What is a Program?
A program is an executable file residing on the disk (secondary storage) in a directory. It is also termed as a set of instructions stored in the secondary storage device that are intended to carry out a specific job. It is read into the primary memory and executed by the kernel.
Therefore, a program is termed as a ‘passive entity’ which exists in the secondary storage persistently even if the machine reboots.
On a Microsoft Windows® system: The ‘Calculator’ program is stored at “:\windows\system32\calc.exe”.
On a Linux system: The ‘ls’ program is available at: “/bin/ls”.
What is a 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 termed as an ‘active entity’ since it is always stored in the main memory and disappears if the machine is power cycled. 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 producing an illusion of concurrency.
Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.
What is a thread?
A thread is called a ‘lightweight process’. 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.
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 within the process gains unprecedented importance.
What is the relationship of a thread, a process and a program?
The below image portrays the relationship between a process and threads. A process can be composed of several threads executing in parallel.
The image below depicts the relationship between a program and processes. A program can be subdivided into multiple processes executing in parallel.
Consider an example: Microsoft Word is a massive program that when loaded into the primary memory in-turn loads several processes which in-turn create several threads.
Advanced Programming in UNIX Environment, Second Edition (by W. Richard Stevens and Stephen A. Rago) – http://www.apuebook.com/index.html