 rescue Errno::ENOENT, Errno::ECONNREFUSED => e
   File.delete(PATH) if File.socket?(PATH)
   if pid = fork()
     Process.detach(pid)
     sleep 1
     retry
   else
     server = UNIXServer.new(PATH)
     loop do
       sock = server.accept
       sock.puts("Echo: "+sock.gets)
     end
   end
 end

 sock.puts("HI!")
 puts sock.gets

Here's a GLIB event loop for an erlang port program (that communicates with the VM over stdio). The port is set up for {packet, 2} mode which means all messages (sending and receiving) are prefixed with a 2 byte (16 bit) 'length' field.

#include <sys/uio.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <glib.h>

gboolean socket_data(GIOChannel *source, GIOCondition condition, gpointer data) {
 uint16_t len;
 uint16_t inlen;
 char *inbuf;
 
 if (condition == G_IO_IN) {
  read(0, &len, 2);
  inlen = ntohs(len);
  inbuf = malloc(inlen + 1);
  
  memset(inbuf, 0, inlen+1);
 
  read(0, inbuf, inlen);
 
  write(1, &len, sizeof(uint16_t));
  write(1, inbuf, strlen(inbuf));
  free(inbuf);
 } else {
  g_main_loop_quit((GMainLoop*) data);
  return FALSE;
 }

 return TRUE;
}

int main(int argc, char** argv) {
 GMainLoop *mainloop = g_main_loop_new (NULL, FALSE);
 GIOChannel *chan = g_io_channel_unix_new(0);
 g_io_add_watch(chan, G_IO_IN | G_IO_HUP | G_IO_ERR, socket_data, mainloop);
 
 g_main_loop_run (mainloop);
 
 return 0;
}