What is the difference between a process and a thread

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What is the technical difference between a process and a thread   I get the feeling a word like  process  is overused and there are also hardware and software threads  How about light-weight processes in languages like Erlang  Is there a definitive reason to use one term over the other

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An application consists of one or more processes. A process, in the simplest terms, is an executing program. One or more threads run in the context of the process. A thread is the basic unit to which the operating system allocates processor time. A thread can execute any part of the process code, including parts currently being executed by another thread. A fiber is a unit of execution that must be manually scheduled by the application. Fibers run in the context of the threads that schedule them.

Stolen from here.

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Trying to answer it from Linux Kernel s OS View  A program becomes a process when launched into memory  A process has its own address space meaning having various segments in memory such as  text segement for storing compiled code   bss for storing uninitialized static or global variables  etc  Each process would have its own program counter and user-space stack  Inside kernel  each process would have its own kernel stack  which is separated from user space stack for security issues  and a structure named task struct which is generally abstracted as the process control block  storing all the information regarding the process such as its priority  state  and a whole lot of other chunk   A process can have multiple threads of execution   Coming to threads  they reside inside a process and share the address space of the parent process along with other resources which can be passed during thread creation such as filesystem resources  sharing pending signals  sharing data variables and instructions  therefore making threads lightweight and hence allowing faster context switching  Inside kernel  each thread has its own kernel stack along with the task struct structure which defines the thread  Therefore kernel views threads of same process as different entities and are schedulable in themselves  Threads in same process share a common id called as thread group id tgid   also they have a unique id called as the process id  pid

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Both threads and processes are atomic units of OS resource allocation  i e  there is a concurrency model describing how CPU time is divided between them  and the model of owning other OS resources   There is a difference in    Shared resources  threads are sharing memory by definition  they do not own anything except stack and local variables  processes could also share memory  but there is a separate mechanism for that  maintained by OS  Allocation space  kernel space for processes vs  user space for threads    Greg Hewgill above was correct about the Erlang meaning of the word  process   and here there s a discussion of why Erlang could do processes lightweight

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An application consists of one or more processes. A process, in the simplest terms, is an executing program. One or more threads run in the context of the process. A thread is the basic unit to which the operating system allocates processor time. A thread can execute any part of the process code, including parts currently being executed by another thread. A fiber is a unit of execution that must be manually scheduled by the application. Fibers run in the context of the threads that schedule them.

Stolen from here.

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While building an algorithm in Python  interpreted language  that incorporated multi-threading I was surprised to see that execution time was not any better when compared to the sequential algorithm I had previously built  In an effort to understand the reason for this result I did some reading  and believe what I learned offers an interesting context from which to better understand the differences between multi-threading and multi-processes    Multi-core systems may exercise multiple threads of execution  and so Python should support multi-threading  But Python is not a compiled language and instead is an interpreted language1  This means that the program must be interpreted in order to run  and the interpreter is not aware of the program before it begins execution  What it does know  however  are the rules of Python and it then dynamically applies those rules  Optimizations in Python must then be principally optimizations of the interpreter itself  and not the code that is to be run  This is in contrast to compiled languages such as C    and has consequences for multi-threading in Python  Specifically  Python uses the Global Interpreter Lock to manage multi-threading   On the other hand a compiled language is  well  compiled  The program is processed  entirely   where first it is interpreted according to its syntactical definitions  then mapped to a language agnostic intermediate representation  and finally linked into an executable code  This process allows the code to be highly optimized because it is all available at the time of compilation  The various program interactions and relationships are defined at the time the executable is created and robust decisions about optimization can be made   In modern environments Python s interpreter must permit multi-threading  and this must both be safe and efficient  This is where the difference between being an interpreted language versus a compiled language enters the picture  The interpreter must not to disturb internally shared data from different threads  while at the same time optimizing the use of processors for computations   As has been noted in the previous posts both a process and a thread are independent sequential executions with the primary difference being that memory is shared across multiple threads of a process  while processes isolate their  memory spaces   In Python data is protected from simultaneous access by different threads by the Global Interpreter Lock  It requires that in any Python program only one thread can be executed at any time  On the other hand it is possible to run multiple processes since the memory for each process is isolated from any other process  and processes can run on multiple cores     1 Donald Knuth has a good explanation of interpretive routines in The Art of Computer Programming  Fundamental Algorithms

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Basically  a thread is a part of a process without process thread wouldn t able to work   A thread is lightweight whereas the process is heavyweight  communication between process requires some Time whereas thread requires less time  Threads can share the same memory area whereas process lives in separate

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Threads within the same process share the Memory  but each thread has its own stack and registers  and threads store thread-specific data in the heap  Threads never execute independently  so the inter-thread communication is much faster when compared to inter-process communication   Processes never share the same memory  When a child process creates it duplicates the memory location of the parent process  Process communication is done by using pipe  shared memory  and message parsing  Context switching between threads is very slow

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Difference between Thread and Process   A process is an executing instance of an application and A thread is a path of execution within a process  Also  a process can contain multiple threads It   s important to note that a thread can do anything a process can do  But since a process can consist of multiple threads  a thread could be considered a    lightweight    process  Thus  the essential difference between a thread and a process is the work that each one is used to accomplish  Threads are used for small tasks  whereas processes are used for more    heavyweight    tasks     basically the execution of applications   Another difference between a thread and a process is that threads within the same process share the same address space  whereas different processes do not  This allows threads to read from and write to the same data structures and variables  and also facilitates communication between threads  Communication between processes     also known as IPC  or inter-process communication     is quite difficult and resource-intensive      Here   s a summary of the differences between threads and processes     Threads are easier to create than processes since they  don t require a separate address space  Multithreading requires careful programming since threads  share data strucures that should only be modified by one thread at a time   Unlike threads  processes don t share the same  address space  Threads are considered lightweight because they use far  less resources than processes  Processes are independent of each other   Threads  since they  share the same address space are interdependent  so caution  must be taken so that different threads don t step on each other  This is really another way of stating  2 above  A process can consist of multiple threads

