What is the meaning of the term thread-safe

Question

Does it mean that two threads can t change the underlying data simultaneously  Or does it mean that the given code segment will run with predictable results when multiple threads are executing that code segment

User · Answer

Yes and no   Thread safety is a little bit more than just making sure your shared data is accessed by only one thread at a time   You have to ensure sequential access to shared data  while at the same time avoiding race conditions  deadlocks  livelocks  and resource starvation   Unpredictable results when multiple threads are running is not a required condition of thread-safe code  but it is often a by-product   For example  you could have a producer-consumer scheme set up with a shared queue  one producer thread  and few consumer threads  and the data flow might be perfectly predictable   If you start to introduce more consumers you ll see more random looking results

User · Answer

An easier way to understand it  is what make code not thread-safe   There s two main issue that will make a threaded application to have unwanted behavior    Accessing shared variable without locking This variable could be modified by another thread while executing the function  You want to prevent it with a locking mechanism to be sure of the behavior of your function  General rule of thumb is to keep the lock for the shortest time possible  Deadlock caused by mutual dependency on shared variable If you have two shared variable A and B   In one function  you lock A first then later you lock B  In another function  you start locking B and after a while  you lock A  This is a potential deadlock where first function will wait for B to be unlocked when second function will wait for A to be unlocked  This issue will probably not occur in your development environment and only from time to time  To avoid it  all locks must always be in the same order

User · Answer

In essence  many things can go wrong in a multi threaded environment  instructions reordering  partially constructed objects  same variable having different values in different threads because of caching at the CPU level etc     I like the definition given by Java Concurrency in Practice      A  portion of code  is thread-safe if it behaves correctly when accessed from multiple threads  regardless of the scheduling or interleaving of the execution of those threads by the runtime environment  and with no additional synchronization or other coordination on the part of the calling code    By correctly they mean that the program behaves in compliance with its specifications   Contrived example  Imagine that you implement a counter  You could say that it behaves correctly if    counter next   never returns a value that has already been returned before  we assume no overflow etc  for simplicity  all values from 0 to the current value have been returned at some stage  no value is skipped    A thread safe counter would behave according to those rules regardless of how many threads access it concurrently  which would typically not be the case of a naive implementation    Note  cross-post on Programmers

User · Answer

I like the definition from Brian Goetz s Java Concurrency in Practice for its comprehensiveness   A class is thread-safe if it behaves correctly when accessed from multiple threads  regardless of the scheduling or interleaving of the execution of those threads by the runtime environment  and with no additional synchronization or other coordination on the part of the calling code

User · Answer

As others have pointed out  thread safety means that a piece of code will work without errors if it s used by more than one thread at once   It s worth being aware that this sometimes comes at a cost  of computer time and more complex coding  so it isn t always desirable  If a class can be safely used on only one thread  it may be better to do so   For example  Java has two classes that are almost equivalent  StringBuffer and StringBuilder  The difference is that StringBuffer is thread-safe  so a single instance of a StringBuffer may be used by multiple threads at once  StringBuilder is not thread-safe  and is designed as a higher-performance replacement for those cases  the vast majority  when the String is built by only one thread

User · Answer

Yes and yes   It implies that data is not modified by more than one thread simultaneously    However  your program might work as expected  and appear thread-safe  even if it is fundamentally not        Note that the unpredictablility of results is a consequence of  race-conditions  that probably result in data being modified in an order other than the expected one

User · Answer

An easier way to understand it  is what make code not thread-safe   There s two main issue that will make a threaded application to have unwanted behavior    Accessing shared variable without locking This variable could be modified by another thread while executing the function  You want to prevent it with a locking mechanism to be sure of the behavior of your function  General rule of thumb is to keep the lock for the shortest time possible  Deadlock caused by mutual dependency on shared variable If you have two shared variable A and B   In one function  you lock A first then later you lock B  In another function  you start locking B and after a while  you lock A  This is a potential deadlock where first function will wait for B to be unlocked when second function will wait for A to be unlocked  This issue will probably not occur in your development environment and only from time to time  To avoid it  all locks must always be in the same order

User · Answer

Thread-safe-code works as specified  even when entered simultaneously by different threads  This often means  that internal data-structures or operations that should run uninterrupted are protected against different modifications at the same time

