Ops, forgot this.

[platform/upstream/glib.git] / docs / reference / glib / tmpl / threads.sgml

<!-- ##### SECTION Title ##### -->

Threads

<!-- ##### SECTION Short_Description ##### -->

thread abstraction; including threads, different mutexes, conditions
and thread private data.

<!-- ##### SECTION Long_Description ##### -->

<para>
Threads act almost like processes, but unlike processes all threads of
one process share the same memory. This is good, as it provides easy
communication between the involved threads via this shared memory, and
it is bad, because strange things (so called Heisenbugs) might happen,
when the program is not carefully designed. Especially bad is, that due
to the concurrent nature of threads no assumptions on the order of
execution of different threads can be done unless explicitly forced by
the programmer through synchronization primitives.
</para>

<para>
The aim of the thread related functions in GLib is to provide a
portable means for writing multi-threaded software. There are
primitives for mutexes to protect the access to portions of memory
(#GMutex, #GStaticMutex, #G_LOCK_DEFINE, #GStaticRecMutex and
#GStaticRWLock), there are primitives for condition variables to allow
synchronization of threads (#GCond) and finally there are primitives
for thread-private data, that every thread has a private instance of
(#GPrivate, #GStaticPrivate). Last but definitely not least there are
primitives to portably create and manage threads (#GThread).
</para>

<!-- ##### SECTION See_Also ##### -->
<para>
<variablelist>

<varlistentry>
<term>#GThreadPool</term>
<listitem><para>Thread pools.</para></listitem>
</varlistentry>

<varlistentry>
<term>#GAsyncQueue</term>
<listitem><para>Send asynchronous messages between threads.</para></listitem>
</varlistentry>

</variablelist>
</para>

<!-- ##### MACRO G_THREADS_ENABLED ##### -->

<para>
This macro is defined, if GLib was compiled with thread support. This
does not necessarily mean, that there is a thread implementation
available, but the infrastructure is in place and once you provide a
thread implementation to g_thread_init(), GLib will be multi-thread
safe. It isn't and cannot be, if #G_THREADS_ENABLED is not defined.
</para>



<!-- ##### MACRO G_THREADS_IMPL_POSIX ##### -->

<para>
This macro is defined, if POSIX style threads are used.
</para>



<!-- ##### MACRO G_THREADS_IMPL_SOLARIS ##### -->

<para>
This macro is defined, if the Solaris thread system is used.
</para>



<!-- ##### MACRO G_THREADS_IMPL_NONE ##### -->

<para>
This macro is defined, if no thread implementation is used. You can
however provide one to g_thread_init() to make GLib multi-thread safe.
</para>



<!-- ##### MACRO G_THREAD_ERROR ##### -->
<para>
The error domain of the GLib thread subsystem.
</para>



<!-- ##### ENUM GThreadError ##### -->
<para>
Possible errors of thread related functions.
</para>

@G_THREAD_ERROR_AGAIN: a thread couldn't be created due to resource
shortage. Try again later.

<!-- ##### STRUCT GThreadFunctions ##### -->

<para>
This function table is used by g_thread_init() to initialize the
thread system. The functions in that table are directly used by their
g_* prepended counterparts, that are described here, e.g. if you call
g_mutex_new() then mutex_new() from the table provided to
g_thread_init() will be called.
</para>

<note>
<para>
This struct should only be used, if you know, what you are doing.
</para>
</note>

@mutex_new: 
@mutex_lock: 
@mutex_trylock: 
@mutex_unlock: 
@mutex_free: 
@cond_new: 
@cond_signal: 
@cond_broadcast: 
@cond_wait: 
@cond_timed_wait: 
@cond_free: 
@private_new: 
@private_get: 
@private_set: 
@thread_create: 
@thread_yield: 
@thread_join: 
@thread_exit: 
@thread_set_priority: 
@thread_self: 

<!-- ##### FUNCTION g_thread_init ##### -->

<para>
Before you use a thread related function in GLib, you should
initialize the thread system. This is done by calling
g_thread_init(). Most of the time you will only have to call
g_thread_init(NULL). 
</para>

<note>
<para>
You should only call g_thread_init() with a non-NULL parameter, if you
really know, what you are doing.
</para>
</note>

<note>
<para>
g_thread_init() must not be called directly or indirectly as a
call-back from GLib. Also no mutexes may be currently locked, while
calling g_thread_init().
</para>
</note>

<para>
g_thread_init() might only be called once. On the second call
it will abort with an error. If you want to make sure, that the thread
system is initialized, you can do that too:
</para>

<para>
<informalexample>
<programlisting>
if (!g_thread_supported ()) g_thread_init (NULL);
</programlisting>
</informalexample>
</para>

<para>
After that line either the thread system is initialized or the program
will abort, if no thread system is available in GLib, i.e. either
#G_THREADS_ENABLED is not defined or #G_THREADS_IMPL_NONE is defined.
</para>

<para>
If no thread system is available and @vtable is NULL or if not all
elements of @vtable are non-NULL, then g_thread_init() will abort.
</para>

<note>
<para>
To use g_thread_init() in your program, you have to link with the
libraries, that the command "glib-config --libs gthread" outputs. This
is not the case for all the other thread related functions of
GLib. Those can be used without having to link with the thread
libraries.
</para>
</note>