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Consider process like a unit of ownership or what resources are needed by a task  A Process can have resources like memory space  specific input output  specific files  and priority etc   A thread is a dispatchable unit of execution or in simple words the progress through a sequence of instructions

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Thread is a light weight process while  the process is a self contained execution environment  What is meant by  quot self contained execution process quot   Private set of basic runtime resources  What is meant by  quot private set of basic runtime resources quot   The space allocated from memory to run the the process  Simply a memory space

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Example 1  A JVM runs in a single process and threads in a JVM share the heap belonging to that process  That is why several threads may access the same object  Threads share the heap and have their own stack space  This is how one thread   s invocation of a method and its local variables are kept thread safe from other threads  But the heap is not thread-safe and must be synchronized for thread safety

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Process     Process is a heavy weight process  Process is a separate program that has separate memory data resources ect  Process are created using fork   method  Context switch between the process is time consuming     Example   Say  opening any browser  mozilla  Chrome  IE   At this point new process will start to execute   Threads    Threads are light weight processes Threads are bundled inside the process  Threads have a shared memory data resources files etc  Threads are created using clone   method  Context switch between the threads are not much time consuming as Process    Example   Opening multiple tabs in the browser

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An application consists of one or more processes. A process, in the simplest terms, is an executing program. One or more threads run in the context of the process. A thread is the basic unit to which the operating system allocates processor time. A thread can execute any part of the process code, including parts currently being executed by another thread. A fiber is a unit of execution that must be manually scheduled by the application. Fibers run in the context of the threads that schedule them.

Stolen from here.

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Process - Program in execution  Thread  - a thread is execution of the smallest sequence of programmed instructions   Eg- you want to calculate matrix multiplication you will write a program of 3 for loops inside  main and execute it   Now this is your process   Now the same program you can solve by creating threads and assigning each thread to execute result of row  Each thread will work independently and the result will stored in an array   As the threads share the same memory inside a process    In Both the cases result will be same

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A process is a collection of code  memory  data and other resources  A thread is a sequence of code that is executed within the scope of the process  You can  usually  have multiple threads executing concurrently within the same process

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Trying to answer it from Linux Kernel s OS View  A program becomes a process when launched into memory  A process has its own address space meaning having various segments in memory such as  text segement for storing compiled code   bss for storing uninitialized static or global variables  etc  Each process would have its own program counter and user-space stack  Inside kernel  each process would have its own kernel stack  which is separated from user space stack for security issues  and a structure named task struct which is generally abstracted as the process control block  storing all the information regarding the process such as its priority  state  and a whole lot of other chunk   A process can have multiple threads of execution   Coming to threads  they reside inside a process and share the address space of the parent process along with other resources which can be passed during thread creation such as filesystem resources  sharing pending signals  sharing data variables and instructions  therefore making threads lightweight and hence allowing faster context switching  Inside kernel  each thread has its own kernel stack along with the task struct structure which defines the thread  Therefore kernel views threads of same process as different entities and are schedulable in themselves  Threads in same process share a common id called as thread group id tgid   also they have a unique id called as the process id  pid

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An application consists of one or more processes. A process, in the simplest terms, is an executing program. One or more threads run in the context of the process. A thread is the basic unit to which the operating system allocates processor time. A thread can execute any part of the process code, including parts currently being executed by another thread. A fiber is a unit of execution that must be manually scheduled by the application. Fibers run in the context of the threads that schedule them.

Stolen from here.

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Both processes and threads are independent sequences of execution  The typical difference is that threads  of the same process  run in a shared memory space  while processes run in separate memory spaces   I m not sure what  hardware  vs  software  threads you might be referring to  Threads are an operating environment feature  rather than a CPU feature  though the CPU typically has operations that make threads efficient    Erlang uses the term  process  because it does not expose a shared-memory multiprogramming model  Calling them  threads  would imply that they have shared memory

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First  let s look at the theoretical aspect   You need to understand what a process is conceptually to understand the difference between a process and a thread and what s shared between them  We have the following in section 2 2 2 The Classical Thread Model of Modern Operating Systems 3e by Tanenbaum   The process model is based on two independent concepts  resource grouping and execution  Sometimes it is use  ful to separate them  this is where threads come in      He continues   One way of looking at a process is that it is a way to group related resources together  A process has an address space containing program text and data  as well as other resources  These resource may include open files  child processes  pending alarms  signal handlers  accounting information  and more  By putting them together in the form of a process  they can be managed more easily  The other concept a process has is a thread of execution  usually shortened to just thread  The thread has a program counter that keeps track of which instruc  tion to execute next  It has registers  which hold its current working variables  It has a stack  which contains the execution history  with one frame for each proce  dure called but not yet returned from  Although a thread must execute in some process  the thread and its process are different concepts and can be treated sepa  rately  Processes are used to group resources together  threads are the entities scheduled for execution on the CPU   Further down he provides the following table  Per process items               Per thread items ------------------------------ ----------------- Address space                   Program counter Global variables                Registers Open files                      Stack Child processes                 State Pending alarms                  Signals and signal handlers     Accounting information           Let s deal with the hardware multithreading issue   Classically  a CPU would support a single thread of execution  maintaining the thread s state via a single program counter  PC   and set of registers   But what happens when there s a cache miss   It takes a long time to fetch data from main memory  and while that s happening the CPU is just sitting there idle   So someone had the idea to basically have two sets of thread state  PC   registers  so that another thread  maybe in the same process  maybe in a different process  can get work done while the other thread is waiting on main memory   There are multiple names and implementations of this concept  such as Hyper-threading and simultaneous multithreading  SMT for short   Now let s look at the software side   There are basically three ways that threads can be implemented on the software side   User space threads Kernel threads A combination of the two  All you need to implement threads is the ability to save the CPU state and maintain multiple stacks  which can in many cases be done in user space   The advantage of user space threads is super fast thread switching since you don t have to trap into the kernel and the ability to schedule your threads the way you like   The biggest drawback is the inability to do blocking I O  which would block the entire process and all its user threads    which is one of the big reasons we use threads in the first place   Blocking I O using threads greatly simplifies program design in many cases  Kernel threads have the advantage of being able to use blocking I O  in addition to leaving all the scheduling issues to the OS   But each thread switch requires trapping into the kernel which is potentially relatively slow   However  if you re switching threads because of blocked I O this isn t really an issue since the I O operation probably trapped you into the kernel already anyway  Another approach is to combine the two  with multiple kernel threads each having multiple user threads  So getting back to your question of terminology  you can see that a process and a thread of execution are two different concepts and your choice of which term to use depends on what you re talking about   Regarding the term  quot light weight process quot   I don t personally see the point in it since it doesn t really convey what s going on as well as the term  quot thread of execution quot