User · Answer

At least in C    I think of thread-safe as a bit of a misnomer in that it leaves a lot out of the name  To be thread-safe  code typically has to be proactive about it  It s not generally a passive quality  For a class to be thread-safe  it has to have  quot extra quot  features that add overhead  These features are part of the implementation of the class and generally speaking  hidden from the interface  That is  different threads can access any of the class  members without ever having to worry about conflicting with a concurrent access by a different thread AND can do so in a very lazy manner  using some plain old regular human coding style  without having to do all that crazy synchronization stuff that is already rolled into the guts of the code being called  And this is why some people prefer to use the term internally synchronized  Terminology Sets There are three main sets of terminology for these ideas I have encountered  The first and historically more popular  but worse  is   thread safe not thread safe  The second  and better  is   thread proof thread compatible thread hostile  A third is   internally synchronized externally synchronized unsynchronizable  Analogies thread safe   thread proof   internally synchronized An example of an internally synchronized  aka  thread-safe or thread proof  system is a restaurant where a host greets you at the door  and disallows you from queueing yourself  The host is part of the mechanism of the restaurant for dealing with multiple customers  and can use some rather tricky tricks for optimizing the seating of waiting customers  like taking the size of their party into account  or how much time they look like they have  or even taking reservations over the phone  The restaurant is internally synchronized because all of this is included  quot behind the scenes quot  when you interact with it  You  the customer  don t do any of it  The host does all of it for you  not thread-safe  but nice    thread compatible   externally synchronized   free-threaded Suppose that you go to the bank  There is a line  i e  contention for the bank tellers  Because you re not a savage  you recognize that the best thing to do in the midst of contention for a resource is to queue like a civilized being  No one technically makes you do this  We hope you have the necessary social programming to do it on your own  In this sense  the bank lobby is externally synchronized  Should we say that it s thread-unsafe  that s what the implication is if you go with the thread-safe  thread-unsafe bipolar terminology set  It s not a very good set of terms  The better terminology is externally synchronized  The bank lobby is not hostile to being accessed by multiple customers  but it doesn t do the work of synchronizing them either  The customers do that themselves  This is also called  quot free threaded  quot  where  quot free quot  is as in  quot free from lice quot --or in this case  locks  Well  more accurately  synchronization primitives  That doesn t mean the code can run on multiple threads without those primitives  It just means it doesn t come with them already installed and it s up to you  the user of the code  to install them yourself however you see fit  Installing your own synchronization primitives can be difficult and requires thinking hard about the code  but also can lead to the fastest possible program by allowing you to customize how the program executes on today s hyperthreaded CPUs  not thread safe  and bad     thread hostile   unsynchronizable An example everyday analogy of a thread-hostile system is some jerk with a sports car refusing to use their blinkers and changing lanes willy-nilly  Their driving style is thread hostile or unsychronizable because you have no way to coordinate with them  and this can lead to contention for the same lane  without resolution  and thus an accident as two cars attempt to occupy the same space  without any protocol to prevent this  This pattern can also be thought of more broadly as anti-social  which I prefer because it s less specific to threads and so more generally applicable to many areas of programming  Why thread safe et al  are a bad terminology set The first and oldest terminology set fails to make the finer distinction between thread hostility and thread compatibility  Thread compatibility is more passive than so-called thread safety  but that doesn t mean the called code is unsafe for concurrent thread use  It just means it s passive about the synchronization that would allow this  putting it off to the calling code  instead of providing it as part of its internal implementation  Thread compatible is how code should probably be written by default in most cases but this is also sadly often erroneously thought of as thread unsafe  as if it s inherently anti safety  which is a major point of confusion for programmers  NOTE  Many software manuals actually use the term  quot thread-safe quot  to refer to  quot thread-compatible  quot  adding even more confusion to what was already a mess  I avoid the term  quot thread-safe quot  and  quot thread-unsafe quot  at all costs for this very reason  as some sources will call something  quot thread-safe quot  while others will call it  quot thread-unsafe quot  because they can t agree on whether you have to meet some extra standards for safety  synchronization primitives   or just NOT be hostile to be considered  quot safe quot   So avoid those terms and use the smarter terms instead  to avoid dangerous miscommunications with other engineers  Reminder of our goals Essentially  our goal is to subvert chaos  We do that by creating deterministic systems we can rely on  Determinism is expensive  mostly due to the opportunity costs of losing parallelism  pipelining  and reordering  We try to minimize the amount of determinism we need to keep our costs low  while also avoiding making decisions that will further erode what little determinism we can afford  Synchronization of threads is about increasing the order and decreasing the chaos  The levels at which you do this correspond to the terms mentioned above  The highest level means that a system behaves in an entirely predictable manner every time  The second level means that the system behaves well enough that calling code can reliably detect unpredictability  For example  a spurious wakeup in a condition variable or a failure to lock a mutex because it s not ready  The third level means that the system doesn t behave well enough to play with anyone else and can only EVER be run single-threaded without incurring chaos

User · Answer

Simply - code will run fine if many threads are executing this code at the same time

User · Answer

I would like to add some more info on top of other good answers   Thread safety implies multiple threads can write read data in same object without memory inconsistency errors  In highly  multi threaded program  a thread safe program does not cause side effects to shared data   Have a look at this SE question for more details   What does threadsafe mean   Thread safe program guarantees memory consistency   From oracle documentation page on advanced concurrent API    Memory Consistency Properties   Chapter 17 of The Java    Language Specification defines the happens-before relation on memory operations such as reads and writes of shared variables  The results of a write by one thread are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation    The synchronized and volatile constructs  as well as the Thread start   and Thread join   methods  can form happens-before relationships    The methods of all classes in java util concurrent and its subpackages extend these guarantees to higher-level synchronization  In particular    Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or removal of that element from the collection in another thread  Actions in a thread prior to the submission of a Runnable to an Executor happen-before its execution begins  Similarly for Callables submitted to an ExecutorService  Actions taken by the asynchronous computation represented by a Future happen-before actions subsequent to the retrieval of the result via Future get   in another thread  Actions prior to  releasing  synchronizer methods such as Lock unlock  Semaphore release  and CountDownLatch countDown happen-before actions subsequent to a successful  acquiring  method such as Lock lock  Semaphore acquire  Condition await  and CountDownLatch await on the same synchronizer object in another thread  For each pair of threads that successfully exchange objects via an Exchanger  actions prior to the exchange   in each thread happen-before those subsequent to the corresponding exchange   in another thread  Actions prior to calling CyclicBarrier await and Phaser awaitAdvance  as well as its variants  happen-before actions performed by the barrier action  and actions performed by the barrier action happen-before actions subsequent to a successful return from the corresponding await in other threads

