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 runtime 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 runtime is expected to function.