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The following is what I got from one of the articles on The Code Project  I guess it explains everything needed clearly      A thread is another mechanism for splitting the workload into separate   execution streams  A thread is lighter weight than a process  This   means  it offers less flexibility than a full blown process  but can   be initiated faster because there is less for the Operating System to   set up  When a program consists of two or more threads  all the   threads share a single memory space  Processes are given separate address spaces    all the threads share a single heap  But each thread is given its own stack

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Process 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   Thread A thread is an 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     This information was found on Microsoft Docs here  About Processes and Threads          Microsoft Windows supports preemptive multitasking  which creates the effect of simultaneous execution of multiple threads from multiple processes  On a multiprocessor computer  the system can simultaneously execute as many threads as there are processors on the computer

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Both processes and threads are independent sequences of execution  The typical difference is that threads  of the same process  run in a shared memory space  while processes run in separate memory spaces               Process  Is a program in execution  it has text section i e the program code  current activity as represented by the value of program counter  amp  content of processors register  It also includes the process stack that contains temporary data such as function parameters  return addressed and local variables   and a data section  which contains global variables  A process may also include a heap  which is memory that is dynamically allocated during process run time   Thread  A thread is a basic unit of CPU utilisation  it comprises a thread ID  a program counter  register set  and a stack  it shared with other threads belonging to the same process its code section  data section and other operating system resources such as open files and signals   -- Taken from Operating System by Galvin

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Consider process like a unit of ownership or what resources are needed by a task  A Process can have resources like memory space  specific input output  specific files  and priority etc   A thread is a dispatchable unit of execution or in simple words the progress through a sequence of instructions

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Both threads and processes are atomic units of OS resource allocation  i e  there is a concurrency model describing how CPU time is divided between them  and the model of owning other OS resources   There is a difference in    Shared resources  threads are sharing memory by definition  they do not own anything except stack and local variables  processes could also share memory  but there is a separate mechanism for that  maintained by OS  Allocation space  kernel space for processes vs  user space for threads    Greg Hewgill above was correct about the Erlang meaning of the word  process   and here there s a discussion of why Erlang could do processes lightweight

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From the point of view of an interviewer  there are basically just 3 main things that I want to hear  besides obvious things like a process can have multiple threads    Threads share same memory space  which means a thread can access memory from other s thread memory  Processes normally can not  Resources  Resources  memory  handles  sockets  etc  are release at process termination  not thread termination  Security  A process has a fixed security token  A thread  on the other hand  can impersonate different users tokens    If you want more  Scott Langham s response pretty much covers everything  All these are from the perspective of an operating system  Different languages can implement different concepts  like tasks  light-wigh threads and so on  but they are just ways of using threads  of fibers on Windows   There are no hardware and software threads  There are hardware and software exceptions and interrupts  or user-mode and kernel threads

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Process   Process is basically a program in execution  It is an active entity  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  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    ref-https   practice geeksforgeeks org problems difference-between-process-and-thread

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To explain more with respect to concurrent programming  A process has a self-contained execution environment  A process generally has a complete  private set of basic run-time resources  in particular  each process has its own memory space   Threads exist within a process     every process has at least one  Threads share the process s resources  including memory and open files  This makes for efficient  but potentially problematic  communication    An example keeping the average person in mind  On your computer  open Microsoft Word and a web browser  We call these two processes  In Microsoft Word  you type something and it gets automatically saved  Now  you have observed editing and saving happens in parallel - editing on one thread and saving on the other thread

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Process  program under execution is known as process  Thread   Thread is a functionality which is executed with the other part of the program based on the concept of  one with other so thread is a part of process

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I believe the easiest way to understand the difference is to visualise how threads and processes execute their jobs      Threads are running in parallel  in a shared memory space  of the process that created them    Thread 1              Thread 2              Thread 3                                                                                                                                                                                                                                                                                                                              Complete             Complete              Complete   Note  The above can be interpreted as a process  i e  one process with 3 threads     Processes are running in parallel and concurrently    Process 1              Process 2              Process 3                                                                                                                                                                                                                                                                                                                         Complete               Complete               Complete

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Process  program under execution is known as process  Thread   Thread is a functionality which is executed with the other part of the program based on the concept of  one with other so thread is a part of process

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From the point of view of an interviewer  there are basically just 3 main things that I want to hear  besides obvious things like a process can have multiple threads    Threads share same memory space  which means a thread can access memory from other s thread memory  Processes normally can not  Resources  Resources  memory  handles  sockets  etc  are release at process termination  not thread termination  Security  A process has a fixed security token  A thread  on the other hand  can impersonate different users tokens    If you want more  Scott Langham s response pretty much covers everything  All these are from the perspective of an operating system  Different languages can implement different concepts  like tasks  light-wigh threads and so on  but they are just ways of using threads  of fibers on Windows   There are no hardware and software threads  There are hardware and software exceptions and interrupts  or user-mode and kernel threads

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A process is a collection of code  memory  data and other resources  A thread is a sequence of code that is executed within the scope of the process  You can  usually  have multiple threads executing concurrently within the same process