@vtable: a function table of type #GThreadFunctions, that provides the
entry points to the thread system to be used


<!-- ##### FUNCTION g_thread_supported ##### -->
<para>
This function returns, whether the thread system is initialized or
not.
</para>

<note>
<para>
This function is actually a macro. Apart from taking the address of it
you can however use it as if it was a function.
</para>
</note>

@Returns: TRUE, if the thread system is initialized


<!-- ##### USER_FUNCTION GThreadFunc ##### -->
<para>
Specifies the type of the @func functions passed to
g_thread_create().
</para>

@data: data passed to the thread
@Returns: the return value of the thread, which will be returned by
g_thread_join()


<!-- ##### ENUM GThreadPriority ##### -->
<para>
Specifies the priority of a thread. 
</para>

<note>
<para>
It is not guaranteed, that threads with different priorities really
behave accordingly. On some systems (e.g. Linux) only root can increase
priorities. On other systems (e.g. Solaris) there doesn't seem to be
different scheduling for different priorities. All in all try to avoid
being dependent on priorities.
</para>
</note>

@G_THREAD_PRIORITY_LOW: a priority lower than normal
@G_THREAD_PRIORITY_NORMAL: the default priority
@G_THREAD_PRIORITY_HIGH: a priority higher than normal
@G_THREAD_PRIORITY_URGENT: the highest priority

<!-- ##### STRUCT GThread ##### -->
<para>
The #GThread struct represents a running thread. It has three public
read-only members, but the underlying struct is bigger, so you must
not copy this struct.
</para>

<note>
<para>
Resources for a joinable thread are not fully released until
g_thread_join() is called for that thread.
</para>
</note>

@func: the function executing in that thread
@data: the argument to the function
@joinable: is this thread joinable?
@bound: is this thread bound to a system thread?
@priority: the priority of the thread

<!-- ##### FUNCTION g_thread_create ##### -->
<para>
This function creates a new thread with the priority @priority. The
stack gets the size @stack_size or the default value for the current
platform, if @stack_size is 0.
</para>

<para>
If @joinable is #TRUE, you can wait for this threads termination
calling g_thread_wait(). Otherwise the thread will just disappear, when
ready. If @bound is #TRUE, this thread will be scheduled in the system
scope, otherwise the implementation is free to do scheduling in the
process scope. The first variant is more expensive resource-wise, but
generally faster. On some systems (e.g. Linux) all threads are bound.
</para>

<para>
The new thread executes the function @func with the argument
@data. If the thread was created successfully, it is returned.
</para>

<para>
@error can be NULL to ignore errors, or non-NULL to report errors. The
error is set, if and only if the function returns #NULL.
</para>

<note>
<para>
It is not guaranteed, that threads with different priorities really
behave accordingly. On some systems (e.g. Linux) only root can increase
priorities. On other systems (e.g. Solaris) there doesn't seem to be
different scheduling for different priorities. All in all try to avoid
being dependent on priorities. Use %G_THREAD_PRIORITY_NORMAL here as a
default.
</para>
</note>

@func: a function to execute in the new thread
@data: an argument to supply to the new thread
@stack_size: a stack size for the new thread
@joinable: should this thread be joinable?
@bound: should this thread be bound to a system thread?
@priority: a priority for the thread
@error: return location for error.
@Returns: the new #GThread on success


<!-- ##### FUNCTION g_thread_self ##### -->
<para>
This functions returns the #GThread corresponding to the calling thread.
</para>

@Returns: the current thread


<!-- ##### FUNCTION g_thread_join ##### -->
<para>
Waits until @thread finishes, i.e. the function @func, as given
to g_thread_create, returns or g_thread_exit() is called by
@thread. All resources of @thread including the #GThread struct are
released. @thread must have been created with @joinable=#TRUE in
g_thread_create(). The value returned by @func or given to
g_thread_exit() by @thread is returned by this function.
</para>

@thread: a #GThread to be waited for
@Returns: the return value of the thread


<!-- ##### FUNCTION g_thread_set_priority ##### -->
<para>
Change the priority of @thread to @priority.
</para>

<note>
<para>
It is not guaranteed, that threads with different priorities really
behave accordingly. On some systems (e.g. Linux) only root can increase
priorities. On other systems (e.g. Solaris) there doesn't seem to be
different scheduling for different priorities. All in all try to avoid
being dependent on priorities.
</para>
</note>

@thread: a #GThread
@priority: a new priority for @thread


<!-- ##### FUNCTION g_thread_yield ##### -->
<para>
Give way to other threads waiting to be scheduled. 
</para>

<para>
This function is often used as a method to make busy wait less
evil. But in most cases, you will encounter, there are better methods
to do that. So in general you shouldn't use that function.
</para>



<!-- ##### FUNCTION g_thread_exit ##### -->
<para>
Exit the current thread. If another thread is waiting for that thread
using g_thread_join() and the current thread is joinable, the waiting
thread will be woken up and getting @retval as the return value of
g_thread_join(). If the current thread is not joinable, @retval is
ignored. Calling
</para>

<para>
<informalexample>
<programlisting>
g_thread_join (retval);
</programlisting>
</informalexample>
</para>

