Examples&Exercises:

• Complete the following codes if necessary, then compile and run the code.
• Analyze the code and output.

1. Local Sockets; http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/socket-server.csocket-server.c, http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/socket-client.csocket-client
   • Study and analyze the code.
   • During/after execution, give the following commands

     $ ./socket-server /yourdirectory/socketname
     $ ./socket-client /yourdirectory/socketname "Hello, world."
     $ ./socket-client /yourdirectory/socketname "This is a test."
     $ ./socket-client /yourdirectory/socketname "quit"
     

   • Modify the code to correct as return non-zero if the client sent a "quit" message.
2. Internet-Domain Sockets; http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/socket-inet.csocket-inet.c
   • Study and analyze the code.
   • During/after execution, give the following command

     $ ./socket-inet siber.cankaya.edu.tr
     

   • Change the content of the "buffer" (not GET),
   • Change the port number (not 80),
   • Try some other addresses (e.g., ari.cankaya.edu.tr, www.cankaya.edu.tr, www.metu.edu.tr); what could be the reason if it does not succeed?
3. About Devices; study the following commands:
   • Device Entries

     $ mknod /yourdirectory/yourdevice ? ? ?
     $ ls -l yourdevice
     $ rm /yourdirectory/yourdevice
     

   • The /dev Directory

     $ ls -l /dev/hda /dev/hda1
     $ ls -l /dev/lp0
     

     • What are the major and minor numbers of these devices?
     • How to understand a device as if block or character device?
     • What about special devices (which are not hardware devices).

     $ ls -l /dev/
     

   • Accessing Devices by Opening Files

     $ cat /usr/share/sndconfig/sample.au > /dev/audio
     $ cat yourdocument.txt > /dev/lp0
     

     • This is the general way to access to a device.
     • You should have privileged to access to printer.
     • Be Super User and try again.

     $ cat yourdocument.txt >> logfile  2>&1
     

     What about 2 and 1, which devices are these?
   • /dev/null

     $ verbose_command > /dev/null
     

     Where for the verbose_command use any command (e.g., ls).

     $ cp /dev/null empty-file
     $ ls -l empty-file
     

   • /dev/zero

     $ hexdump -v /dev/zero
     $ hexdump anyfile
     

   • /dev/full

     $ cp /etc/fstab /dev/full
     

   • Random Number Devices

     $ od -t a (d2,x1) /dev/random
     $ od -t x1 /dev/urandom
     $ man od
     

   • Loopback Devices

     $ dd if=/dev/zero of=/yourdirectory/disk-image count=20480
     $ ls -l /yourdirectory/disk-image
     $ mke2fs -q /yourdirectory/disk-image
     $ mkdir /yourdirectory/virtual-fs
     $ mount -o loop=/dev/loop0 /yourdirectory/disk-image
        /yourdirectory/virtual-fs
     $ df -h /yourdirectory/virtual-fs
     $ cd /yourdirectory/virtual-fs
     $ echo "Hello, world!" > test.txt
     $ ls -l
     $ cat test.txt
     $ cd /yourdirectory
     $ umount /yourdirectory/virtual-fs
     

4. TO BE GRADED; Write a C program to access a device by openning file.
   • Say, you are sending a file to the printer,
   • Open a device and return to a file descriptor,
   • Write some buffer to this file descriptor,
   • Test your code as priviliged user.
5. TO BE GRADED; Write a C program to use random numbers;
   • Complete the http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/random_number.ccode.