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I ve perused almost all answers there  alas  as an undergraduate student taking OS course currently I can t comprehend thoroughly the two concepts  I mean most of guys read from some OS books the differences i e  threads are able to access to global variables in the transaction unit since they make use of their process  address space  Yet  the newly question arises why there are processes  cognizantly we know already threads are more lightweight vis-  -vis processes  Let s glance at the following example by making use of the image excerpted from one of the prior answers    We have 3 threads working at once on a word document e g  Libre Office  The first does spellchecking by underlining if the word is misspelt  The second takes and prints letters from keyboard  And the last does save document in every short times not to lose the document worked at if something goes wrong  In this case  the 3 threads cannot be 3 processes since they share a common memory which is the address space of their process and thus all have access to the document being edited  So  the road is the word document along with two bulldozers which are the threads though one of them is lack in the image

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Both processes and threads are independent sequences of execution  The typical difference is that threads  of the same process  run in a shared memory space  while processes run in separate memory spaces   I m not sure what  hardware  vs  software  threads you might be referring to  Threads are an operating environment feature  rather than a CPU feature  though the CPU typically has operations that make threads efficient    Erlang uses the term  process  because it does not expose a shared-memory multiprogramming model  Calling them  threads  would imply that they have shared memory

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Process 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   Thread A thread is an 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     This information was found on Microsoft Docs here  About Processes and Threads          Microsoft Windows supports preemptive multitasking  which creates the effect of simultaneous execution of multiple threads from multiple processes  On a multiprocessor computer  the system can simultaneously execute as many threads as there are processors on the computer

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Example 1  A JVM runs in a single process and threads in a JVM share the heap belonging to that process  That is why several threads may access the same object  Threads share the heap and have their own stack space  This is how one thread   s invocation of a method and its local variables are kept thread safe from other threads  But the heap is not thread-safe and must be synchronized for thread safety

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http   lkml iu edu hypermail linux kernel 9608 0191 html     Linus Torvalds  torvalds cs helsinki fi       Tue  6 Aug 1996 12 47 31  0300  EET DST       Messages sorted by    date    thread    subject    author        Next message  Bernd P  Ziller   Re  Oops in get hash table       Previous message  Linus Torvalds   Re  I O request ordering       On Mon  5 Aug 1996  Peter P  Eiserloh wrote            We need to keep a clear the concept of threads  Too many people     seem to confuse a thread with a process  The following discussion     does not reflect the current state of linux  but rather is an     attempt to stay at a high level discussion          NO       There is NO reason to think that  threads  and  processes  are   separate entities  That s how it s traditionally done  but I   personally think it s a major mistake to think that way  The only   reason to think that way is historical baggage        Both threads and processes are really just one thing  a  context of   execution   Trying to artificially distinguish different cases is just   self-limiting        A  context of execution   hereby called COE  is just the conglomerate   of  all the state of that COE  That state includes things like CPU   state   registers etc   MMU state  page mappings   permission state    uid  gid   and various  communication states   open files  signal   handlers etc   Traditionally  the difference between a  thread  and a    process  has been mainly that a threads has CPU state    possibly   some other minimal state   while all the other context comes from the   process  However  that s just   one way of dividing up the total state of the COE  and there is nothing that says that it s the right way to do it  Limiting yourself   to that kind of image is just plain stupid        The way Linux thinks about this  and the way I want things to work  is   that there is no such thing as a  process  or a  thread   There is   only the totality of the COE  called  task  by Linux   Different COE s   can share parts of their context with each other  and one subset of   that sharing is the traditional  thread   process  setup  but that   should really be seen as ONLY a subset  it s an important subset  but   that importance comes not from design  but from standards  we obviusly   want to run standards-conforming threads programs on top of Linux   too         In short  do NOT design around the thread process way of thinking  The   kernel should be designed around the COE way of thinking  and then the   pthreads library can export the limited pthreads interface to users   who  want to use that way of looking at COE s       Just as an example of what becomes possible when you think COE as   opposed  to thread process          You can do a external  cd  program  something that is traditionally impossible in UNIX and or process thread  silly example  but the idea    is that you can have these kinds of  modules  that aren t limited to    the traditional UNIX threads setup   Do a          clone CLONE VM CLONE FS        child  execve  external-cd            the  execve    will disassociate the VM  so the only reason we    used CLONE VM was to make the act of cloning faster            You can do  vfork    naturally  it meeds minimal kernel support  but  that support fits the CUA way of thinking perfectly           clone CLONE VM        child  continue to run  eventually execve        mother  wait for execve         you can do external  IO deamons           clone CLONE FILES        child  open file descriptors etc      mother  use the fd s the child opened and vv       All of the above work because you aren t tied to the thread process   way of thinking  Think of a web server for example  where the CGI   scripts are done as  threads of execution   You can t do that with   traditional threads  because traditional threads always have to share   the whole address space  so you d have to link in everything you ever   wanted to do in the web server itself  a  thread  can t run another   executable         Thinking of this as a  context of execution  problem instead  your   tasks can now chose to execute external programs    separate the   address space from the parent  etc if they want to  or they can for   example share everything with the parent except for the file   descriptors  so that the sub- threads  can open lots of files without   the parent needing to worry about them  they close automatically when   the sub- thread  exits  and it doesn t use up fd s in the parent         Think of a threaded  inetd   for example  You want low overhead   fork exec  so with the Linux way you can instead of using a  fork      you write a multi-threaded inetd where each thread is created with   just CLONE VM  share address space  but don t share file descriptors   etc   Then the child can execve if it was a external service  rlogind    for example   or maybe it was one of the internal inetd services    echo  timeofday  in which case it just does it s thing and exits        You can t do that with  thread   process        Linus

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From Erlang Programming  2009   Erlang concurrency is fast and scalable  Its processes are lightweight in that the Erlang virtual machine does not create an OS thread for every created process  They are created  scheduled  and handled in the VM  independent of underlying operating system   Erlang implements a preemptive scheduler  which allows each process to run for a set period of time without blocking a system thread for too long  which gives each process some cpu time to be executed  The number of system threads depends on the number of cores if I m not mistaking  and processes can be removed from one thread and moved to another if the load becomes uneven  this is all handled by the Erlang scheduler