<para>
is equivalent to calling 
</para>

<para>
<informalexample>
<programlisting>
return retval;
</programlisting>
</informalexample>
</para>

<para>
in the function @func, as given to g_thread_create().
</para>

<note>
<para>
Never call g_thread_exit from within a thread of a #GThreadPool, as
that will mess up the bookkeeping and lead to funny and unwanted
results.
</para>
</note>

@retval: the return value of this thread


<!-- ##### STRUCT GMutex ##### -->

<para>
The #GMutex struct is an opaque data structure to represent a mutex
(mutual exclusion). It can be used to protect data against shared
access. Take for example the following function:

<example>
<title>A function which will not work in a threaded environment</title>
<programlisting>
  int give_me_next_number ()
  {
    static int current_number = 0;

    /* now do a very complicated calculation to calculate the new number,
       this might for example be a random number generator */
    current_number = calc_next_number (current_number); 
    return current_number;
  }
</programlisting>
</example>
</para>

<para>
It is easy to see, that this won't work in a multi-threaded
application. There current_number must be protected against shared
access. A first naive implementation would be:
</para>

<para>
<example>
<title>The wrong way to write a thread-safe function</title>
<programlisting>
  int give_me_next_number ()
  {
    static int current_number = 0;
    int ret_val;
    static GMutex * mutex = NULL;

    if (!mutex)
      mutex = g_mutex_new ();
    g_mutex_lock (mutex);
    ret_val = current_number = calc_next_number (current_number); 
    g_mutex_unlock (mutex);
    return ret_val;
  }
</programlisting>
</example>
</para>

<para>
This looks like it would work, but there is a race condition while
constructing the mutex and this code cannot work reliable. So please do
not use such constructs in your own programs. One working solution is:
</para>

<para>
<example>
<title>A correct thread-safe function</title>
<programlisting>
  static GMutex *give_me_next_number_mutex = NULL;

  /* this function must be called before any call to give_me_next_number ()
     it must be called exactly once. */
  void init_give_me_next_number () 
  {
    g_assert (give_me_next_number_mutex == NULL);
    give_me_next_number_mutex = g_mutex_new ();
  }

  int give_me_next_number ()
  {
    static int current_number = 0;
    int ret_val;

    g_mutex_lock (give_me_next_number_mutex);
    ret_val = current_number = calc_next_number (current_number); 
    g_mutex_unlock (give_me_next_number_mutex);
    return ret_val;
  }
</programlisting>
</example>
</para>

<para>
#GStaticMutex provides a simpler and safer way of doing this.
</para>

<para>
If you want to use a mutex, but your code should also work without
calling g_thread_init() first, you can not use a #GMutex, as
g_mutex_new() requires that. Use a #GStaticMutex instead.
</para>

<para>
A #GMutex should only be accessed via the following functions.
</para>

<note>
<para>
All of the g_mutex_* functions are actually macros. Apart from taking
the addresses of them, you can however use them as if they were functions.
</para>
</note>


<!-- ##### FUNCTION g_mutex_new ##### -->

<para>
Creates a new #GMutex. 
</para>

<note>
<para>
This function will abort, if g_thread_init() has not been called yet.
</para>
</note>

@Returns: a new #GMutex


<!-- ##### FUNCTION g_mutex_lock ##### -->

<para>
Locks @mutex. If @mutex is already locked by another thread, the
current thread will block until @mutex is unlocked by the other
thread.
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will do nothing then.
</para>

<note>
<para>
#GMutex is not recursive, i.e. a thread will deadlock, if it already
has locked @mutex while calling g_mutex_lock(). Use
#GStaticRecMutex instead, if you need recursive mutexes.
</para>
</note>

@mutex: a #GMutex


<!-- ##### FUNCTION g_mutex_trylock ##### -->

<para>
Tries to lock @mutex. If @mutex is already locked by another
thread, it immediately returns FALSE. Otherwise it locks @mutex
and returns TRUE.
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will immediately return TRUE then.
</para>

<note>
<para>
#GMutex is not recursive, i.e. g_mutex_trylock() will return FALSE,
if the current thread already has locked @mutex. Use
#GStaticRecMutex instead, if you need recursive mutexes.
</para>
</note>

@mutex: a #GMutex
@Returns: TRUE, if @mutex could be locked


<!-- ##### FUNCTION g_mutex_unlock ##### -->

<para>
Unlocks @mutex. If another thread is blocked in a g_mutex_lock() call
for @mutex, it will be woken and can lock @mutex itself.
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will do nothing then.
</para>

@mutex: a #GMutex


<!-- ##### FUNCTION g_mutex_free ##### -->

<para>
Destroys @mutex.
</para>

@mutex: a #GMutex


<!-- ##### STRUCT GStaticMutex ##### -->

<para>
A #GStaticMutex works like a #GMutex, but it has one significant
advantage. It doesn't need to be created at run-time like a #GMutex,
but can be defined at compile-time. Here is a shorter, easier and
safer version of our give_me_next_number() example:
</para>

