What is the difference between a process and a thread

The major difference between threads and processes is:

  1. Threads share the address space of the process that created it; processes have their own address space.

  2. Threads have direct access to the data segment of its process; processes have their own copy of the data segment of the parent process.

  3. Threads can directly communicate with other threads of its process; processes must use interprocess communication to communicate with sibling processes.

  4. Threads have almost no overhead; processes have considerable overhead.

  5. New threads are easily created; new processes require duplication of the parent process.

  6. Threads can exercise considerable control over threads of the same process; processes can only exercise control over child processes.

  7. Changes to the main thread (cancellation, priority change, etc.) may affect the behavior of the other threads of the process; changes to the parent process does not affect child processes.


Each process provides the resources needed to execute a program. A process has a virtual address space, executable code, open handles to system objects, a security context, a unique process identifier, environment variables, a priority class, minimum and maximum working set sizes, and at least one thread of execution. Each process is started with a single thread, often called the primary thread, but can create additional threads from any of its threads.


A thread is the entity within a process that can be scheduled for execution. All threads of a process share its virtual address space and system resources. In addition, each thread maintains exception handlers, a scheduling priority, thread local storage, a unique thread identifier, and a set of structures the system will use to save the thread context until it is scheduled. The thread context includes the thread's set of machine registers, the kernel stack, a thread environment block, and a user stack in the address space of the thread's process. Threads can also have their own security context, which can be used for impersonating clients.


  • 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.


  • 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.
  • 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 descriptors, 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.

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