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The best answer I ve found so far is Michael Kerrisk s  The Linux Programming Interface       In modern UNIX implementations  each process can have multiple threads   of execution  One way of envisaging threads is as a set of processes   that share the same virtual memory  as well as a range of other   attributes  Each thread is executing the same program code and shares   the same data area and heap  However  each thread has it own stack   containing local variables and function call linkage information   LPI 2 12    This book is a source of great clarity  Julia Evans mentioned its help in clearing up how Linux groups really work in this article

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Both threads and processes are atomic units of OS resource allocation  i e  there is a concurrency model describing how CPU time is divided between them  and the model of owning other OS resources   There is a difference in    Shared resources  threads are sharing memory by definition  they do not own anything except stack and local variables  processes could also share memory  but there is a separate mechanism for that  maintained by OS  Allocation space  kernel space for processes vs  user space for threads    Greg Hewgill above was correct about the Erlang meaning of the word  process   and here there s a discussion of why Erlang could do processes lightweight

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Process 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   Thread A thread is an 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     This information was found on Microsoft Docs here  About Processes and Threads          Microsoft Windows supports preemptive multitasking  which creates the effect of simultaneous execution of multiple threads from multiple processes  On a multiprocessor computer  the system can simultaneously execute as many threads as there are processors on the computer

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For those who are more comfortable with learning by visualizing  here is a handy diagram I created to explain Process and Threads  I used the information from MSDN - About Processes and Threads

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They are almost as same    But the key difference is a thread is lightweight and a process is heavy-weight in terms of context switching  work load and so on

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To explain more with respect to concurrent programming  A process has a self-contained execution environment  A process generally has a complete  private set of basic run-time resources  in particular  each process has its own memory space   Threads exist within a process     every process has at least one  Threads share the process s resources  including memory and open files  This makes for efficient  but potentially problematic  communication    An example keeping the average person in mind  On your computer  open Microsoft Word and a web browser  We call these two processes  In Microsoft Word  you type something and it gets automatically saved  Now  you have observed editing and saving happens in parallel - editing on one thread and saving on the other thread

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Difference between process and thread are given below     Process is an executing instance of a program whereas Thread is the smallest unit of process   Process can be divided into multiple threads whereas Thread can not be divided  Process may be considered as a task whereas Thread may be considered as a task lightweight process  Process allocate separate memory space whereas Thread allocate shared memory space  Process is maintained by operating system whereas Thread is maintained by programmer

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Every process is a thread  primary thread   But every thread is not a process  It is a part entity  of a process

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Coming from the embedded world  I would like to add that the concept of processes only exists in  big  processors  desktop CPUs  ARM Cortex A-9  that have MMU  memory management unit    and operating systems that support using MMUs  such as Linux   With small old processors and microcontrollers and small RTOS operating system  real time operating system   such as freeRTOS  there is no MMU support and thus no processes but only threads   Threads can access each others memory  and they are scheduled by OS in an interleaved manner so they appear to run in parallel  or with multi-core they really run in parallel    Processes  on the other hand  live in their private sandbox of virtual memory  provided and guarded by MMU  This is handy because it enables     keeping buggy process from crashing the entire system  Maintaining security by making other processes data invisible and unreachable   The actual work inside the process is taken care by one or more threads

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Process     Process is a heavy weight process  Process is a separate program that has separate memory data resources ect  Process are created using fork   method  Context switch between the process is time consuming     Example   Say  opening any browser  mozilla  Chrome  IE   At this point new process will start to execute   Threads    Threads are light weight processes Threads are bundled inside the process  Threads have a shared memory data resources files etc  Threads are created using clone   method  Context switch between the threads are not much time consuming as Process    Example   Opening multiple tabs in the browser

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Real world example for Process and Thread This will give you the basic idea about thread and process   I borrowed the above info from Scott Langham s Answer - thanks

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Process   Process is basically a program in execution  It is an active entity  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  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    ref-https   practice geeksforgeeks org problems difference-between-process-and-thread

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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  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 within the process gains unprecedented importance    I borrowed the above info from the Knowledge Quest  blog

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Every process is a thread  primary thread   But every thread is not a process  It is a part entity  of a process

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Both processes and threads are independent sequences of execution  The typical difference is that threads  of the same process  run in a shared memory space  while processes run in separate memory spaces   I m not sure what  hardware  vs  software  threads you might be referring to  Threads are an operating environment feature  rather than a CPU feature  though the CPU typically has operations that make threads efficient    Erlang uses the term  process  because it does not expose a shared-memory multiprogramming model  Calling them  threads  would imply that they have shared memory

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I ve perused almost all answers there  alas  as an undergraduate student taking OS course currently I can t comprehend thoroughly the two concepts  I mean most of guys read from some OS books the differences i e  threads are able to access to global variables in the transaction unit since they make use of their process  address space  Yet  the newly question arises why there are processes  cognizantly we know already threads are more lightweight vis-  -vis processes  Let s glance at the following example by making use of the image excerpted from one of the prior answers    We have 3 threads working at once on a word document e g  Libre Office  The first does spellchecking by underlining if the word is misspelt  The second takes and prints letters from keyboard  And the last does save document in every short times not to lose the document worked at if something goes wrong  In this case  the 3 threads cannot be 3 processes since they share a common memory which is the address space of their process and thus all have access to the document being edited  So  the road is the word document along with two bulldozers which are the threads though one of them is lack in the image