<para>
<example>
<title>Using GStaticMutex to simplify thread-safe programming</title>
<programlisting>
  int give_me_next_number ()
  {
    static int current_number = 0;
    int ret_val;
    static GStaticMutex mutex = G_STATIC_MUTEX_INIT;

    g_static_mutex_lock (&amp;mutex);
    ret_val = current_number = calc_next_number (current_number); 
    g_static_mutex_unlock (&amp;mutex);
    return ret_val;
  }
</programlisting>
</example>
</para>

<para>
Sometimes you would like to dynamically create a mutex. If you don't
want to require prior calling to g_thread_init(), because your code
should also be usable in non-threaded programs, you are not able to
use g_mutex_new() and thus #GMutex, as that requires a prior call to
g_thread_init(). In theses cases you can also use a #GStaticMutex. It
must be initialized with g_static_mutex_init() before using it and
freed with with g_static_mutex_free() when not needed anymore to free
up any allocated recourses.
</para>

<para>
Even though #GStaticMutex is not opaque, it should only be used with
the following functions, as it is defined differently on different
platforms.
</para>

<para>
All of the g_static_mutex_* functions can also be used, if
g_thread_init() has not yet been called.
</para>

<note>
<para>
All of the g_static_mutex_* functions are actually macros. Apart from
taking the addresses of them, you can however use them as if they were
functions.
</para>
</note>


<!-- ##### MACRO G_STATIC_MUTEX_INIT ##### -->

<para>
A #GStaticMutex must be initialized with this macro, before it can be
used. This macro can used be to initialize a variable, but it cannot
be assigned to a variable. In that case you have to use
g_static_mutex_init().
</para>

<para>
<informalexample>
<programlisting>
GStaticMutex my_mutex = G_STATIC_MUTEX_INIT;
</programlisting>
</informalexample>
</para>



<!-- ##### FUNCTION g_static_mutex_init ##### -->
<para>
Initializes @mutex. Alternatively you can initialize it with
#G_STATIC_MUTEX_INIT.
</para>

@mutex: a #GStaticMutex to be initialized


<!-- ##### FUNCTION g_static_mutex_lock ##### -->
<para>
Works like g_mutex_lock(), but for a #GStaticMutex.
</para>

@mutex: a #GStaticMutex


<!-- ##### FUNCTION g_static_mutex_trylock ##### -->

<para>
Works like g_mutex_trylock(), but for a #GStaticMutex.
</para>

@mutex: a #GStaticMutex
@Returns: TRUE, if the #GStaticMutex could be locked


<!-- ##### FUNCTION g_static_mutex_unlock ##### -->

<para>
Works like g_mutex_unlock(), but for a #GStaticMutex.
</para>

@mutex: a #GStaticMutex


<!-- ##### FUNCTION g_static_mutex_get_mutex ##### -->

<para>
For some operations (like g_cond_wait()) you must have a #GMutex
instead of a #GStaticMutex. This function will return the
corresponding #GMutex for @mutex.
</para>

@mutex: a #GStaticMutex
@Returns: the #GMutex corresponding to @mutex


<!-- ##### FUNCTION g_static_mutex_free ##### -->
<para>
Releases all resources allocated to @mutex. 
</para>

<para>
You don't have to call this functions for a #GStaticMutex with an
unbounded lifetime, i.e. objects declared 'static', but if you have a
#GStaticMutex as a member of a structure and the structure is freed,
you should also free the #GStaticMutex.
</para>

@mutex: a #GStaticMutex to be freed


<!-- ##### MACRO G_LOCK_DEFINE ##### -->

<para>
The G_LOCK_* macros provide a convenient interface to #GStaticMutex
with the advantage that they will expand to nothing in programs
compiled against a thread-disabled GLib, saving code and memory
there. #G_LOCK_DEFINE defines a lock. It can appear, where variable
definitions may appear in programs, i.e. in the first block of a
function or outside of functions. The @name parameter will be mangled
to get the name of the #GStaticMutex. This means, that you can use
names of existing variables as the parameter, e.g. the name of the
variable you intent to protect with the lock. Look at our
give_me_next_number() example using the G_LOCK_* macros:
</para>

<para>
<example>
<title>Using the G_LOCK_* convenience macros</title>
<programlisting>
G_LOCK_DEFINE (current_number);

int give_me_next_number ()
  {
    static int current_number = 0;
    int ret_val;

    G_LOCK (current_number);
    ret_val = current_number = calc_next_number (current_number); 
    G_UNLOCK (current_number);
    return ret_val;
  }
</programlisting>
</example>
</para>

@name: the name of the lock


<!-- ##### MACRO G_LOCK_DEFINE_STATIC ##### -->

<para>
This works like #G_LOCK_DEFINE, but it creates a static object.
</para>

@name: the name of the lock


<!-- ##### MACRO G_LOCK_EXTERN ##### -->

<para>
This declares a lock, that is defined with #G_LOCK_DEFINE in another module.
</para>

@name: the name of the lock


<!-- ##### MACRO G_LOCK ##### -->

<para>
Works like g_mutex_lock(), but for a lock defined with #G_LOCK_DEFINE.
</para>

@name: the name of the lock


<!-- ##### MACRO G_TRYLOCK ##### -->

<para>
Works like g_mutex_trylock(), but for a lock defined with #G_LOCK_DEFINE.
</para>

