Thread-Local Storage(TLS)

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本文介绍了GCC中线程局部存储(TLS)的概念及其实现机制。TLS是一种为每个线程分配独立变量实例的机制，它起源于IA-64处理器特定的ABI，并已扩展到其他处理器。文章详细解释了TLS的运行时模型、地址模型及如何在C/C++中使用__thread关键字声明TLS变量。

http://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/Thread-Local.html

Thread-local storage (TLS ) is a mechanism by which variables are allocated such that there is one instance of the variable per extant thread. The run-time model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (ld ), dynamic linker (ld.so ), and system libraries (libc.so and libpthread.so ), so it is not available everywhere.

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 __thread specifier may be used alone, with the extern or static specifiers, but with no other storage class specifier. When used with extern or static , __thread must appear immediately after the other storage class specifier.

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.

When the address-of operator is applied to a thread-local variable, it is evaluated at run-time and returns the address of the current thread's instance of that variable. An address so obtained may be used by any thread. When a thread terminates, any pointers to thread-local variables in that thread become invalid.

No static initialization may refer to the address of a thread-local variable.

In C++, if an initializer is present for a thread-local variable, it must be a constant-expression , as defined in 5.19.2 of the ANSI/ISO C++ standard.

See ELF Handling For Thread-Local Storage for a detailed explanation of the four thread-local storage addressing models, and how the run-time is expected to function.