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While building an algorithm in Python  interpreted language  that incorporated multi-threading I was surprised to see that execution time was not any better when compared to the sequential algorithm I had previously built  In an effort to understand the reason for this result I did some reading  and believe what I learned offers an interesting context from which to better understand the differences between multi-threading and multi-processes    Multi-core systems may exercise multiple threads of execution  and so Python should support multi-threading  But Python is not a compiled language and instead is an interpreted language1  This means that the program must be interpreted in order to run  and the interpreter is not aware of the program before it begins execution  What it does know  however  are the rules of Python and it then dynamically applies those rules  Optimizations in Python must then be principally optimizations of the interpreter itself  and not the code that is to be run  This is in contrast to compiled languages such as C    and has consequences for multi-threading in Python  Specifically  Python uses the Global Interpreter Lock to manage multi-threading   On the other hand a compiled language is  well  compiled  The program is processed  entirely   where first it is interpreted according to its syntactical definitions  then mapped to a language agnostic intermediate representation  and finally linked into an executable code  This process allows the code to be highly optimized because it is all available at the time of compilation  The various program interactions and relationships are defined at the time the executable is created and robust decisions about optimization can be made   In modern environments Python s interpreter must permit multi-threading  and this must both be safe and efficient  This is where the difference between being an interpreted language versus a compiled language enters the picture  The interpreter must not to disturb internally shared data from different threads  while at the same time optimizing the use of processors for computations   As has been noted in the previous posts both a process and a thread are independent sequential executions with the primary difference being that memory is shared across multiple threads of a process  while processes isolate their  memory spaces   In Python data is protected from simultaneous access by different threads by the Global Interpreter Lock  It requires that in any Python program only one thread can be executed at any time  On the other hand it is possible to run multiple processes since the memory for each process is isolated from any other process  and processes can run on multiple cores     1 Donald Knuth has a good explanation of interpretive routines in The Art of Computer Programming  Fundamental Algorithms

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Coming from the embedded world  I would like to add that the concept of processes only exists in  big  processors  desktop CPUs  ARM Cortex A-9  that have MMU  memory management unit    and operating systems that support using MMUs  such as Linux   With small old processors and microcontrollers and small RTOS operating system  real time operating system   such as freeRTOS  there is no MMU support and thus no processes but only threads   Threads can access each others memory  and they are scheduled by OS in an interleaved manner so they appear to run in parallel  or with multi-core they really run in parallel    Processes  on the other hand  live in their private sandbox of virtual memory  provided and guarded by MMU  This is handy because it enables     keeping buggy process from crashing the entire system  Maintaining security by making other processes data invisible and unreachable   The actual work inside the process is taken care by one or more threads

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Trying to answer this question relating to Java world   A process is an execution of a program but a thread is a single execution sequence within the process  A process can contain multiple threads  A thread is sometimes called a lightweight process   For example   Example 1  A JVM runs in a single process and threads in a JVM share the heap belonging to that process  That is why several threads may access the same object  Threads share the heap and have their own stack space  This is how one thread   s invocation of a method and its local variables are kept thread safe from other threads  But the heap is not thread-safe and must be synchronized for thread safety   Example 2  A program might not be able to draw pictures by reading keystrokes  The program must give its full attention to the keyboard input and lacking the ability to handle more than one event at a time will lead to trouble  The ideal solution to this problem is the seamless execution of two or more sections of a program at the same time  Threads allows us to do this  Here Drawing picture is a process and reading keystroke is sub process  thread

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From Erlang Programming  2009   Erlang concurrency is fast and scalable  Its processes are lightweight in that the Erlang virtual machine does not create an OS thread for every created process  They are created  scheduled  and handled in the VM  independent of underlying operating system   Erlang implements a preemptive scheduler  which allows each process to run for a set period of time without blocking a system thread for too long  which gives each process some cpu time to be executed  The number of system threads depends on the number of cores if I m not mistaking  and processes can be removed from one thread and moved to another if the load becomes uneven  this is all handled by the Erlang scheduler

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Both processes and threads are independent sequences of execution  The typical difference is that threads  of the same process  run in a shared memory space  while processes run in separate memory spaces               Process  Is a program in execution  it has text section i e the program code  current activity as represented by the value of program counter  amp  content of processors register  It also includes the process stack that contains temporary data such as function parameters  return addressed and local variables   and a data section  which contains global variables  A process may also include a heap  which is memory that is dynamically allocated during process run time   Thread  A thread is a basic unit of CPU utilisation  it comprises a thread ID  a program counter  register set  and a stack  it shared with other threads belonging to the same process its code section  data section and other operating system resources such as open files and signals   -- Taken from Operating System by Galvin

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A thread runs in a shared memory space  but a process runs in a separate memory space A thread is a light-weight process  but a process is a heavy-weight process  A thread is a subtype of process

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For those who are more comfortable with learning by visualizing  here is a handy diagram I created to explain Process and Threads  I used the information from MSDN - About Processes and Threads

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The following is what I got from one of the articles on The Code Project  I guess it explains everything needed clearly      A thread is another mechanism for splitting the workload into separate   execution streams  A thread is lighter weight than a process  This   means  it offers less flexibility than a full blown process  but can   be initiated faster because there is less for the Operating System to   set up  When a program consists of two or more threads  all the   threads share a single memory space  Processes are given separate address spaces    all the threads share a single heap  But each thread is given its own stack

