8.10.1 Thread Qualifier

The __thread qualifier indicates to the compiler that the qualified object must be stored locally for each thread executed by the program. The compiler's Thread-Local Storage (TLS) feature ensures that __thread-qualified objects are accessed without data corruption or unexpected behavior across multiple threads, which are known as thread-safe objects.

Thread-safe objects can be initialized when they are defined, if required. They cannot use the persistent or address attributes, but they can, however, be placed in a user-defined section using the section attribute. Objects with automatic storage duration are inherently thread safe, so using the __thread qualifier with them makes no sense, but objects with permanent storage duration can be made thread-safe. Use of the space(data) attribute with the __thread qualifier is valid but has no effect; use of the space(prog) attribute and this qualifier will result in an error.

The following examples are valid definitions of thread-safe objects that use the TLS feature implemented by the compiler.

__thread int result;
__thread __attribute__((section("my_section"))) int var = 7;

If the TLS is not enabled, the __thread qualifier is ignored and any object using it is linked and accessed like an ordinary C object.