User · Answer

As others have pointed out  thread safety means that a piece of code will work without errors if it s used by more than one thread at once   It s worth being aware that this sometimes comes at a cost  of computer time and more complex coding  so it isn t always desirable  If a class can be safely used on only one thread  it may be better to do so   For example  Java has two classes that are almost equivalent  StringBuffer and StringBuilder  The difference is that StringBuffer is thread-safe  so a single instance of a StringBuffer may be used by multiple threads at once  StringBuilder is not thread-safe  and is designed as a higher-performance replacement for those cases  the vast majority  when the String is built by only one thread

User · Answer

Thread-safe code is code that will work even if many Threads are executing it simultaneously    http   mindprod com jgloss threadsafe html

User · Answer

To complete other answers   Synchronization is only a worry when the code in your method does one of two things     works with some outside resource that isn t thread safe   Reads or changes a persistent object or class field   This means that variables defined WITHIN your method are always threadsafe   Every call to a method has its own version of these variables   If the method is called by another thread  or by the same thread  or even if the method calls itself  recursion   the values of these variables are not shared    Thread scheduling is not guaranteed to be round-robin  A task may totally hog the CPU at the expense of threads of the same priority  You can use Thread yield   to have a conscience  You can use  in java  Thread setPriority Thread NORM PRIORITY-1  to lower a thread s priority  Plus beware of    the large runtime cost  already mentionned by others  on applications that iterate over these  thread-safe  structures   Thread sleep 5000  is supposed to sleep for 5 seconds  However  if somebody changes the system time  you may sleep for a very long time or no time at all  The OS records the wake up time in absolute form  not relative

User · Answer

Yes and yes   It implies that data is not modified by more than one thread simultaneously    However  your program might work as expected  and appear thread-safe  even if it is fundamentally not        Note that the unpredictablility of results is a consequence of  race-conditions  that probably result in data being modified in an order other than the expected one

User · Answer

I would like to add some more info on top of other good answers   Thread safety implies multiple threads can write read data in same object without memory inconsistency errors  In highly  multi threaded program  a thread safe program does not cause side effects to shared data   Have a look at this SE question for more details   What does threadsafe mean   Thread safe program guarantees memory consistency   From oracle documentation page on advanced concurrent API    Memory Consistency Properties   Chapter 17 of The Java    Language Specification defines the happens-before relation on memory operations such as reads and writes of shared variables  The results of a write by one thread are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation    The synchronized and volatile constructs  as well as the Thread start   and Thread join   methods  can form happens-before relationships    The methods of all classes in java util concurrent and its subpackages extend these guarantees to higher-level synchronization  In particular    Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or removal of that element from the collection in another thread  Actions in a thread prior to the submission of a Runnable to an Executor happen-before its execution begins  Similarly for Callables submitted to an ExecutorService  Actions taken by the asynchronous computation represented by a Future happen-before actions subsequent to the retrieval of the result via Future get   in another thread  Actions prior to  releasing  synchronizer methods such as Lock unlock  Semaphore release  and CountDownLatch countDown happen-before actions subsequent to a successful  acquiring  method such as Lock lock  Semaphore acquire  Condition await  and CountDownLatch await on the same synchronizer object in another thread  For each pair of threads that successfully exchange objects via an Exchanger  actions prior to the exchange   in each thread happen-before those subsequent to the corresponding exchange   in another thread  Actions prior to calling CyclicBarrier await and Phaser awaitAdvance  as well as its variants  happen-before actions performed by the barrier action  and actions performed by the barrier action happen-before actions subsequent to a successful return from the corresponding await in other threads

User · Answer

A more informative question is what makes code not thread safe- and the answer is that there are four conditions that must be true    Imagine the following code  and it s machine language translation   totalRequests   totalRequests   1 MOV EAX   totalRequests       load memory for tot Requests into register INC EAX                       update register MOV  totalRequests   EAX      store updated value back to memory    The first condition is that there are memory locations that are accessible from more than one thread  Typically  these locations are global static variables  or are heap memory reachable from global static variables   Each thread gets it s own stack frame for function method scoped local variables  so these local function method variables  otoh   which are on the stack  are accessible only from the one thread that owns that stack    The second condition is that there is a property  often called an invariant   which is associated with these shared memory locations  that must be true  or valid  for the program to function correctly  In the above example  the property is that    totalRequests must accurately represent the total number of times any thread has executed any part of the increment statement     Typically  this invariant property needs to hold true  in this case  totalRequests must hold an accurate count  before an update occurs for the update to be correct  The third condition is that the invariant property does NOT hold during some part of the actual update   It is transiently invalid or false during some portion of the processing    In this particular case  from the time totalRequests is fetched until the time the updated value is stored  totalRequests does not satisfy the invariant  The fourth and final condition that must occur for a race to happen  and for the code to therefore NOT be  thread-safe   is that another thread must be able to access the shared memory while the invariant is broken  thereby causing inconsistent or incorrect behavior