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First  let s look at the theoretical aspect   You need to understand what a process is conceptually to understand the difference between a process and a thread and what s shared between them  We have the following in section 2 2 2 The Classical Thread Model of Modern Operating Systems 3e by Tanenbaum   The process model is based on two independent concepts  resource grouping and execution  Sometimes it is use  ful to separate them  this is where threads come in      He continues   One way of looking at a process is that it is a way to group related resources together  A process has an address space containing program text and data  as well as other resources  These resource may include open files  child processes  pending alarms  signal handlers  accounting information  and more  By putting them together in the form of a process  they can be managed more easily  The other concept a process has is a thread of execution  usually shortened to just thread  The thread has a program counter that keeps track of which instruc  tion to execute next  It has registers  which hold its current working variables  It has a stack  which contains the execution history  with one frame for each proce  dure called but not yet returned from  Although a thread must execute in some process  the thread and its process are different concepts and can be treated sepa  rately  Processes are used to group resources together  threads are the entities scheduled for execution on the CPU   Further down he provides the following table  Per process items               Per thread items ------------------------------ ----------------- Address space                   Program counter Global variables                Registers Open files                      Stack Child processes                 State Pending alarms                  Signals and signal handlers     Accounting information           Let s deal with the hardware multithreading issue   Classically  a CPU would support a single thread of execution  maintaining the thread s state via a single program counter  PC   and set of registers   But what happens when there s a cache miss   It takes a long time to fetch data from main memory  and while that s happening the CPU is just sitting there idle   So someone had the idea to basically have two sets of thread state  PC   registers  so that another thread  maybe in the same process  maybe in a different process  can get work done while the other thread is waiting on main memory   There are multiple names and implementations of this concept  such as Hyper-threading and simultaneous multithreading  SMT for short   Now let s look at the software side   There are basically three ways that threads can be implemented on the software side   User space threads Kernel threads A combination of the two  All you need to implement threads is the ability to save the CPU state and maintain multiple stacks  which can in many cases be done in user space   The advantage of user space threads is super fast thread switching since you don t have to trap into the kernel and the ability to schedule your threads the way you like   The biggest drawback is the inability to do blocking I O  which would block the entire process and all its user threads    which is one of the big reasons we use threads in the first place   Blocking I O using threads greatly simplifies program design in many cases  Kernel threads have the advantage of being able to use blocking I O  in addition to leaving all the scheduling issues to the OS   But each thread switch requires trapping into the kernel which is potentially relatively slow   However  if you re switching threads because of blocked I O this isn t really an issue since the I O operation probably trapped you into the kernel already anyway  Another approach is to combine the two  with multiple kernel threads each having multiple user threads  So getting back to your question of terminology  you can see that a process and a thread of execution are two different concepts and your choice of which term to use depends on what you re talking about   Regarding the term  quot light weight process quot   I don t personally see the point in it since it doesn t really convey what s going on as well as the term  quot thread of execution quot

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Difference between process and thread are given below     Process is an executing instance of a program whereas Thread is the smallest unit of process   Process can be divided into multiple threads whereas Thread can not be divided  Process may be considered as a task whereas Thread may be considered as a task lightweight process  Process allocate separate memory space whereas Thread allocate shared memory space  Process is maintained by operating system whereas Thread is maintained by programmer

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A thread runs in a shared memory space  but a process runs in a separate memory space A thread is a light-weight process  but a process is a heavy-weight process  A thread is a subtype of process

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Difference between Thread and Process   A process is an executing instance of an application and A thread is a path of execution within a process  Also  a process can contain multiple threads It   s important to note that a thread can do anything a process can do  But since a process can consist of multiple threads  a thread could be considered a    lightweight    process  Thus  the essential difference between a thread and a process is the work that each one is used to accomplish  Threads are used for small tasks  whereas processes are used for more    heavyweight    tasks     basically the execution of applications   Another difference between a thread and a process is that threads within the same process share the same address space  whereas different processes do not  This allows threads to read from and write to the same data structures and variables  and also facilitates communication between threads  Communication between processes     also known as IPC  or inter-process communication     is quite difficult and resource-intensive      Here   s a summary of the differences between threads and processes     Threads are easier to create than processes since they  don t require a separate address space  Multithreading requires careful programming since threads  share data strucures that should only be modified by one thread at a time   Unlike threads  processes don t share the same  address space  Threads are considered lightweight because they use far  less resources than processes  Processes are independent of each other   Threads  since they  share the same address space are interdependent  so caution  must be taken so that different threads don t step on each other  This is really another way of stating  2 above  A process can consist of multiple threads

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A process is a collection of code  memory  data and other resources  A thread is a sequence of code that is executed within the scope of the process  You can  usually  have multiple threads executing concurrently within the same process

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Thread is a light weight process while  the process is a self contained execution environment  What is meant by  quot self contained execution process quot   Private set of basic runtime resources  What is meant by  quot private set of basic runtime resources quot   The space allocated from memory to run the the process  Simply a memory space

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The best answer I ve found so far is Michael Kerrisk s  The Linux Programming Interface       In modern UNIX implementations  each process can have multiple threads   of execution  One way of envisaging threads is as a set of processes   that share the same virtual memory  as well as a range of other   attributes  Each thread is executing the same program code and shares   the same data area and heap  However  each thread has it own stack   containing local variables and function call linkage information   LPI 2 12    This book is a source of great clarity  Julia Evans mentioned its help in clearing up how Linux groups really work in this article

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Both processes and threads are independent sequences of execution  The typical difference is that threads  of the same process  run in a shared memory space  while processes run in separate memory spaces   I m not sure what  hardware  vs  software  threads you might be referring to  Threads are an operating environment feature  rather than a CPU feature  though the CPU typically has operations that make threads efficient    Erlang uses the term  process  because it does not expose a shared-memory multiprogramming model  Calling them  threads  would imply that they have shared memory

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They are almost as same    But the key difference is a thread is lightweight and a process is heavy-weight in terms of context switching  work load and so on

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Trying to answer this question relating to Java world   A process is an execution of a program but a thread is a single execution sequence within the process  A process can contain multiple threads  A thread is sometimes called a lightweight process   For example   Example 1  A JVM runs in a single process and threads in a JVM share the heap belonging to that process  That is why several threads may access the same object  Threads share the heap and have their own stack space  This is how one thread   s invocation of a method and its local variables are kept thread safe from other threads  But the heap is not thread-safe and must be synchronized for thread safety   Example 2  A program might not be able to draw pictures by reading keystrokes  The program must give its full attention to the keyboard input and lacking the ability to handle more than one event at a time will lead to trouble  The ideal solution to this problem is the seamless execution of two or more sections of a program at the same time  Threads allows us to do this  Here Drawing picture is a process and reading keystroke is sub process  thread

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Both threads and processes are atomic units of OS resource allocation  i e  there is a concurrency model describing how CPU time is divided between them  and the model of owning other OS resources   There is a difference in    Shared resources  threads are sharing memory by definition  they do not own anything except stack and local variables  processes could also share memory  but there is a separate mechanism for that  maintained by OS  Allocation space  kernel space for processes vs  user space for threads    Greg Hewgill above was correct about the Erlang meaning of the word  process   and here there s a discussion of why Erlang could do processes lightweight

