xref: /external/bluetooth/glib/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 if the program is not carefully designed. In particular, due to
the concurrent nature of threads, no assumptions on the order of
execution of code running in different threads can be made, unless
order is 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).  There are primitives
for thread-private data - 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>

<para>
You must call g_thread_init() before executing any other GLib
functions (except g_mem_set_vtable()) in a GLib program if
g_thread_init() will be called at all. This is a requirement even if
no threads are in fact ever created by the process. It is enough that
g_thread_init() is called. If other GLib functions have been called
before that, the behaviour of the program is undefined. An exception
is g_mem_set_vtable() which may be called before g_thread_init().

Failing this requirement can lead to hangs or crashes, apparently more
easily on Windows than on Linux, for example.

Please note that if you call functions in some GLib-using library, in
particular those above the GTK+ stack, that library might well call
g_thread_init() itself, or call some other library that calls
g_thread_init(). Thus, if you use some GLib-based library that is
above the GTK+ stack, it is safest to call g_thread_init() in your
application's main() before calling any GLib functions or functions in
GLib-using libraries.

After calling g_thread_init(), GLib is completely
thread safe (all global data is automatically locked), but individual
data structure instances are not automatically locked for performance
reasons. So, for example you must coordinate accesses to the same
#GHashTable from multiple threads.  The two notable exceptions from
this rule are #GMainLoop and #GAsyncQueue,
which <emphasis>are</emphasis> threadsafe and needs no further
application-level locking to be accessed from multiple threads.
</para>

<para>
To help debugging problems in multithreaded applications, GLib supports
error-checking mutexes that will give you helpful error messages on
common problems. To use error-checking mutexes, define the symbol
#G_ERRORCHECK_MUTEXES when compiling the application.
</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>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### 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 it does mean that the infrastructure is in place and
that once you provide a thread implementation to g_thread_init(), GLib
will be multi-thread safe. If #G_THREADS_ENABLED is not defined, then
Glib is not, and cannot be, multi-thread safe.
</para>



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

<para>
This macro is defined if POSIX style threads are 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 the table are directly used by their
g_* prepended counterparts (described in this document).  For example,
if you call g_mutex_new() then mutex_new() from the table provided to
g_thread_init() will be called.
</para>

<note>
<para>
Do not use this struct unless 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:
@thread_equal:

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

<para>
If you use GLib from more than one thread, you must initialize
the thread system by calling g_thread_init(). Most of the time you
will only have to call <literal>g_thread_init (NULL)</literal>.
</para>

<note><para>
Do not call g_thread_init() with a non-%NULL parameter unless you
really know what you are doing.
</para></note>

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

<note><para>
g_thread_init() changes the way in which #GTimer measures elapsed time.
As a consequence, timers that are running while g_thread_init() is called
may report unreliable times.
</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 this:
</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, if no
thread system is available in GLib (i.e. either #G_THREADS_ENABLED is
not defined or #G_THREADS_IMPL_NONE is defined), the program will
abort.
</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 <command>pkg-config --libs gthread-2.0</command>
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 %TRUE if the thread system is initialized, and
%FALSE if it is 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.


<!-- ##### FUNCTION g_thread_get_initialized ##### -->
<para>

</para>

@Returns:


<!-- ##### USER_FUNCTION GThreadFunc ##### -->
<para>
Specifies the type of the @func functions passed to
g_thread_create() or g_thread_create_full().
</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) there are no thread
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>


<!-- ##### FUNCTION g_thread_create ##### -->
<para>
This function creates a new thread with the default priority.
</para>

<para>
If @joinable is %TRUE, you can wait for this threads termination
calling g_thread_join(). Otherwise the thread will just disappear when
it terminates.
</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>

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


<!-- ##### FUNCTION g_thread_create_full ##### -->
<para>
This function creates a new thread with the priority @priority. If the
underlying thread implementation supports it, the thread gets a stack
size of @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_join(). Otherwise the thread will just disappear when
it terminates. 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) there are no thread
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>

<note>
<para>
Only use g_thread_create_full() if you really can't use
g_thread_create() instead. g_thread_create() does not take
@stack_size, @bound, and @priority as arguments, as they should only
be used in cases in which it is unavoidable.
</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>
Changes 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) there are no thread
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>
Gives 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 this function.
</para>



<!-- ##### FUNCTION g_thread_exit ##### -->
<para>
Exits 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 get @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_exit (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.