User · Answer

To complete other answers   Synchronization is only a worry when the code in your method does one of two things     works with some outside resource that isn t thread safe   Reads or changes a persistent object or class field   This means that variables defined WITHIN your method are always threadsafe   Every call to a method has its own version of these variables   If the method is called by another thread  or by the same thread  or even if the method calls itself  recursion   the values of these variables are not shared    Thread scheduling is not guaranteed to be round-robin  A task may totally hog the CPU at the expense of threads of the same priority  You can use Thread yield   to have a conscience  You can use  in java  Thread setPriority Thread NORM PRIORITY-1  to lower a thread s priority  Plus beware of    the large runtime cost  already mentionned by others  on applications that iterate over these  thread-safe  structures   Thread sleep 5000  is supposed to sleep for 5 seconds  However  if somebody changes the system time  you may sleep for a very long time or no time at all  The OS records the wake up time in absolute form  not relative

User · Answer

Thread-safe code is code that will work even if many Threads are executing it simultaneously    http   mindprod com jgloss threadsafe html

User · Answer

Don t confuse thread safety with determinism   Thread-safe code can also be non-deterministic   Given the difficulty of debugging problems with threaded code  this is probably the normal case    -   Thread safety simply ensures that when a thread is modifying or reading shared data  no other thread can access it in a way that changes the data   If your code depends on a certain order for execution for correctness  then you need other synchronization mechanisms beyond those required for thread safety to ensure this

User · Answer

Yes and no   Thread safety is a little bit more than just making sure your shared data is accessed by only one thread at a time   You have to ensure sequential access to shared data  while at the same time avoiding race conditions  deadlocks  livelocks  and resource starvation   Unpredictable results when multiple threads are running is not a required condition of thread-safe code  but it is often a by-product   For example  you could have a producer-consumer scheme set up with a shared queue  one producer thread  and few consumer threads  and the data flow might be perfectly predictable   If you start to introduce more consumers you ll see more random looking results

User · Answer

Thread-safe-code works as specified  even when entered simultaneously by different threads  This often means  that internal data-structures or operations that should run uninterrupted are protected against different modifications at the same time

User · Answer

Thread-safe-code works as specified  even when entered simultaneously by different threads  This often means  that internal data-structures or operations that should run uninterrupted are protected against different modifications at the same time

User · Answer

A more informative question is what makes code not thread safe- and the answer is that there are four conditions that must be true    Imagine the following code  and it s machine language translation   totalRequests   totalRequests   1 MOV EAX   totalRequests       load memory for tot Requests into register INC EAX                       update register MOV  totalRequests   EAX      store updated value back to memory    The first condition is that there are memory locations that are accessible from more than one thread  Typically  these locations are global static variables  or are heap memory reachable from global static variables   Each thread gets it s own stack frame for function method scoped local variables  so these local function method variables  otoh   which are on the stack  are accessible only from the one thread that owns that stack    The second condition is that there is a property  often called an invariant   which is associated with these shared memory locations  that must be true  or valid  for the program to function correctly  In the above example  the property is that    totalRequests must accurately represent the total number of times any thread has executed any part of the increment statement     Typically  this invariant property needs to hold true  in this case  totalRequests must hold an accurate count  before an update occurs for the update to be correct  The third condition is that the invariant property does NOT hold during some part of the actual update   It is transiently invalid or false during some portion of the processing    In this particular case  from the time totalRequests is fetched until the time the updated value is stored  totalRequests does not satisfy the invariant  The fourth and final condition that must occur for a race to happen  and for the code to therefore NOT be  thread-safe   is that another thread must be able to access the shared memory while the invariant is broken  thereby causing inconsistent or incorrect behavior

User · Answer

Thread-safe code is code that will work even if many Threads are executing it simultaneously    http   mindprod com jgloss threadsafe html

