Can you use thread local variables inside a class or structure

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Like this.

struct some_struct
{
 // Other fields
 .....
 __thread int tl;
}

I'm trying to do that but the compiler is giving me this error.

./cv.h:16:2: error: '__thread' is only allowed on variable declarations
        __thread int tl;

In C and C++, thread-local storage applies to static variables or to variables with external linkage only.

Local (automatic) variables are usually created on the stack and therefore are specific to the thread that executes the code, but global and static variables are shared among all threads since they reside in the data or BSS segment. TLS provides a mechanism to make those global variables local to the thread and that's what the __thread keyword achieves - it instructs the compiler to create a separate copy of the variable in each thread while lexically it remains a global or static one (e.g., it can be accessed by different functions called within the same thread of execution).

Non-static class members and structure members are placed where the object (class or structure) is allocated - either on the stack if an automatic variable is declared or on the heap if new or malloc() is used. Either way, each thread receives a unique storage location for the variable and __thread is just not applicable in this case, hence the compiler error you get.

When and How to Use ThreadLocal Variables, Inside ThreadLocal Class? The Java Concurrency API provides a clean mechanism for thread-local variables using ThreadLocal class with a very good� Local variables are always thread safe. The only thing which may prevent you to do this is your application design constraints. In wabapp server, it may be keep a thread pool, so a ThreadLocal var should be removed before response to the client, since current thread may be reused by next request.

gcc imposes the following restrictions on the use of __thread:

The __thread specifier may be applied to any global, file-scoped static, function-scoped static, or static data member of a class. It may not be applied to block-scoped automatic or non-static data member.

The __thread modifier is supported by multiple compiler. It is not inconceivable that the exact restrictions vary somewhat from compiler to compiler.

C++, Thread-local storage can be created using the thread_local keyword. Each thread in a program has its own copy of each thread-local variable. A thread- local variable with namespace or class (non-local) scope will be initialized as one copy of that variable per thread, rather than one copy per (thread, instance) pair. With the exception of Dispose (), all public and protected members of ThreadLocal<T> are thread-safe and may be used concurrently from multiple threads. The value returned for the Value and IsValueCreated properties is specific for the thread on which the property is accessed.

C11 standard Section 6.7.1 Paragraph 2

At most, one storage-class specifier may be given in the declaration specifiers in a declaration, except that _Thread_local may appear with static or extern.120)

C11 standard Section 6.7.1 Paragraph 3

In the declaration of an object with block scope, if the declaration specifiers include _Thread_local, they shall also include either static or extern. If _Thread_local appears in any declaration of an object, it shall be present in every declaration of that object.

Thread-Local (Using the GNU Compiler Collection (GCC)), Thread-local storage ( TLS ) is a mechanism by which variables are allocated The runtime model GCC uses to implement this originates in the IA-64 At the user level, the extension is visible with a new storage class keyword: __thread . For example: __thread int i; extern __thread struct state s; static __thread char *p; . The local variables of a function are unique to each thread that runs the function. However, the static and global variables are shared by all threads in the process. With thread local storage (TLS), you can provide unique data for each thread that the process can access using a global index.

You should change __thread int tl; to thread_local static int tl;

Thread-local storage, Thread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread. While the use of global variables is generally discouraged in modern If errno were a global variable, a call of a system function on one thread may overwrite the value previously set by a call of a system� The variables that are defined inside the class but outside the method can be accessed within the class(all methods included) using the instance of a class. For Example – self.var_name. If you want to use that variable even outside the class, you must declared that variable as a global. Then the variable can be accessed using its name inside and outside the class and not using the instance of the class.

According to the old Petzold book 'Programming Windows' (page 1241) you mark a variable as thread local by using the keywords: __declspec (thread). So for instance: __declspec (thread) int iGlobal = 1;

I doubt this could be done in a class though. You can also make the variable static too. [edit] just realized you are probably not running on windows... So I guess for anyone who needs an windows answer, this may be relevant.

Java.lang.ThreadLocal class in Java, No two thread can see each others thread local variable. These variable are generally private static field in classes and maintain its state inside thread. you can also write an article using contribute.geeksforgeeks.org or Learn; Algorithms � Data Structures � Languages � CS Subjects � Video Tutorials. Well, if you declare a local variable you won't be able to refer to that anywhere other than in classes created within that method. Where are you implementing Runnable ? If it's in the same class, then you could either make it an instance variable and make main set the variable on the same instance that you're creating the thread from, or make it a static variable.

I understand using a global variable is sometimes the most convenient thing to do, especially in cases where usage of class makes the easiest thing so much harder (e.g., multiprocessing). I ran into the same problem with declaring global variables and figured it out with some experiments.

Local Variable Local Variable is defined as a type of variable declared within programming block or subroutines. It can only be used inside the subroutine or code block in which it is declared. The local variable exists until the block of the function is under execution.

In unmanaged C++, you use TlsAlloc to allocate slots dynamically and __declspec(thread) to declare that a variable should be allocated in thread-relative storage. Thread-relative static fields and data slots provide the managed version of this behavior. In the .NET Framework 4, you can use the System.Threading.ThreadLocal<T> class to create

Comments
  • Please post the compiler error you're getting.
  • AFAICS __thread is not standard. Which compiler are you using?
  • Not with the current meaning of __thread or thread_local but there are quite some situations where it makes sense to have a different member for each thread, wrapped up nicely behind some interface.
  • I have to disagree -- just because an object is created by a single thread doesn't mean it will be manipulated solely by one thread (e.g. lock-free data structures). Admittedly, this is relatively rare, however.
  • I would appreciate if any of the downvoters would care to post a contra-example or a better answer or provide suggestions on how to edit the question or edit the question themselves.
  • @BeeOnRope I appreciate your comment and have removed the generalisation.
  • I guess you mean C11, no? C99 didn't have a thread model and all that.
  • Corrected. Habit of writing C99.
  • Then you can't have multiple instances of the struct/class having different values in this field.
  • @rustyx: Of course you can have different values in that field: Every thread will have its own value. What you cannot have with standard C++ is a thread_local normal member: Such would have a different value for each object and each thread.
  • I don't know, maybe it'd be nice to have different values per thread-class combination.
  • It does not seem allowed, I got C2720 with VS2015: msdn.microsoft.com/en-us/library/w8x1t4f0.aspx