Clarification on Glib Thread-Safe Operation

David Judkovics <djudkovi@xxxxxxxxxx> · Thu, 02 Oct 2003 10:43:01 -0400

My project makes extensive use of what I believe are glibs non thread related
functions and types.  The wording of the documentation concerning glib thread
safe operation has left me confused.

The documentation states glib must be compiled with the macro 'G_THREADS_ENABLED'
defined for thread safe operation.

..."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."...

Later on the documentation for 'g_thread_init()'  states, 

..."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)."...

To me thread safe operation and the use of glib thread related functions
are two distinct configuration issues.  The documentation seems to mix them.

This is my confusion.

My project is a library that uses the 'g_slist_' calls extensively, calls
I believe that are not thread related.  My project library must be thread
safe.  The version of glib we are using does not have 'G_THREADS_ENABLED'
defined.  Is my project library thread safe within the context of its use
of glib's 'g_slist_' calls?

Put another way,  is glibs 'g_slist_' calls thread safe without 'G_THREADS_ENABLED'
defined?

Cheers

David Judkovics

dmjudkov@xxxxxxxxxx

Adv. Software Engineer

LTC xSeries Linux

IBM Server -- Endicott, NY

Tel: 607-429-4745 - Tie 620-4745

Title: 
Threads

GLib Reference Manual 

Threads

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

#include <glib.h>

#define     G_THREADS_ENABLED
#define     G_THREADS_IMPL_POSIX
#define     G_THREADS_IMPL_SOLARIS
#define     G_THREADS_IMPL_NONE

#define     G_THREAD_ERROR
enum        GThreadError;

struct      GThreadFunctions;
void        g_thread_init                   (GThreadFunctions *vtable);
gboolean    g_thread_supported              ();

gpointer    (*GThreadFunc)                  (gpointer data);
enum        GThreadPriority;
struct      GThread;
GThread*    g_thread_create                 (GThreadFunc func,
                                             gpointer data,
                                             gboolean joinable,
                                             GError **error);
GThread*    g_thread_create_full            (GThreadFunc func,
                                             gpointer data,
                                             gulong stack_size,
                                             gboolean joinable,
                                             gboolean bound,
                                             GThreadPriority priority,
                                             GError **error);
GThread*    g_thread_self                   (void);
gpointer    g_thread_join                   (GThread *thread);
void        g_thread_set_priority           (GThread *thread,
                                             GThreadPriority priority);
void        g_thread_yield                  ();
void        g_thread_exit                   (gpointer retval);

struct      GMutex;
GMutex*     g_mutex_new                     ();
void        g_mutex_lock                    (GMutex *mutex);
gboolean    g_mutex_trylock                 (GMutex *mutex);
void        g_mutex_unlock                  (GMutex *mutex);
void        g_mutex_free                    (GMutex *mutex);

struct      GStaticMutex;
#define     G_STATIC_MUTEX_INIT
void        g_static_mutex_init             (GStaticMutex *mutex);
void        g_static_mutex_lock             (GStaticMutex *mutex);
gboolean    g_static_mutex_trylock          (GStaticMutex *mutex);
void        g_static_mutex_unlock           (GStaticMutex *mutex);
GMutex*     g_static_mutex_get_mutex        (GStaticMutex *mutex);
void        g_static_mutex_free             (GStaticMutex *mutex);

#define     G_LOCK_DEFINE                   (name)
#define     G_LOCK_DEFINE_STATIC            (name)
#define     G_LOCK_EXTERN                   (name)
#define     G_LOCK                          (name)
#define     G_TRYLOCK                       (name)
#define     G_UNLOCK                        (name)

struct      GStaticRecMutex;
#define     G_STATIC_REC_MUTEX_INIT
void        g_static_rec_mutex_init         (GStaticRecMutex *mutex);
void        g_static_rec_mutex_lock         (GStaticRecMutex *mutex);
gboolean    g_static_rec_mutex_trylock      (GStaticRecMutex *mutex);
void        g_static_rec_mutex_unlock       (GStaticRecMutex *mutex);
void        g_static_rec_mutex_lock_full    (GStaticRecMutex *mutex,
                                             guint depth);
guint       g_static_rec_mutex_unlock_full  (GStaticRecMutex *mutex);
void        g_static_rec_mutex_free         (GStaticRecMutex *mutex);

struct      GStaticRWLock;
#define     G_STATIC_RW_LOCK_INIT
void        g_static_rw_lock_init           (GStaticRWLock *lock);
void        g_static_rw_lock_reader_lock    (GStaticRWLock *lock);
gboolean    g_static_rw_lock_reader_trylock (GStaticRWLock *lock);
void        g_static_rw_lock_reader_unlock  (GStaticRWLock *lock);
void        g_static_rw_lock_writer_lock    (GStaticRWLock *lock);
gboolean    g_static_rw_lock_writer_trylock (GStaticRWLock *lock);
void        g_static_rw_lock_writer_unlock  (GStaticRWLock *lock);
void        g_static_rw_lock_free           (GStaticRWLock *lock);