User · Answer

At least in C    I think of thread-safe as a bit of a misnomer in that it leaves a lot out of the name  To be thread-safe  code typically has to be proactive about it  It s not generally a passive quality  For a class to be thread-safe  it has to have  quot extra quot  features that add overhead  These features are part of the implementation of the class and generally speaking  hidden from the interface  That is  different threads can access any of the class  members without ever having to worry about conflicting with a concurrent access by a different thread AND can do so in a very lazy manner  using some plain old regular human coding style  without having to do all that crazy synchronization stuff that is already rolled into the guts of the code being called  And this is why some people prefer to use the term internally synchronized  Terminology Sets There are three main sets of terminology for these ideas I have encountered  The first and historically more popular  but worse  is   thread safe not thread safe  The second  and better  is   thread proof thread compatible thread hostile  A third is   internally synchronized externally synchronized unsynchronizable  Analogies thread safe   thread proof   internally synchronized An example of an internally synchronized  aka  thread-safe or thread proof  system is a restaurant where a host greets you at the door  and disallows you from queueing yourself  The host is part of the mechanism of the restaurant for dealing with multiple customers  and can use some rather tricky tricks for optimizing the seating of waiting customers  like taking the size of their party into account  or how much time they look like they have  or even taking reservations over the phone  The restaurant is internally synchronized because all of this is included  quot behind the scenes quot  when you interact with it  You  the customer  don t do any of it  The host does all of it for you  not thread-safe  but nice    thread compatible   externally synchronized   free-threaded Suppose that you go to the bank  There is a line  i e  contention for the bank tellers  Because you re not a savage  you recognize that the best thing to do in the midst of contention for a resource is to queue like a civilized being  No one technically makes you do this  We hope you have the necessary social programming to do it on your own  In this sense  the bank lobby is externally synchronized  Should we say that it s thread-unsafe  that s what the implication is if you go with the thread-safe  thread-unsafe bipolar terminology set  It s not a very good set of terms  The better terminology is externally synchronized  The bank lobby is not hostile to being accessed by multiple customers  but it doesn t do the work of synchronizing them either  The customers do that themselves  This is also called  quot free threaded  quot  where  quot free quot  is as in  quot free from lice quot --or in this case  locks  Well  more accurately  synchronization primitives  That doesn t mean the code can run on multiple threads without those primitives  It just means it doesn t come with them already installed and it s up to you  the user of the code  to install them yourself however you see fit  Installing your own synchronization primitives can be difficult and requires thinking hard about the code  but also can lead to the fastest possible program by allowing you to customize how the program executes on today s hyperthreaded CPUs  not thread safe  and bad     thread hostile   unsynchronizable An example everyday analogy of a thread-hostile system is some jerk with a sports car refusing to use their blinkers and changing lanes willy-nilly  Their driving style is thread hostile or unsychronizable because you have no way to coordinate with them  and this can lead to contention for the same lane  without resolution  and thus an accident as two cars attempt to occupy the same space  without any protocol to prevent this  This pattern can also be thought of more broadly as anti-social  which I prefer because it s less specific to threads and so more generally applicable to many areas of programming  Why thread safe et al  are a bad terminology set The first and oldest terminology set fails to make the finer distinction between thread hostility and thread compatibility  Thread compatibility is more passive than so-called thread safety  but that doesn t mean the called code is unsafe for concurrent thread use  It just means it s passive about the synchronization that would allow this  putting it off to the calling code  instead of providing it as part of its internal implementation  Thread compatible is how code should probably be written by default in most cases but this is also sadly often erroneously thought of as thread unsafe  as if it s inherently anti safety  which is a major point of confusion for programmers  NOTE  Many software manuals actually use the term  quot thread-safe quot  to refer to  quot thread-compatible  quot  adding even more confusion to what was already a mess  I avoid the term  quot thread-safe quot  and  quot thread-unsafe quot  at all costs for this very reason  as some sources will call something  quot thread-safe quot  while others will call it  quot thread-unsafe quot  because they can t agree on whether you have to meet some extra standards for safety  synchronization primitives   or just NOT be hostile to be considered  quot safe quot   So avoid those terms and use the smarter terms instead  to avoid dangerous miscommunications with other engineers  Reminder of our goals Essentially  our goal is to subvert chaos  We do that by creating deterministic systems we can rely on  Determinism is expensive  mostly due to the opportunity costs of losing parallelism  pipelining  and reordering  We try to minimize the amount of determinism we need to keep our costs low  while also avoiding making decisions that will further erode what little determinism we can afford  Synchronization of threads is about increasing the order and decreasing the chaos  The levels at which you do this correspond to the terms mentioned above  The highest level means that a system behaves in an entirely predictable manner every time  The second level means that the system behaves well enough that calling code can reliably detect unpredictability  For example  a spurious wakeup in a condition variable or a failure to lock a mutex because it s not ready  The third level means that the system doesn t behave well enough to play with anyone else and can only EVER be run single-threaded without incurring chaos

User · Answer

I like the definition from Brian Goetz s Java Concurrency in Practice for its comprehensiveness   A class is thread-safe if it behaves correctly when accessed from multiple threads  regardless of the scheduling or interleaving of the execution of those threads by the runtime environment  and with no additional synchronization or other coordination on the part of the calling code

User · Answer

A more informative question is what makes code not thread safe- and the answer is that there are four conditions that must be true    Imagine the following code  and it s machine language translation   totalRequests   totalRequests   1 MOV EAX   totalRequests       load memory for tot Requests into register INC EAX                       update register MOV  totalRequests   EAX      store updated value back to memory    The first condition is that there are memory locations that are accessible from more than one thread  Typically  these locations are global static variables  or are heap memory reachable from global static variables   Each thread gets it s own stack frame for function method scoped local variables  so these local function method variables  otoh   which are on the stack  are accessible only from the one thread that owns that stack    The second condition is that there is a property  often called an invariant   which is associated with these shared memory locations  that must be true  or valid  for the program to function correctly  In the above example  the property is that    totalRequests must accurately represent the total number of times any thread has executed any part of the increment statement     Typically  this invariant property needs to hold true  in this case  totalRequests must hold an accurate count  before an update occurs for the update to be correct  The third condition is that the invariant property does NOT hold during some part of the actual update   It is transiently invalid or false during some portion of the processing    In this particular case  from the time totalRequests is fetched until the time the updated value is stored  totalRequests does not satisfy the invariant  The fourth and final condition that must occur for a race to happen  and for the code to therefore NOT be  thread-safe   is that another thread must be able to access the shared memory while the invariant is broken  thereby causing inconsistent or incorrect behavior