<!-- ##### FUNCTION g_thread_foreach ##### -->
<para>

</para>

@thread_func:
@user_data:


<!-- ##### 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. 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, and your code should also work without
calling g_thread_init() first, then you can not use a #GMutex, as
g_mutex_new() requires that the thread system be initialized. Use a
#GStaticMutex instead.
</para>

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

<note>
<para>
All of the <function>g_mutex_*</function> functions are actually macros.
Apart from taking their addresses, 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 be used even if g_thread_init() has not yet been
called, and, in that case, will do nothing.
</para>

<note>
<para>
#GMutex is neither guaranteed to be recursive nor to be non-recursive,
i.e. a thread could deadlock while calling g_mutex_lock(), if it
already has locked @mutex. Use #GStaticRecMutex, 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 be used even if g_thread_init() has not yet been
called, and, in that case, will immediately return %TRUE.
</para>

<note>
<para>
#GMutex is neither guaranteed to be recursive nor to be non-recursive,
i.e. the return value of g_mutex_trylock() could be both %FALSE or
%TRUE, if the current thread already has locked @mutex. Use
#GStaticRecMutex, 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 be used even if g_thread_init() has not yet been
called, and, in that case, will do nothing.
</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 <function>give_me_next_number()</function> example:
</para>

<para>
<example>
<title>Using <structname>GStaticMutex</structname> 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 resources.
</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 <function>g_static_mutex_*</function> functions apart from
<function>g_static_mutex_get_mutex</function> can also be used even if
g_thread_init() has not yet been called. Then they do nothing, apart
from <function>g_static_mutex_trylock</function>, which does nothing
but returning %TRUE.
</para>

<note>
<para>
All of the <function>g_static_mutex_*</function> functions are actually
macros. Apart from taking their addresses, 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 anywhere 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
<function>give_me_next_number()</function> 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, 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 of the #GStaticRecMutex 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 <function>g_static_rec_mutex_*</function> functions can be
used even if g_thread_init() has not been called. Then they do
nothing, apart from <function>g_static_rec_mutex_trylock</function>,
which does nothing but returning %TRUE.
</para>


<!-- ##### 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 thread will be allowed to lock @mutex only
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 @depth 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 &lt; 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 <function>my_array_get()</function> function) simultaneously,
whereas the writers (the <function>my_array_set()</function> function)
will only be allowed once at a time and only if no readers currently access
the array. This is because of the potentially dangerous resizing of the
array. Using these functions is fully multi-thread safe now.
</para>

<para>
Most of the time, writers should have precedence over 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 <function>g_static_rw_lock_*</function> functions can be
used even if g_thread_init() has not been called. Then they do
nothing, apart from <function>g_static_rw_lock_*_trylock</function>,
which does nothing but returning %TRUE.
</para>

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


<!-- ##### 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 is not recursive. It might seem to be possible to
recursively lock for reading, but that can result in a deadlock, due
to writer preference.
</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, immediately returns %FALSE. Otherwise 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 is waiting 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 is waiting to lock
@lock for writing, and some thread or threads are waiting 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 that represents a
condition. Threads can block on a #GCond if they find a certain
condition to be false. If other threads change the state of this
condition they signal the #GCond, and that causes the waiting threads
to be 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 <function>pop_data()</function> now, it will
wait until current_data is non-%NULL, i.e. until some other thread
has called <function>push_data()</function>.
</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.  It is not guaranteed that the waiting thread will find the
condition fulfilled after it wakes up, even if the signaling thread
left the condition in that state: another thread may 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 <function>g_cond_*</function> functions are actually macros.
Apart from taking their addresses, 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 be used even if g_thread_init() has not yet been called,
and, in that case, will do nothing.
</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 be used even if g_thread_init() has not yet been called,
and, in that case, will do nothing.
</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 be used even if g_thread_init() has not yet been
called, and, in that case, will immediately return.
</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 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 be used even if g_thread_init() has not yet been
called, and, in that case, will immediately return %TRUE.
</para>

<para>
To easily calculate @abs_time a combination of g_get_current_time()
and g_time_val_add() can be used.
</para>

@cond: a #GCond.
@mutex: a #GMutex that is currently locked.
@abs_time: a #GTimeVal, determining the final time.
@Returns: %TRUE if @cond was signalled, or %FALSE on timeout.