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Real world example for Process and Thread This will give you the basic idea about thread and process   I borrowed the above info from Scott Langham s Answer - thanks

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Process - Program in execution  Thread  - a thread is execution of the smallest sequence of programmed instructions   Eg- you want to calculate matrix multiplication you will write a program of 3 for loops inside  main and execute it   Now this is your process   Now the same program you can solve by creating threads and assigning each thread to execute result of row  Each thread will work independently and the result will stored in an array   As the threads share the same memory inside a process    In Both the cases result will be same

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Basically  a thread is a part of a process without process thread wouldn t able to work   A thread is lightweight whereas the process is heavyweight  communication between process requires some Time whereas thread requires less time  Threads can share the same memory area whereas process lives in separate

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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  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 within the process gains unprecedented importance    I borrowed the above info from the Knowledge Quest  blog

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A process is a collection of code  memory  data and other resources  A thread is a sequence of code that is executed within the scope of the process  You can  usually  have multiple threads executing concurrently within the same process

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I believe the easiest way to understand the difference is to visualise how threads and processes execute their jobs      Threads are running in parallel  in a shared memory space  of the process that created them    Thread 1              Thread 2              Thread 3                                                                                                                                                                                                                                                                                                                              Complete             Complete              Complete   Note  The above can be interpreted as a process  i e  one process with 3 threads     Processes are running in parallel and concurrently    Process 1              Process 2              Process 3                                                                                                                                                                                                                                                                                                                         Complete               Complete               Complete

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Threads within the same process share the Memory  but each thread has its own stack and registers  and threads store thread-specific data in the heap  Threads never execute independently  so the inter-thread communication is much faster when compared to inter-process communication   Processes never share the same memory  When a child process creates it duplicates the memory location of the parent process  Process communication is done by using pipe  shared memory  and message parsing  Context switching between threads is very slow

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Process 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   Thread A thread is an 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     This information was found on Microsoft Docs here  About Processes and Threads          Microsoft Windows supports preemptive multitasking  which creates the effect of simultaneous execution of multiple threads from multiple processes  On a multiprocessor computer  the system can simultaneously execute as many threads as there are processors on the computer

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http   lkml iu edu hypermail linux kernel 9608 0191 html     Linus Torvalds  torvalds cs helsinki fi       Tue  6 Aug 1996 12 47 31  0300  EET DST       Messages sorted by    date    thread    subject    author        Next message  Bernd P  Ziller   Re  Oops in get hash table       Previous message  Linus Torvalds   Re  I O request ordering       On Mon  5 Aug 1996  Peter P  Eiserloh wrote            We need to keep a clear the concept of threads  Too many people     seem to confuse a thread with a process  The following discussion     does not reflect the current state of linux  but rather is an     attempt to stay at a high level discussion          NO       There is NO reason to think that  threads  and  processes  are   separate entities  That s how it s traditionally done  but I   personally think it s a major mistake to think that way  The only   reason to think that way is historical baggage        Both threads and processes are really just one thing  a  context of   execution   Trying to artificially distinguish different cases is just   self-limiting        A  context of execution   hereby called COE  is just the conglomerate   of  all the state of that COE  That state includes things like CPU   state   registers etc   MMU state  page mappings   permission state    uid  gid   and various  communication states   open files  signal   handlers etc   Traditionally  the difference between a  thread  and a    process  has been mainly that a threads has CPU state    possibly   some other minimal state   while all the other context comes from the   process  However  that s just   one way of dividing up the total state of the COE  and there is nothing that says that it s the right way to do it  Limiting yourself   to that kind of image is just plain stupid        The way Linux thinks about this  and the way I want things to work  is   that there is no such thing as a  process  or a  thread   There is   only the totality of the COE  called  task  by Linux   Different COE s   can share parts of their context with each other  and one subset of   that sharing is the traditional  thread   process  setup  but that   should really be seen as ONLY a subset  it s an important subset  but   that importance comes not from design  but from standards  we obviusly   want to run standards-conforming threads programs on top of Linux   too         In short  do NOT design around the thread process way of thinking  The   kernel should be designed around the COE way of thinking  and then the   pthreads library can export the limited pthreads interface to users   who  want to use that way of looking at COE s       Just as an example of what becomes possible when you think COE as   opposed  to thread process          You can do a external  cd  program  something that is traditionally impossible in UNIX and or process thread  silly example  but the idea    is that you can have these kinds of  modules  that aren t limited to    the traditional UNIX threads setup   Do a          clone CLONE VM CLONE FS        child  execve  external-cd            the  execve    will disassociate the VM  so the only reason we    used CLONE VM was to make the act of cloning faster            You can do  vfork    naturally  it meeds minimal kernel support  but  that support fits the CUA way of thinking perfectly           clone CLONE VM        child  continue to run  eventually execve        mother  wait for execve         you can do external  IO deamons           clone CLONE FILES        child  open file descriptors etc      mother  use the fd s the child opened and vv       All of the above work because you aren t tied to the thread process   way of thinking  Think of a web server for example  where the CGI   scripts are done as  threads of execution   You can t do that with   traditional threads  because traditional threads always have to share   the whole address space  so you d have to link in everything you ever   wanted to do in the web server itself  a  thread  can t run another   executable         Thinking of this as a  context of execution  problem instead  your   tasks can now chose to execute external programs    separate the   address space from the parent  etc if they want to  or they can for   example share everything with the parent except for the file   descriptors  so that the sub- threads  can open lots of files without   the parent needing to worry about them  they close automatically when   the sub- thread  exits  and it doesn t use up fd s in the parent         Think of a threaded  inetd   for example  You want low overhead   fork exec  so with the Linux way you can instead of using a  fork      you write a multi-threaded inetd where each thread is created with   just CLONE VM  share address space  but don t share file descriptors   etc   Then the child can execve if it was a external service  rlogind    for example   or maybe it was one of the internal inetd services    echo  timeofday  in which case it just does it s thing and exits        You can t do that with  thread   process        Linus

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