User · Answer

An easier way to understand it  is what make code not thread-safe   There s two main issue that will make a threaded application to have unwanted behavior    Accessing shared variable without locking This variable could be modified by another thread while executing the function  You want to prevent it with a locking mechanism to be sure of the behavior of your function  General rule of thumb is to keep the lock for the shortest time possible  Deadlock caused by mutual dependency on shared variable If you have two shared variable A and B   In one function  you lock A first then later you lock B  In another function  you start locking B and after a while  you lock A  This is a potential deadlock where first function will wait for B to be unlocked when second function will wait for A to be unlocked  This issue will probably not occur in your development environment and only from time to time  To avoid it  all locks must always be in the same order

User · Answer

Simply - code will run fine if many threads are executing this code at the same time

User · Answer

Yes and no   Thread safety is a little bit more than just making sure your shared data is accessed by only one thread at a time   You have to ensure sequential access to shared data  while at the same time avoiding race conditions  deadlocks  livelocks  and resource starvation   Unpredictable results when multiple threads are running is not a required condition of thread-safe code  but it is often a by-product   For example  you could have a producer-consumer scheme set up with a shared queue  one producer thread  and few consumer threads  and the data flow might be perfectly predictable   If you start to introduce more consumers you ll see more random looking results

User · Answer

Don t confuse thread safety with determinism   Thread-safe code can also be non-deterministic   Given the difficulty of debugging problems with threaded code  this is probably the normal case    -   Thread safety simply ensures that when a thread is modifying or reading shared data  no other thread can access it in a way that changes the data   If your code depends on a certain order for execution for correctness  then you need other synchronization mechanisms beyond those required for thread safety to ensure this

User · Answer

Don t confuse thread safety with determinism   Thread-safe code can also be non-deterministic   Given the difficulty of debugging problems with threaded code  this is probably the normal case    -   Thread safety simply ensures that when a thread is modifying or reading shared data  no other thread can access it in a way that changes the data   If your code depends on a certain order for execution for correctness  then you need other synchronization mechanisms beyond those required for thread safety to ensure this

User · Answer

As others have pointed out  thread safety means that a piece of code will work without errors if it s used by more than one thread at once   It s worth being aware that this sometimes comes at a cost  of computer time and more complex coding  so it isn t always desirable  If a class can be safely used on only one thread  it may be better to do so   For example  Java has two classes that are almost equivalent  StringBuffer and StringBuilder  The difference is that StringBuffer is thread-safe  so a single instance of a StringBuffer may be used by multiple threads at once  StringBuilder is not thread-safe  and is designed as a higher-performance replacement for those cases  the vast majority  when the String is built by only one thread

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Yes and no   Thread safety is a little bit more than just making sure your shared data is accessed by only one thread at a time   You have to ensure sequential access to shared data  while at the same time avoiding race conditions  deadlocks  livelocks  and resource starvation   Unpredictable results when multiple threads are running is not a required condition of thread-safe code  but it is often a by-product   For example  you could have a producer-consumer scheme set up with a shared queue  one producer thread  and few consumer threads  and the data flow might be perfectly predictable   If you start to introduce more consumers you ll see more random looking results

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As others have pointed out  thread safety means that a piece of code will work without errors if it s used by more than one thread at once   It s worth being aware that this sometimes comes at a cost  of computer time and more complex coding  so it isn t always desirable  If a class can be safely used on only one thread  it may be better to do so   For example  Java has two classes that are almost equivalent  StringBuffer and StringBuilder  The difference is that StringBuffer is thread-safe  so a single instance of a StringBuffer may be used by multiple threads at once  StringBuilder is not thread-safe  and is designed as a higher-performance replacement for those cases  the vast majority  when the String is built by only one thread

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Yes and yes   It implies that data is not modified by more than one thread simultaneously    However  your program might work as expected  and appear thread-safe  even if it is fundamentally not        Note that the unpredictablility of results is a consequence of  race-conditions  that probably result in data being modified in an order other than the expected one

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Let s answer this by example  class NonThreadSafe        private int count   0       public boolean countTo10             count   count   1          return  count    10          The countTo10 method adds one to the counter and then returns true if the count has reached 10  It should only return true once  This will work as long as only one thread is running the code  If two threads run the code at the same time various problems can occur  For example  if count starts as 9  one thread could add 1 to count  making 10  but then a second thread could enter the method and add 1 again  making 11  before the first thread has a chance to execute the comparison with 10  Then both threads do the comparison and find that count is 11 and neither returns true  So this code is not thread safe  In essence  all multi-threading problems are caused by some variation of this kind of problem  The solution is to ensure that the addition and the comparison cannot be separated  for example by surrounding the two statements by some kind of synchronization code  or by devising a solution that does not require two operations  Such code would be thread-safe

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Simply - code will run fine if many threads are executing this code at the same time

