Mutual Exclusion

A way of making sure that if one process is using a shared modifiable data, the other processes will be excluded from doing the same thing.

Formally, while one process executes the shared variable, all other processes desiring to do so at the same time moment should be kept waiting; when that process has finished executing the shared variable, one of the processes waiting; while that process has finished executing the shared variable, one of the processes waiting to do so should be allowed to proceed. In this fashion, each process executing the shared data (variables) excludes all others from doing so simultaneously. This is called Mutual Exclusion.

Note that mutual exclusion needs to be enforced only when processes access shared modifiable data - when processes are performing operations that do not conflict with one another they should be allowed to proceed concurrently.

Mutual Exclusion Conditions:

If we could arrange matters such that no two processes were ever in their critical sections simultaneously, we could avoid race conditions. We need four conditions to hold to have a good solution for the critical section problem (mutual exclusion).

• No two processes may at the same moment inside their critical sections.
• No assumptions are made about relative speeds of processes or number of CPUs.
• No process should outside its critical section should block other processes.
• No process should wait arbitrary long to enter its critical section.

Linux Kernel

Linux Kernel

Linux kernel is the very basic thing which we should have in our hand. Kernel is the one which is doing scheduling, task management, memory management and input output management. When you are getting kernel means you are getting collection of modules which are doing all above mentioned tasks. Now question is what I will get if I am purchasing kernel from market

You will get kernel as a library. Some companies are selling source code of kernel but for that you have to pay more. And royalty is also involved with some proprietary kernels.

Linux Kernel Version Numbering

According to this website at the time of writing this article, kernel release version is 2.6.34.

Let us see what these numbers are, below is the description 

-First number 2, represent linux version

-Second number 6, represent major revision-Third number 34, represent minor revision

If third number is odd, it means that this refer to unstable version and is under development. It means that when we are going to download kernel we should select those version which has even number  at third place. For example v2.6.24 is stable release and 2.6.25 is unstable release.

Here in above screen shot you can see other versions like 2.6.35-rc3, this means that 2.6.35 is a stable kernel and is under development and will be next released. And rc-3 represent release candidate -3, so we can say that this is a prerelease version.

Why should we choose Linux OS?

The reasons why most of the people chose Linux OS for the embedded device are,

·         Linux is a free and open source operating system

·         Linux can run on various kind of CPUs

·         Linux has a very well structured kernel

·         Linux is a highly secure OS

·         Linux OS never crash unless there is any hardware problems

·         Easy to write device drivers in Linux kernel

·         Wide range of software available which are supported by Linux

6 level stages of a typical Linux boot process

Press the power button on your system, and after few moments you see the Linux login prompt.

Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?

1. BIOS

   BIOS stands for Basic Input/Output System

§  Performs some system integrity checks

§  Searches, loads, and executes the boot loader program.

§  It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.

§  Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.

§  So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

§  MBR stands for Master Boot Record.

§  It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda

§  MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.

§  It contains information about GRUB (or LILO in old systems).

§  So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

§  GRUB stands for Grand Unified Bootloader.

§  If you have multiple kernel images installed on your system, you can choose which one to be executed.

§  GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

§  GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).

§  Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

§  As you notice from the above info, it contains kernel and initrd image.

§  So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

§  Mounts the root file system as specified in the “root=” in grub.conf

§  Kernel executes the /sbin/init program

§  Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.

§  initrd stands for Initial RAM Disk.

§  initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

§  Looks at the /etc/inittab file to decide the Linux run level.

§  Following are the available run levels

§  0 – halt

§  1 – Single user mode

§  2 – Multiuser, without NFS

§  3 – Full multiuser mode

§  4 – unused

§  5 – X11

§  6 – reboot

§  Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.

§  Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level

§  If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.

§  Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

§  When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.

§  Depending on your default init level setting, the system will execute the programs from one of the following directories.

§  Run level 0 – /etc/rc.d/rc0.d/

§  Run level 1 – /etc/rc.d/rc1.d/

§  Run level 2 – /etc/rc.d/rc2.d/

§  Run level 3 – /etc/rc.d/rc3.d/

§  Run level 4 – /etc/rc.d/rc4.d/

§  Run level 5 – /etc/rc.d/rc5.d/

§  Run level 6 – /etc/rc.d/rc6.d/

§  Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.

§  Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.

§  Programs starts with S are used during startup. S for startup.

§  Programs starts with K are used during shutdown. K for kill.

§  There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.

§  For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

The differences between binary semaphore and mutex are:

• Mutex is used exclusively for mutual exclusion. Both mutual exclusion and synchronization can be used by binary.
• A task that took mutex can only give mutex.
• From an ISR a mutex can not be given.
• Recursive taking of mutual exclusion semaphores is possible. This means that a task that holds before finally releasing a semaphore, can take the semaphore more than once.
• Options for making the task which takes as DELETE_SAFE are provided by Mutex, which means the task deletion is not possible when holding the mutex.

Swap Values Using Different Methods using c

Swapping of two numbers in c Using Temp Variable
#include <stdio.h>

int main()
{
   int x, y, temp;

   printf("Enter the value of x and y\n");
   scanf("%d%d", &x, &y);

   printf("Before Swapping\nx = %d\ny = %d\n",x,y);

   temp = x;
   x    = y;
   y    = temp;

   printf("After Swapping\nx = %d\ny = %d\n",x,y);

   return 0;
}

Swapping of two numbers without third variable

#include <stdio.h>

int main()
{
   int a, b;

   printf("Enter two integers to swap\n");
   scanf("%d%d", &a, &b);

   a = a + b;
   b = a - b;
   a = a - b;

   printf("a = %d\nb = %d\n",a,b);
   return 0;
}

Swap two numbers using pointers
#include <stdio.h>

int main()
{
   int x, y, *a, *b, temp;

   printf("Enter the value of x and y\n");
   scanf("%d%d", &x, &y);

   printf("Before Swapping\nx = %d\ny = %d\n", x, y);

   a = &x;
   b = &y;

   temp = *b;
   *b   = *a;
   *a   = temp;

   printf("After Swapping\nx = %d\ny = %d\n", x, y);

   return 0;
}

Swapping numbers using call by reference

#include <stdio.h>

void swap(int*, int*);

int main()
{
   int x, y;

   printf("Enter the value of x and y\n");
   scanf("%d%d",&x,&y);

   printf("Before Swapping\nx = %d\ny = %d\n", x, y);

   swap(&x, &y);

   printf("After Swapping\nx = %d\ny = %d\n", x, y);

   return 0;
}

void swap(int *a, int *b)
{
   int temp;

   temp = *b;
   *b   = *a;
   *a   = temp;  
}

C programming code to swap using bitwise XOR
#include <stdio.h>

int main()
{
  int x, y;

  scanf("%d%d", &x, &y);

  printf("x = %d\ny = %d\n", x, y);

  x = x ^ y;
  y = x ^ y;
  x = x ^ y;

  printf("x = %d\ny = %d\n", x, y);

  return 0;
}