<!-- ##### 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 <function>give_me_next_number()</function> example from above.
Suppose we don't want <literal>current_number</literal> to be shared
between the threads, but instead 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 <literal>current_number_key</literal>
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 <function>g_private_*</function> functions are actually macros.
Apart from taking their addresses, 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 used 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 destroy 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.
If g_private_set() hasn't been called for the
current @private_key and thread yet, this pointer will be %NULL.
</para>

<para>
This function can be used even if g_thread_init() has not yet been called, and,
in that case, will return the value of @private_key casted to #gpointer.
Note however, that private data set <emphasis>before</emphasis> g_thread_init() will
<emphasis>not</emphasis> be retained <emphasis>after</emphasis> the call. Instead, %NULL
will be returned in all threads directly after g_thread_init(), regardless of
any g_private_set() calls issued before threading system intialization.
</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 be used even if g_thread_init() has not yet been
called, and, in that case, will set @private_key to @data casted to #GPrivate*.
See g_private_get() for resulting caveats.
</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
<function>give_me_next_number()</function> 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>


<!-- ##### 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 works even 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 works even 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 used 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.


<!-- ##### STRUCT GOnce ##### -->
<para>
A <structname>GOnce</structname> struct controls a one-time initialization
function. Any one-time initialization function must have its own unique
<structname>GOnce</structname> struct.
</para>

@status: the status of the #GOnce
@retval: the value returned by the call to the function, if @status
  is %G_ONCE_STATUS_READY
@Since: 2.4

<!-- ##### ENUM GOnceStatus ##### -->
<para>
The possible statuses of a one-time initialization function controlled by a #GOnce struct.
</para>

@G_ONCE_STATUS_NOTCALLED: the function has not been called yet.
@G_ONCE_STATUS_PROGRESS: the function call is currently in progress.
@G_ONCE_STATUS_READY: the function has been called.
@Since: 2.4

<!-- ##### MACRO G_ONCE_INIT ##### -->
<para>
A #GOnce must be initialized with this macro before it can be used.
</para>
<para>
<informalexample>
<programlisting>
GOnce my_once = G_ONCE_INIT;
</programlisting>
</informalexample>
</para>

@Since: 2.4


<!-- ##### MACRO g_once ##### -->
<para>
The first call to this routine by a process with a given #GOnce struct calls
@func with the given argument. Thereafter, subsequent calls to g_once()  with
the same #GOnce struct do not call @func again, but return the stored result
of the first call. On return from g_once(), the status of @once will be
%G_ONCE_STATUS_READY.
</para>
<para>
For example, a mutex or a thread-specific data key must be created exactly
once. In a threaded environment, calling g_once() ensures that the
initialization is serialized across multiple threads.
</para>
<note><para>
Calling g_once() recursively on the same #GOnce struct in @func will lead
to a deadlock.
</para></note>
<para>
<informalexample>
<programlisting>
gpointer
get_debug_flags ()
{
  static GOnce my_once = G_ONCE_INIT;

  g_once (&amp;my_once, parse_debug_flags, NULL);

  return my_once.retval;
}
</programlisting>
</informalexample>
</para>

@once: a #GOnce structure
@func: the #GThreadFunc function associated to @once. This function is
       called only once, regardless of the number of times it and its
       associated #GOnce struct are passed to g_once() .
@arg:  data to be passed to @func
@Since: 2.4


<!-- ##### FUNCTION g_once_init_enter ##### -->
<para>
Function to be called when starting a critical initialization section.
The argument @value_location must point to a static 0-initialized variable
that will be set to a value other than 0 at the end of the initialization
section.
In combination with g_once_init_leave() and the unique address @value_location,
it can be ensured that an initialization section will be executed only once
during a program's life time, and that concurrent threads are blocked until
initialization completed. To be used in constructs like this:
</para>

<para>
<informalexample><programlisting>
static gsize initialization_value = 0;
if (g_once_init_enter (&amp;initialization_value))              /* section start */
  {
    gsize setup_value = 42; /* initialization code here */
    g_once_init_leave (&amp;initialization_value, setup_value); /* section end */
  }
/* use initialization_value here */
</programlisting></informalexample>
</para>

@value_location: location of a static initializable variable containing 0.
@Returns: %TRUE if the initialization section should be entered, %FALSE and blocks otherwise
@Since: 2.14


<!-- ##### FUNCTION g_once_init_leave ##### -->
<para>
Counterpart to g_once_init_enter(). Expects a location of a static
0-initialized initialization variable, and an initialization value other
than 0. Sets the variable to the initialization value, and releases
concurrent threads blocking in g_once_init_enter() on this initialization
variable.
</para>

@value_location: location of a static initializable variable containing 0.
@initialization_value: new non-0 value for *@value_location.
@Since: 2.14