What resources are shared between threads

Question

Recently  I have been asked a question in an interview what s the difference between a process and a thread  Really  I did not know the answer  I thought for a minute and gave a very weird answer    Threads share the same memory  processes do not  After answering this  the interviewer gave me an evil smile and fired the following questions at me   Q  Do you know the segments in which a program gets divided   My answer  yep  thought it was an easy one  Stack  Data  Code  Heap  Q  So  tell me  which segments do threads share   I could not answer this and ended up in saying all of them   Please  can anybody present the correct and impressive answers for the difference between a process and a thread

User · Accepted Answer

You re pretty much correct  but threads share all segments except the stack  Threads have independent call stacks  however the memory in other thread stacks is still accessible and in theory you could hold a pointer to memory in some other thread s local stack frame  though you probably should find a better place to put that memory

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Threads share data and code while processes do not  The stack is not shared for both   Processes can also share memory  more precisely code  for example after a Fork    but this is an implementation detail and  operating system  optimization  Code shared by multiple processes will  hopefully  become duplicated on the first write to the code - this is known as copy-on-write  I am not sure about the exact semantics for the code of threads  but I assume shared code               Process   Thread     Stack   private   private    Data    private   shared    Code    private1  shared2   1 The code is logically private but might be shared for performance reasons  2 I am not 100  sure

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Besides global memory  threads also share a number of other attributes    i e   these attributes are global to a process  rather than specific   to a thread   These attributes include the following          process ID and parent process ID    process group ID and session ID    controlling terminal    process credentials  user and group IDs     open file descriptors    record locks created using fcntl      signal dispositions    file system   related information  umask  current working directory  and root directory    interval timers  setitimer    and POSIX timers  timer create       System V semaphore undo  semadj  values  Section 47 8     resource limits    CPU time consumed  as returned by times       resources consumed  as returned by getrusage     and   nice value  set by setpriority   and nice             Among the attributes that are distinct for each thread are the   following          thread ID  Section 29 5     signal mask    thread-specific data  Section 31 3     alternate signal stack  sigaltstack       the errno variable    floating-point environment  see fenv 3      realtime scheduling policy and priority  Sections 35 2 and 35 3     CPU affinity  Linux-specific  described in Section 35 4     capabilities  Linux-specific  described in Chapter 39   and   stack  local variables and function call linkage information        Excerpt From  The Linux Programming Interface  A Linux and UNIX System Programming Handbook   Michael Kerrisk  page 619

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Threads share the code and data segments and the heap  but they don t share the stack

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From Wikipedia  I think that would make a really good answer for the interviewer  P      Threads differ from traditional   multitasking operating system   processes in that          processes are typically independent  while threads exist as subsets of a   process   processes carry considerable state information  whereas multiple threads   within a process share state as well   as memory and other resources   processes have separate address spaces  whereas threads share their   address space   processes interact only through system-provided inter-process   communication mechanisms    Context switching between threads in the same process is typically faster   than context switching between   processes

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Thread share the heap  there is a research about thread specific heap  but current  implementation share the heap   and of course the code

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Tell the interviewer that it depends entirely on the implementation of the OS   Take Windows x86 for example  There are only 2 segments  1   Code and Data  And they re both mapped to the whole 2GB  linear  user  address space  Base 0  Limit 2GB  They would ve made one but x86 doesn t allow a segment to be both Read Write and Execute  So they made two  and set CS to point to the code descriptor  and the rest  DS  ES  SS  etc  to point to the other  2   But both point to the same stuff   The person interviewing you had made a hidden assumption that he she did not state  and that is a stupid trick to pull   So regarding      Q  So tell me which segment thread   share    The segments are irrelevant to the question  at least on Windows  Threads share the whole address space  There is only 1 stack segment  SS  and it points to the exact same stuff that DS  ES  and CS do  2   I e  the whole bloody user space  0-2GB  Of course  that doesn t mean threads only have 1 stack  Naturally each has its own stack  but x86 segments are not used for this purpose   Maybe  nix does something different  Who knows  The premise the question was based on was broken      At least for user space  From ntsd notepad  cs 001b  ss 0023  ds 0023  es 0023