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Let s answer this by example  class NonThreadSafe        private int count   0       public boolean countTo10             count   count   1          return  count    10          The countTo10 method adds one to the counter and then returns true if the count has reached 10  It should only return true once  This will work as long as only one thread is running the code  If two threads run the code at the same time various problems can occur  For example  if count starts as 9  one thread could add 1 to count  making 10  but then a second thread could enter the method and add 1 again  making 11  before the first thread has a chance to execute the comparison with 10  Then both threads do the comparison and find that count is 11 and neither returns true  So this code is not thread safe  In essence  all multi-threading problems are caused by some variation of this kind of problem  The solution is to ensure that the addition and the comparison cannot be separated  for example by surrounding the two statements by some kind of synchronization code  or by devising a solution that does not require two operations  Such code would be thread-safe

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Thread-safe-code works as specified  even when entered simultaneously by different threads  This often means  that internal data-structures or operations that should run uninterrupted are protected against different modifications at the same time

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I like the definition from Brian Goetz s Java Concurrency in Practice for its comprehensiveness   A class is thread-safe if it behaves correctly when accessed from multiple threads  regardless of the scheduling or interleaving of the execution of those threads by the runtime environment  and with no additional synchronization or other coordination on the part of the calling code

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Thread-safe code is code that will work even if many Threads are executing it simultaneously    http   mindprod com jgloss threadsafe html

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In simplest words  P  If it is safe to execute multiple threads on a block of code it is thread safe    conditions apply  Conditions are mentioned by other answeres like 1  The result should be same if you execute one thread or multiple threads over it etc

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In essence  many things can go wrong in a multi threaded environment  instructions reordering  partially constructed objects  same variable having different values in different threads because of caching at the CPU level etc     I like the definition given by Java Concurrency in Practice      A  portion of code  is thread-safe if it behaves correctly when accessed from multiple threads  regardless of the scheduling or interleaving of the execution of those threads by the runtime environment  and with no additional synchronization or other coordination on the part of the calling code    By correctly they mean that the program behaves in compliance with its specifications   Contrived example  Imagine that you implement a counter  You could say that it behaves correctly if    counter next   never returns a value that has already been returned before  we assume no overflow etc  for simplicity  all values from 0 to the current value have been returned at some stage  no value is skipped    A thread safe counter would behave according to those rules regardless of how many threads access it concurrently  which would typically not be the case of a naive implementation    Note  cross-post on Programmers

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A more informative question is what makes code not thread safe- and the answer is that there are four conditions that must be true    Imagine the following code  and it s machine language translation   totalRequests   totalRequests   1 MOV EAX   totalRequests       load memory for tot Requests into register INC EAX                       update register MOV  totalRequests   EAX      store updated value back to memory    The first condition is that there are memory locations that are accessible from more than one thread  Typically  these locations are global static variables  or are heap memory reachable from global static variables   Each thread gets it s own stack frame for function method scoped local variables  so these local function method variables  otoh   which are on the stack  are accessible only from the one thread that owns that stack    The second condition is that there is a property  often called an invariant   which is associated with these shared memory locations  that must be true  or valid  for the program to function correctly  In the above example  the property is that    totalRequests must accurately represent the total number of times any thread has executed any part of the increment statement     Typically  this invariant property needs to hold true  in this case  totalRequests must hold an accurate count  before an update occurs for the update to be correct  The third condition is that the invariant property does NOT hold during some part of the actual update   It is transiently invalid or false during some portion of the processing    In this particular case  from the time totalRequests is fetched until the time the updated value is stored  totalRequests does not satisfy the invariant  The fourth and final condition that must occur for a race to happen  and for the code to therefore NOT be  thread-safe   is that another thread must be able to access the shared memory while the invariant is broken  thereby causing inconsistent or incorrect behavior

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In simplest words  P  If it is safe to execute multiple threads on a block of code it is thread safe    conditions apply  Conditions are mentioned by other answeres like 1  The result should be same if you execute one thread or multiple threads over it etc

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An easier way to understand it  is what make code not thread-safe   There s two main issue that will make a threaded application to have unwanted behavior    Accessing shared variable without locking This variable could be modified by another thread while executing the function  You want to prevent it with a locking mechanism to be sure of the behavior of your function  General rule of thumb is to keep the lock for the shortest time possible  Deadlock caused by mutual dependency on shared variable If you have two shared variable A and B   In one function  you lock A first then later you lock B  In another function  you start locking B and after a while  you lock A  This is a potential deadlock where first function will wait for B to be unlocked when second function will wait for A to be unlocked  This issue will probably not occur in your development environment and only from time to time  To avoid it  all locks must always be in the same order

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Yes and yes   It implies that data is not modified by more than one thread simultaneously    However  your program might work as expected  and appear thread-safe  even if it is fundamentally not        Note that the unpredictablility of results is a consequence of  race-conditions  that probably result in data being modified in an order other than the expected one

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To complete other answers   Synchronization is only a worry when the code in your method does one of two things     works with some outside resource that isn t thread safe   Reads or changes a persistent object or class field   This means that variables defined WITHIN your method are always threadsafe   Every call to a method has its own version of these variables   If the method is called by another thread  or by the same thread  or even if the method calls itself  recursion   the values of these variables are not shared    Thread scheduling is not guaranteed to be round-robin  A task may totally hog the CPU at the expense of threads of the same priority  You can use Thread yield   to have a conscience  You can use  in java  Thread setPriority Thread NORM PRIORITY-1  to lower a thread s priority  Plus beware of    the large runtime cost  already mentionned by others  on applications that iterate over these  thread-safe  structures   Thread sleep 5000  is supposed to sleep for 5 seconds  However  if somebody changes the system time  you may sleep for a very long time or no time at all  The OS records the wake up time in absolute form  not relative