@name: the name of the lock
@Returns: TRUE, if the lock could be locked


<!-- ##### MACRO G_UNLOCK ##### -->

<para>
Works like g_mutex_unlock(), but for a lock defined with #G_LOCK_DEFINE.
</para>

@name: the name of the lock


<!-- ##### STRUCT GStaticRecMutex ##### -->
<para>
A #GStaticRecMutex works like a #GStaticMutex, but it can be locked
multiple times by one thread. If you enter it n times, however, you
have to unlock it n times again to let other threads lock it. An
exception is the function g_static_rec_mutex_unlock_full(), that
allows you to unlock a #GStaticRecMutex completely returning the depth,
i.e. the number of times this mutex was locked. The depth can later be
used to restore the state by calling g_static_rec_mutex_lock_full().
</para>

<para>
Even though #GStaticRecMutex is not opaque, it should only be used with
the following functions.
</para>

<para>
All of the g_static_rec_mutex_* functions can also be used, if
g_thread_init() has not been called.
</para>

@mutex: 
@depth: 
@owner: 

<!-- ##### MACRO G_STATIC_REC_MUTEX_INIT ##### -->
<para>
A #GStaticRecMutex must be initialized with this macro, before it can
be used. This macro can used be to initialize a variable, but it
cannot be assigned to a variable. In that case you have to use
g_static_rec_mutex_init().
</para>

<para>
<informalexample>
<programlisting>
GStaticRecMutex my_mutex = G_STATIC_REC_MUTEX_INIT;
</programlisting>
</informalexample>
</para>



<!-- ##### FUNCTION g_static_rec_mutex_init ##### -->
<para>
A #GStaticRecMutex must be initialized with this function, before it
can be used. Alternatively you can initialize it with
#G_STATIC_REC_MUTEX_INIT.
</para>

@mutex: a #GStaticRecMutex to be initialized


<!-- ##### FUNCTION g_static_rec_mutex_lock ##### -->
<para>
Locks @mutex. If @mutex is already locked by another thread, the
current thread will block until @mutex is unlocked by the other
thread. If @mutex is already locked by the calling thread, this
functions increases the depth of @mutex and returns immediately.
</para>

@mutex: a #GStaticRecMutex to lock


<!-- ##### FUNCTION g_static_rec_mutex_trylock ##### -->
<para>
Tries to lock @mutex. If @mutex is already locked by another thread,
it immediately returns #FALSE. Otherwise it locks @mutex and returns
#TRUE. If @mutex is already locked by the calling thread, this
functions increases the depth of @mutex and immediately  returns #TRUE.
</para>

@mutex: a #GStaticRecMutex to lock
@Returns: TRUE, if @mutex could be locked


<!-- ##### FUNCTION g_static_rec_mutex_unlock ##### -->
<para>
Unlocks @mutex. Another threads can, however, only lock @mutex when it
has been unlocked as many times, as it had been locked before. If
@mutex is completely unlocked and another thread is blocked in a
g_static_rec_mutex_lock() call for @mutex, it will be woken and can
lock @mutex itself.
</para>

@mutex: a #GStaticRecMutex to unlock


<!-- ##### FUNCTION g_static_rec_mutex_lock_full ##### -->
<para>
Works like calling g_static_rec_mutex_lock() for @mutex n times.
</para>

@mutex: a #GStaticRecMutex to lock
@depth: number of times this mutex has to be unlocked to be completely unlocked


<!-- ##### FUNCTION g_static_rec_mutex_unlock_full ##### -->
<para>
Completely unlocks @mutex. If another thread is blocked in a
g_static_rec_mutex_lock() call for @mutex, it will be woken and can
lock @mutex itself. This function returns the number of times, that
@mutex has been locked by the current thread. To restore the state
before the call to g_static_rec_mutex_unlock_full() you can call
g_static_rec_mutex_lock_full() with the depth returned by this
function.
</para>

@mutex: a #GStaticRecMutex to completely unlock
@Returns: number of times @mutex has been locked by the current thread


<!-- ##### FUNCTION g_static_rec_mutex_free ##### -->
<para>
Releases all resources allocated to a #GStaticRecMutex.
</para>

<para>
You don't have to call this functions for a #GStaticRecMutex with an
unbounded lifetime, i.e. objects declared 'static', but if you have a
#GStaticRecMutex as a member of a structure and the structure is
freed, you should also free the #GStaticRecMutex.
</para>

@mutex: a #GStaticRecMutex to be freed


<!-- ##### STRUCT GStaticRWLock ##### -->
<para>
The #GStaticRWLock struct represents a read-write lock. A read-write
lock can be used for protecting data, that some portions of code only
read from, while others also write. In such situations it is
desirable, that several readers can read at once, whereas of course
only one writer may write at a time. Take a look at the following
example:

<example>
<title>An array with access functions</title>
<programlisting>
  GStaticRWLock rwlock = G_STATIC_RW_LOCK_INIT;

