The __thread storage class specifier (IBM extension)

The __thread storage class marks a static variable as having thread-local storage duration. This means that, in a multi-threaded application, a unique instance of the variable is created for each thread that uses it, and destroyed when the thread terminates. The __thread storage class specifier can provide a convenient way of assuring thread-safety: declaring an object as per-thread allows multiple threads to access the object without the concern of race conditions, while avoiding the need for low-level programming of thread synchronization or significant program restructuring.

The tls_model attribute allows source-level control for the thread-local storage model used for a given variable. The tls_model attribute must specify one of local-exec, initial-exec, local-dynamic, or global-dynamic access method, which overrides the -qtls option for that variable. For example:

__thread int i __attribute__((tls_model("local-exec")));

The tls_model attribute allows the linker to check that the correct thread model has been used to build the application or shared library. The linker/loader behavior is as follows:

Table 1. Link time/runtime behavior for thread access models
Access method	Link-time diagnostic	Runtime diagnostic
`local-exec`	Fails if referenced symbol is imported.	Fails if module is not the main program. Fails if referenced symbol is imported (but the linker should have detected the error already).
`initial-exec`	None.	`dlopen()/load()` fails if referenced symbol is not in the module loaded at execution time.
`local-dynamic`	Fails if referenced symbol is imported.	Fails if referenced symbol is imported (but the linker should have detected the error already).
`global-dynamic`	None.	None.

Note: In order for the __thread keyword to be recognized, you must compile with the -qtls option. See -qtls for details.

The specifier can be applied to any of the following:

global variables
file-scoped static variables
function-scoped static variables
static data members of a class

It cannot be applied to function-scoped automatic variables or non-static data members.

The thread specifier can be either preceded or followed by the static or extern specifier.

__thread int i;      
extern __thread struct state s;      
static __thread char *p;

Variables marked with the __thread specifier can be initialized or uninitialized.

C++ only __thread variables must be initialized with a constant expression, and must not have a static constructor.

Applying the address-of operator (&) to a thread-local variable returns the runtime address of the current thread's instance of the variable. That thread can pass this address to any other thread; however, when the first thread terminates, any pointers to its thread-local variables become invalid.

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