Threads, Process and Program(Application)

http://www.qnx.com/developers/docs/6.4.1/neutrino/getting_started/s1_procs.html
http://stackoverflow.com/questions/200469/what-is-the-difference-between-a-process-and-a-thread
http://ksearch.wordpress.com/2010/09/12/program-vs-process-vs-thread/


Process:

An executing instance of a program is called a process.
Some operating systems use the term ‘task‘ to refer to a program that is being executed.
A process is always stored in the main memory also termed as the primary memory or random access memory.
Therefore, a process is termed as an active entity. It disappears if the machine is rebooted.
Several process may be associated with a same program.
On a multiprocessor system, multiple processes can be executed in parallel.
On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency.
Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.
Thread:

A thread is a subset of the process.
It is termed as a ‘lightweight process’, since it is similar to a real process but executes within the context of a process and shares the same resources allotted to the process by the kernel (See kquest.co.cc/2010/03/operating-system for more info on the term ‘kernel’).
Usually, a process has only one thread of control – one set of machine instructions executing at a time.
A process may also be made up of multiple threads of execution that execute instructions concurrently.
Multiple threads of control can exploit the true parallelism possible on multiprocessor systems.
On a uni-processor system, a thread scheduling algorithm is applied and the processor is scheduled to run each thread one at a time.
All the threads running within a process share the same address space, file descriptor, stack and other process related attributes.
Since the threads of a process share the same memory, synchronizing the access to the shared data withing the process gains unprecedented importance.