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Something that really needs to be pointed out is that there are really two aspects to this question - the theoretical aspect and the implementations aspect   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 from section 2 2 2 The Classical Thread Model in 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            The above is what you need for threads to work  As others have pointed out  things like segments are OS dependant implementation details

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Threads share everything  1   There is one address space for the whole process   Each thread has its own stack and registers  but all threads  stacks are visible in the shared address space   If one thread allocates some object on its stack  and sends the address to another thread  they ll both have equal access to that object     Actually  I just noticed a broader issue  I think you re confusing two uses of the word segment   The file format for an executable  eg  ELF  has distinct sections in it  which may be referred to as segments  containing compiled code  text   initialized data  linker symbols  debug info  etc  There are no heap or stack segments here  since those are runtime-only constructs   These binary file segments may be mapped into the process address space seperately  with different permissions  eg  read-only executable for code text  and copy-on-write non-executable for initialized data    Areas of this address space are used for different purposes  like heap allocation and thread stacks  by convention  enforced by your language runtime libraries    It is all just memory though  and probably not segmented unless you re running in virtual 8086 mode   Each thread s stack is a chunk of memory allocated at thread creation time  with the current stack top address stored in a stack pointer register  and each thread keeps its own stack pointer along with its other registers      1  OK  I know  signal masks  TSS TSD etc   The address space  including all its mapped program segments  are still shared though

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In process all threads share system resource like heap Memory etc  while Thread has its own stack  So your ans should be heap memory which all threads share for a process

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A process has code  data  heap and stack segments  Now  the Instruction Pointer  IP  of a thread OR threads points to the code segment of the process  The data and heap segments are shared by all the threads  Now what about the stack area  What is actually the stack area  Its an area created by the process just for its thread to use    because stacks can be used in a much faster way than heaps etc  The stack area of the process is divided among threads  i e  if there are 3 threads  then the stack area of the process is divided into 3 parts and each is given to the 3 threads  In other words  when we say that each thread has its own stack  that stack is actually a part of the process stack area allocated to each thread  When a thread finishes its execution  the stack of the thread is reclaimed by the process  In fact  not only the stack of a process is divided among threads  but all the set of registers that a thread uses like SP  PC and state registers are the registers of the process   So when it comes to sharing  the code  data and heap areas are shared  while the stack area is just divided among threads

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Generally  Threads are called light weight process  If we divide memory into three sections then it will be  Code  data and Stack  Every process has its own code  data and stack sections and due to this context switch time is a little high  To reduce context switching time  people have come up with concept of thread  which shares Data and code segment with other thread process and it has its own STACK segment

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In an x86 framework  one can divide as many segments  up to 2 16-1   The ASM directives SEGMENT ENDS allows this  and the operators SEG and OFFSET allows initialization of segment registers  CS IP are usually initialized by the loader  but for DS  ES  SS the application is responsible with initialization  Many environments allow the so-called  simplified segment definitions  like  code   data   bss   stack etc  and  depending also on the  memory model   small  large  compact etc   the loader initializes segment registers accordingly  Usually  data   bss   stack and other usual segments  I haven t done this since 20 years so I don t remember all  are grouped in one single group - that is why usually DS  ES and SS points to teh same area  but this is only to simplify things   In general  all segment registers can have different values upon run-time  So  the interview question was right  which one of the CODE  DATA  and STACK are shared between threads  Heap management is something else - it is simply a sequence of calls to the OS  But what if you don t have an OS at all  like in an embedded system - can you still have new delete in your code   My advice to the young people - read some good assembly programming book  It seems that university curriculae are quite poor in this respect

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