  GPtrArray *array;

  gpointer my_array_get (guint index)
  {
    gpointer retval = NULL;

    if (!array)
      return NULL;

    g_static_rw_lock_reader_lock (&amp;rwlock);

    if (index < array->len)
      retval = g_ptr_array_index (array, index);

    g_static_rw_lock_reader_unlock (&amp;rwlock);

    return retval;
  }

  void my_array_set (guint index, gpointer data)
  {
    g_static_rw_lock_writer_lock (&amp;rwlock);

    if (!array)
      array = g_ptr_array_new ();

    if (index >= array->len)
      g_ptr_array_set_size (array, index+1);

    g_ptr_array_index (array, index) = data; 

    g_static_rw_lock_writer_unlock (&amp;rwlock);
  }
</programlisting>
</example>
</para>

<para>
This example shows an array, which can be accessed by many readers
(the my_array_get function) simultaneously, whereas the writers (the
my_array_set function) only will be allowed once a time and only if no
readers currently access the array. This is because of the potentially
dangerous resizing of the array. Using that functions is fully
multi-thread safe now. 
</para>

<para>
Most of the time the writers should have precedence of readers. That
means for this implementation, that as soon as a writer wants to lock
the data, no other reader is allowed to lock the data, whereas of
course the readers, that already have locked the data are allowed to
finish their operation. As soon as the last reader unlocks the data,
the writer will lock it.
</para>

<para>
Even though #GStaticRWLock is not opaque, it should only be used with
the following functions.
</para>

<para>
All of the g_static_rw_lock_* functions can also be used, if
g_thread_init() has not been called.
</para>

<note>
<para>
A read-write lock has a higher overhead as a mutex. For example both
g_static_rw_lock_reader_lock() and g_static_rw_lock_reader_unlock()
has to lock and unlock a #GStaticMutex, so it takes at least twice the
time to lock and unlock a #GStaticRWLock than to lock and unlock a
#GStaticMutex. So only data structures, that are accessed by multiple
readers, which keep the lock for a considerable time justify a
#GStaticRWLock. The above example most probably would fare better with
a #GStaticMutex.
</para>
</note>

@mutex: 
@read_cond: 
@write_cond: 
@read_counter: 
@write: 
@want_to_write: 

<!-- ##### MACRO G_STATIC_RW_LOCK_INIT ##### -->
<para>
A #GStaticRWLock must be initialized with this macro, before it can
be used. This macro can used be to initialize a variable, but it
cannot be assigned to a variable. In that case you have to use
g_static_rw_lock_init().
</para>

<para>
<informalexample>
<programlisting>
GStaticRWLock my_lock = G_STATIC_RW_LOCK_INIT;
</programlisting>
</informalexample>
</para>



<!-- ##### FUNCTION g_static_rw_lock_init ##### -->
<para>
A #GStaticRWLock must be initialized with this function, before it can
be used. Alternatively you can initialize it with
#G_STATIC_RW_LOCK_INIT.
</para>

@lock: a #GStaticRWLock to be initialized


<!-- ##### FUNCTION g_static_rw_lock_reader_lock ##### -->
<para>
Locks @lock for reading. There may be unlimited concurrent locks for
reading of a #GStaticRWLock at the same time.  If @lock is already
locked for writing by another thread or if another thread is already
waiting to lock @lock for writing, this function will block until
@lock is unlocked by the other writing thread and no other writing
threads want to lock @lock. This lock has to be unlocked by
g_static_rw_lock_reader_unlock().
</para>

<para>
#GStaticRWLock in general is not recursive, but as there may be
unlimited concurrent locks for reading, it effectively is for
recursive for reading, but for reading only. Locking for writing after
locking for reading will deadlock, the same holds true for the
opposite order.
</para>

@lock: a #GStaticRWLock to lock for reading


<!-- ##### FUNCTION g_static_rw_lock_reader_trylock ##### -->
<para>
Tries to lock @lock for reading. If @lock is already locked for
writing by another thread or if another thread is already waiting to
lock @lock for writing, it immediately returns #FALSE. Otherwise it
locks @lock for reading and returns TRUE. This lock has to be unlocked
by g_static_rw_lock_reader_unlock().
</para>

@lock: a #GStaticRWLock to lock for reading
@Returns: TRUE, if @lock could be locked for reading


<!-- ##### FUNCTION g_static_rw_lock_reader_unlock ##### -->
<para>
Unlocks @lock. If a thread waits to lock @lock for writing and all
locks for reading have been unlocked, the waiting thread is woken up
and can lock @lock for writing.
</para>

@lock: a #GStaticRWLock to unlock after reading


<!-- ##### FUNCTION g_static_rw_lock_writer_lock ##### -->
<para>
Locks @lock for writing. If @lock is already locked for writing or
reading by other threads, this function will block until @lock is
completely unlocked and then lock @lock for writing. While this
functions waits to lock @lock, no other thread can lock @lock for
reading. When @lock is locked for writing, no other thread can lock
@lock (neither for reading nor writing). This lock has to be unlocked
by g_static_rw_lock_writer_unlock().
</para>

@lock: a #GStaticRWLock to lock for writing


<!-- ##### FUNCTION g_static_rw_lock_writer_trylock ##### -->
<para>
Tries to lock @lock for writing. If @lock is already locked (for
either reading or writing) by another thread, it immediately returns
#FALSE. Otherwise it locks @lock for writing and returns TRUE. This
lock has to be unlocked by g_static_rw_lock_writer_unlock().
</para>

@lock: a #GStaticRWLock to lock for writing
@Returns: TRUE, if @lock could be locked for writing