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Instead of thinking of code or classes as thread safe or not  I think it is more helpful to think of actions as being thread-safe   Two actions are thread safe if they will be behave as specified when run from arbitrary threading contexts   In many cases  classes will support some combinations of actions in thread-safe fashion and others not   For example  many collections like array-lists and hash sets will guarantee that if they are initially accessed exclusively with one thread  and they are never modified after a reference becomes visible to any other threads  they may be read in arbitrary fashion by any combination of threads without interference   More interestingly  some hash-set collections such as the original non-generic one in  NET  may offer a guarantee that as long as no item is ever removed  and provided that only one thread ever writes to them  any thread that tries to read the collection will behave as though accessing a collection where updates might be delayed and occur in arbitrary order  but which will otherwise behave normally   If thread  1 adds X and then Y  and thread  2 looks for and sees Y and then X  it would be possible for thread  2 to see that Y exists but X doesn t  whether or not such behavior is  thread-safe  would depend upon whether thread  2 is prepared to deal with that possibility   As a final note  some classes--especially blocking communications libraries--may have a  close  or  Dispose  method which is thread-safe with respect to all other methods  but no other methods that are thread-safe with respect to each other   If a thread performs a blocking read request and a user of the program clicks  cancel   there would be no way for a close request to be issued by the thread that s attempting to perform the read   The close dispose request  however  may asynchronously set a flag which will cause the read request to be canceled as soon as possible   Once close is performed on any thread  the object would become useless  and all attempts at future actions would fail immediately  but being able to asynchronously terminate any attempted I O operations is better than require that the close request be synchronized with the read  since if the read blocks forever  the synchronization request would be likewise blocked

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I like the definition from Brian Goetz s Java Concurrency in Practice for its comprehensiveness   A class is thread-safe if it behaves correctly when accessed from multiple threads  regardless of the scheduling or interleaving of the execution of those threads by the runtime environment  and with no additional synchronization or other coordination on the part of the calling code

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Simply - code will run fine if many threads are executing this code at the same time

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To complete other answers   Synchronization is only a worry when the code in your method does one of two things     works with some outside resource that isn t thread safe   Reads or changes a persistent object or class field   This means that variables defined WITHIN your method are always threadsafe   Every call to a method has its own version of these variables   If the method is called by another thread  or by the same thread  or even if the method calls itself  recursion   the values of these variables are not shared    Thread scheduling is not guaranteed to be round-robin  A task may totally hog the CPU at the expense of threads of the same priority  You can use Thread yield   to have a conscience  You can use  in java  Thread setPriority Thread NORM PRIORITY-1  to lower a thread s priority  Plus beware of    the large runtime cost  already mentionned by others  on applications that iterate over these  thread-safe  structures   Thread sleep 5000  is supposed to sleep for 5 seconds  However  if somebody changes the system time  you may sleep for a very long time or no time at all  The OS records the wake up time in absolute form  not relative

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Instead of thinking of code or classes as thread safe or not  I think it is more helpful to think of actions as being thread-safe   Two actions are thread safe if they will be behave as specified when run from arbitrary threading contexts   In many cases  classes will support some combinations of actions in thread-safe fashion and others not   For example  many collections like array-lists and hash sets will guarantee that if they are initially accessed exclusively with one thread  and they are never modified after a reference becomes visible to any other threads  they may be read in arbitrary fashion by any combination of threads without interference   More interestingly  some hash-set collections such as the original non-generic one in  NET  may offer a guarantee that as long as no item is ever removed  and provided that only one thread ever writes to them  any thread that tries to read the collection will behave as though accessing a collection where updates might be delayed and occur in arbitrary order  but which will otherwise behave normally   If thread  1 adds X and then Y  and thread  2 looks for and sees Y and then X  it would be possible for thread  2 to see that Y exists but X doesn t  whether or not such behavior is  thread-safe  would depend upon whether thread  2 is prepared to deal with that possibility   As a final note  some classes--especially blocking communications libraries--may have a  close  or  Dispose  method which is thread-safe with respect to all other methods  but no other methods that are thread-safe with respect to each other   If a thread performs a blocking read request and a user of the program clicks  cancel   there would be no way for a close request to be issued by the thread that s attempting to perform the read   The close dispose request  however  may asynchronously set a flag which will cause the read request to be canceled as soon as possible   Once close is performed on any thread  the object would become useless  and all attempts at future actions would fail immediately  but being able to asynchronously terminate any attempted I O operations is better than require that the close request be synchronized with the read  since if the read blocks forever  the synchronization request would be likewise blocked

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Don t confuse thread safety with determinism   Thread-safe code can also be non-deterministic   Given the difficulty of debugging problems with threaded code  this is probably the normal case    -   Thread safety simply ensures that when a thread is modifying or reading shared data  no other thread can access it in a way that changes the data   If your code depends on a certain order for execution for correctness  then you need other synchronization mechanisms beyond those required for thread safety to ensure this

[multithreading] What is the meaning of the term "thread-safe"?

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