haiku/docs/user/support/TLS.dox

/*
 * Copyright 2007 Haiku Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 *
 * Corresponds to:
 *		headers/os/support/TLS.h	rev 19972
 */


/*!
	\file TLS.h
	\ingroup support
	\ingroup libbe
	\brief Functions to use Thread Local Storage.

	The Thread Local Storage API provides convenient methods to transform global
	variables in to thread-context sensitive variables. Some applications rely on
	global variables as a way of intercommunicating between functions and
	objects, but one of your demands might be that the contents of that variable
	differs between threads.

	The following example demonstrates how an imaginary thread manager that
	stores per thread data would function. The constructor of this 
	\c ThreadManager allocates the TLS variables using tls_allocate(). This only
	has to be done once, and not in every spawned thread! Then, every spawned
	thread that interacts with this thread manager, should call the
	\c InitThread() function. This one associates the supplied thread data with
	the TLS index using tls_set(). Each thread can get their associated data with
	\c GetCurrentThreadData(), which uses tls_get() to retrieve the associated
	thread data at the provided index.

\code
int32 gThreadName;
int32 gThreadData;

class ThreadManager
{
public:
    // General initialisation
    ThreadManager() {
        gThreadName = tls_allocate();
        gThreadStatus = tls_allocate();
    };

    // Called from the thread entry function 
    void InitThread(const char *name, void *data) {
        tls_set(gThreadName, (void *)name);
        tls_set(gThreadData, data);
    };

    // Can be called from any of the threads. The returned data will be that
    // which the thread explicitly set in the InitThread() function
    void *GetCurrentThreadData() {
        printf("Thread %s asked for its data.\n",
            (const char*)tls_get(gThreadName));
        return tls_get(gThreadData);
    };
};
\endcode

	\note
	-# It is impossible to get data other than from your thread.
	-# There is a limit to the number of TLS variables you can allocate. This
		limit is define by #TLS_MAX_KEYS, but do realize that you share this
			limit with all the libraries your application is linked to.
	-# The actual global variables, in the example \c gThreadName and
		\c gThreadData, are only indexes. You cannot use these variables to
		access data without the TLS API.
*/


/*!
	\def TLS_MAX_KEYS
	\brief The maximum number of thread local storage variables. This number is
		   process wide.
*/


/*!
	\fn int32 tls_allocate(void)
	\brief Allocate a unique index to use for storing variables.

	You should only have to do this once to allocate the global index, which
	you can reuse in every thread.

	\return A unique index to which you can associate per thread data. If we
		    overrun the maximum number of keys, as defined by #TLS_MAX_KEYS,
		    the function will return \c B_NO_MEMORY.

	\sa tls_get()
	\sa tls_set()
	\sa tls_address()
*/


/*!
	\fn void *tls_get(int32 index)
	\brief Retrieve the data stored for this thread at the provided \a index.

	\param index The \a index that you retrieved with tls_allocate().

	\return The data you set using tls_set() for this thread, or \c NULL if there
			is no data set, or the \a index is invalid.

	\sa tls_allocate()
	\sa tls_set()
*/


/*!
	\fn void **tls_address(int32 index)
	\brief Retrieve the pointer that refers to the data of this thread at the
    provided \a index.

	You can use this pointer to directly manipulate your thread data.

	\param index The \a index that you retrieved with tls_allocate().
	\return The pointer to where your thread's data is, or \c NULL if the index
			is invalid.
	\sa tls_allocate()
	\sa tls_set()
	\sa tls_get()
*/


/*!
	\fn void tls_set(int32 index, void *value)
	\brief Set the data of this thread at the provided \a index.

	It is up to you to make sure the \a index is valid. Any invalid indices can
	lead to unpredictable results.

	\param index The \a index that you retrieved with tls_allocate().
	\param value The data that should be associated with the index for this
    	   thread.

	\sa tls_allocate()
	\sa tls_get()
*/