<!-- ##### FUNCTION g_static_rw_lock_writer_unlock ##### -->
<para>
Unlocks @lock. If a thread waits to lock @lock for writing and all
locks for reading have been unlocked, the waiting thread is woken up
and can lock @lock for writing. If no thread waits to lock @lock for
writing and threads wait to lock @lock for reading, the waiting
threads are woken up and can lock @lock for reading.
</para>

@lock: a #GStaticRWLock to unlock after writing


<!-- ##### FUNCTION g_static_rw_lock_free ##### -->
<para>
Releases all resources allocated to @lock. 
</para>

<para>
You don't have to call this functions for a #GStaticRWLock with an
unbounded lifetime, i.e. objects declared 'static', but if you have a
#GStaticRWLock as a member of a structure and the structure is freed,
you should also free the #GStaticRWLock.
</para>

@lock: a #GStaticRWLock to be freed


<!-- ##### STRUCT GCond ##### -->

<para>
The #GCond struct is an opaque data structure to represent a
condition. A #GCond is an object, that threads can block on, if they
find a certain condition to be false. If other threads change the
state of this condition they can signal the #GCond, such that the
waiting thread is woken up. 
</para>

<para>
<example>
<title>Using GCond to block a thread until a condition is satisfied</title>
<programlisting>
GCond* data_cond = NULL;   /* Must be initialized somewhere */
GMutex* data_mutex = NULL; /* Must be initialized somewhere */
gpointer current_data = NULL;

void push_data (gpointer data)
{
  g_mutex_lock (data_mutex);
  current_data = data;
  g_cond_signal (data_cond);
  g_mutex_unlock (data_mutex);
}

gpointer pop_data ()
{
  gpointer data;

  g_mutex_lock (data_mutex);
  while (!current_data)
      g_cond_wait (data_cond, data_mutex);
  data = current_data;
  current_data = NULL;
  g_mutex_unlock (data_mutex);
  return data;
}
</programlisting>
</example>
</para>

<para>
Whenever a thread calls pop_data() now, it will wait until
current_data is non-NULL, i.e. until some other thread has called
push_data().
</para>

<note>
<para>
It is important to use the g_cond_wait() and g_cond_timed_wait()
functions only inside a loop, which checks for the condition to be
true as it is not guaranteed that the waiting thread will find it
fulfilled, even if the signaling thread left the condition
in that state. This is because another thread can have altered the
condition, before the waiting thread got the chance to be woken up,
even if the condition itself is protected by a #GMutex, like above.
</para>
</note>

<para>
A #GCond should only be accessed via the following functions.
</para>

<note>
<para>
All of the g_cond_* functions are actually macros. Apart from taking
the addresses of them, you can however use them as if they were functions.
</para>
</note>


<!-- ##### FUNCTION g_cond_new ##### -->

<para>
Creates a new #GCond. This function will abort, if g_thread_init()
has not been called yet.
</para>

@Returns: a new #GCond


<!-- ##### FUNCTION g_cond_signal ##### -->
<para>
If threads are waiting for @cond, exactly one of them is woken up. It
is good practice to hold the same lock as the waiting thread, while
calling this function, though not required.
</para>

<para>
This function can also be used, if g_thread_init() has
not yet been called and will do nothing then.
</para>

@cond: a #GCond


<!-- ##### FUNCTION g_cond_broadcast ##### -->

<para>
If threads are waiting for @cond, all of them are woken up. It is good
practice to lock the same mutex as the waiting threads, while calling
this function, though not required.
</para>

<para>
This function can also be used, if g_thread_init() has
not yet been called and will do nothing then.
</para>

@cond: a #GCond


<!-- ##### FUNCTION g_cond_wait ##### -->

<para>
Waits until this thread is woken up on @cond. The @mutex is unlocked
before falling asleep and locked again before resuming.
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will immediately return then.
</para>

@cond: a #GCond
@mutex: a #GMutex, that is currently locked


<!-- ##### FUNCTION g_cond_timed_wait ##### -->

<para>
Waits until this thread is woken up on @cond, but not longer than
until the time, that is specified by @abs_time. The @mutex is
unlocked before falling asleep and locked again before resuming.
</para>

<para>
If @abs_time is NULL, g_cond_timed_wait() acts like g_cond_wait().
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will immediately return TRUE then.
</para>

@cond: a #GCond
@mutex: a #GMutex, that is currently locked
@abs_time: a #GTimeVal, determining the final time
@Returns: TRUE, if the thread is woken up in time


<!-- ##### FUNCTION g_cond_free ##### -->

<para>
Destroys the #GCond.
</para>

@cond: a #GCond


<!-- ##### STRUCT GPrivate ##### -->
<para>
The #GPrivate struct is an opaque data structure to represent a thread
private data key. Threads can thereby obtain and set a pointer, which
is private to the current thread. Take our give_me_next_number()
example from above.  Now we don't want current_number to be shared
between the threads, but to be private to each thread. This can be
done as follows:

<example>
<title>Using GPrivate for per-thread data</title>
<programlisting>
  GPrivate* current_number_key = NULL; /* Must be initialized somewhere */
                                       /* with g_private_new (g_free); */

  int give_me_next_number ()
  {
    int *current_number = g_private_get (current_number_key);

    if (!current_number)
    {
      current_number = g_new (int,1);
      *current_number = 0;
      g_private_set (current_number_key, current_number);
    }
    *current_number = calc_next_number (*current_number); 
    return *current_number;
  }
</programlisting>
</example>
</para>

<para>
Here the pointer belonging to the key current_number_key is read. If
it is NULL, it has not been set yet. Then get memory for an integer
value, assign this memory to the pointer and write the pointer
back. Now we have an integer value, that is private to the current
thread.
</para>

<para>
The #GPrivate struct should only be accessed via the following functions.
</para>

<note>
<para>
All of the g_private_* functions are actually macros. Apart from taking
the addresses of them, you can however use them as if they were functions.
</para>
</note>


<!-- ##### FUNCTION g_private_new ##### -->

<para>
Creates a new #GPrivate. If @destructor is non-NULL, it is a pointer
to a destructor function. Whenever a thread ends and the corresponding
pointer keyed to this instance of #GPrivate is non-NULL, the
destructor is called with this pointer as the argument.
</para>

<note>
<para>
@destructor is working quite differently from @notify in
g_static_private_set().
</para>
</note>

<note>
<para>
A #GPrivate can not be freed. Reuse it instead, if you can to avoid
shortage or use #GStaticPrivate.
</para>
</note>

<note>
<para>
This function will abort, if g_thread_init() has not been called yet.
</para>
</note>

@destructor: a function to handle the data keyed to #GPrivate, when a
thread ends
@Returns: a new #GPrivate


<!-- ##### FUNCTION g_private_get ##### -->

<para>
Returns the pointer keyed to @private_key for the current thread. This
pointer is NULL, when g_private_set() hasn't been called for the
current @private_key and thread yet.
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will return the value of @private_key casted to #gpointer then.
</para>

@private_key: a #GPrivate
@Returns: the corresponding pointer


<!-- ##### FUNCTION g_private_set ##### -->

<para>
Sets the pointer keyed to @private_key for the current thread.
</para>

<para>
This function can also be used, if g_thread_init() has not yet been
called and will set @private_key to @data casted to #GPrivate* then.
</para>

@private_key: a #GPrivate
@data: the new pointer


<!-- ##### STRUCT GStaticPrivate ##### -->

<para>
A #GStaticPrivate works almost like a #GPrivate, but it has one
significant advantage. It doesn't need to be created at run-time like
a #GPrivate, but can be defined at compile-time. This is similar to
the difference between #GMutex and #GStaticMutex. Now look at our
give_me_next_number() example with #GStaticPrivate:
</para>

<para>
<example>
<title>Using GStaticPrivate for per-thread data</title>
<programlisting>
  int give_me_next_number ()
  {
    static GStaticPrivate current_number_key = G_STATIC_PRIVATE_INIT;
    int *current_number = g_static_private_get (&amp;current_number_key);

    if (!current_number)
    {
      current_number = g_new (int,1);
      *current_number = 0;
      g_static_private_set (&amp;current_number_key, current_number, g_free);
    }
    *current_number = calc_next_number (*current_number); 
    return *current_number;
  }
</programlisting>
</example>
</para>

@index: 

<!-- ##### MACRO G_STATIC_PRIVATE_INIT ##### -->
<para>
Every #GStaticPrivate must be initialized with this macro, before it can
be used.
</para>

<para>
<informalexample>
<programlisting>
GStaticPrivate my_private = G_STATIC_PRIVATE_INIT;
</programlisting>
</informalexample>
</para>



<!-- ##### FUNCTION g_static_private_init ##### -->
<para>
Initializes @private_key. Alternatively you can initialize it with
#G_STATIC_PRIVATE_INIT.
</para>

@private_key: a #GStaticPrivate to be initialized


<!-- ##### FUNCTION g_static_private_get ##### -->
<para>
Works like g_private_get() only for a #GStaticPrivate.
</para>

<para>
This function also works, if g_thread_init() has not yet been called.
</para>

@private_key: a #GStaticPrivate
@Returns: the corresponding pointer


<!-- ##### FUNCTION g_static_private_set ##### -->
<para>
Sets the pointer keyed to @private_key for the current thread and the
function @notify to be called with that pointer (NULL or non-NULL),
whenever the pointer is set again or whenever the current thread ends.
</para>

<para>
This function also works, if g_thread_init() has not yet been
called. If g_thread_init() is called later, the @data keyed to
@private_key will be inherited only by the main thread, i.e. the one that
called g_thread_init().
</para>

<note>
<para>
@notify is working quite differently from @destructor in
g_private_new().
</para>
</note>

@private_key: a #GStaticPrivate
@data: the new pointer
@notify: a function to be called with the pointer, whenever the
current thread ends or sets this pointer again


<!-- ##### FUNCTION g_static_private_free ##### -->
<para>
Releases all resources allocated to @private_key. 
</para>

<para>
You don't have to call this functions for a #GStaticPrivate with an
unbounded lifetime, i.e. objects declared 'static', but if you have a
#GStaticPrivate as a member of a structure and the structure is freed,
you should also free the #GStaticPrivate.
</para>

@private_key: a #GStaticPrivate to be freed