struct      GCond;
GCond*      g_cond_new                      ();
void        g_cond_signal                   (GCond *cond);
void        g_cond_broadcast                (GCond *cond);
void        g_cond_wait                     (GCond *cond,
                                             GMutex *mutex);
gboolean    g_cond_timed_wait               (GCond *cond,
                                             GMutex *mutex,
                                             GTimeVal *abs_time);
void        g_cond_free                     (GCond *cond);

struct      GPrivate;
GPrivate*   g_private_new                   (GDestroyNotify destructor);
gpointer    g_private_get                   (GPrivate *private_key);
void        g_private_set                   (GPrivate *private_key,
                                             gpointer data);

struct      GStaticPrivate;
#define     G_STATIC_PRIVATE_INIT
void        g_static_private_init           (GStaticPrivate *private_key);
gpointer    g_static_private_get            (GStaticPrivate *private_key);
void        g_static_private_set            (GStaticPrivate *private_key,
                                             gpointer data,
                                             GDestroyNotify notify);
void        g_static_private_free           (GStaticPrivate *private_key);

Description

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.

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).
Details
G_THREADS_ENABLED
#define G_THREADS_ENABLED

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.
G_THREADS_IMPL_POSIX
#define G_THREADS_IMPL_POSIX

This macro is defined, if POSIX style threads are used.
G_THREADS_IMPL_SOLARIS
#define G_THREADS_IMPL_SOLARIS

This macro is defined, if the Solaris thread system is used.
G_THREADS_IMPL_NONE
#define G_THREADS_IMPL_NONE

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.
G_THREAD_ERROR
#define G_THREAD_ERROR g_thread_error_quark ()

The error domain of the GLib thread subsystem.
enum GThreadError
typedef enum
{
  G_THREAD_ERROR_AGAIN /* Resource temporarily unavailable */
} GThreadError;

Possible errors of thread related functions.
G_THREAD_ERROR_AGAIN a thread couldn't be created due to resource
shortage. Try again later.

struct GThreadFunctions
struct GThreadFunctions {

  GMutex*  (*mutex_new)           (void);
  void     (*mutex_lock)          (GMutex               *mutex);
  gboolean (*mutex_trylock)       (GMutex               *mutex);
  void     (*mutex_unlock)        (GMutex               *mutex);
  void     (*mutex_free)          (GMutex               *mutex);
  GCond*   (*cond_new)            (void);
  void     (*cond_signal)         (GCond                *cond);
  void     (*cond_broadcast)      (GCond                *cond);
  void     (*cond_wait)           (GCond                *cond,
                                   GMutex               *mutex);
  gboolean (*cond_timed_wait)     (GCond                *cond,
                                   GMutex               *mutex,
                                   GTimeVal             *end_time);
  void      (*cond_free)          (GCond                *cond);
  GPrivate* (*private_new)        (GDestroyNotify        destructor);
  gpointer  (*private_get)        (GPrivate             *private_key);
  void      (*private_set)        (GPrivate             *private_key,
                                   gpointer              data);
  void      (*thread_create)      (GThreadFunc           func,
                                   gpointer              data,
                                   gulong                stack_size,
                                   gboolean              joinable,
                                   gboolean              bound,
                                   GThreadPriority       priority,
                                   gpointer              thread,
                                   GError              **error);
  void      (*thread_yield)       (void);
  void      (*thread_join)        (gpointer              thread);
  void      (*thread_exit)        (void);
  void      (*thread_set_priority)(gpointer              thread,
                                   GThreadPriority       priority);
  void      (*thread_self)        (gpointer              thread);
  gboolean  (*thread_equal)       (gpointer              thread1,
				   gpointer              thread2);
};

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.
Note

This struct should only be used, if you know, what you are doing.
g_thread_init ()
void        g_thread_init                   (GThreadFunctions *vtable);

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). 
Note

You should only call g_thread_init() with a non-NULL parameter if you
really know what you are doing.
Note

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().

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:

if (!g_thread_supported ()) g_thread_init (NULL);

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.

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.
Note

To use g_thread_init() in your program, you have to link with the
libraries that the command pkg-config --libs gthread-2.0 
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.
vtable : a function table of type GThreadFunctions, that provides the
entry points to the thread system to be used.

g_thread_supported ()
gboolean    g_thread_supported              ();

This function returns, whether the thread system is initialized or
not.
Note

This function is actually a macro. Apart from taking the address of it
you can however use it as if it was a function.
Returns : TRUE, if the thread system is initialized.

GThreadFunc ()
gpointer    (*GThreadFunc)                  (gpointer data);

Specifies the type of the func functions passed to
g_thread_create() or g_thread_create_full().
data : data passed to the thread.
Returns : the return value of the thread, which will be returned by
g_thread_join().

enum GThreadPriority
typedef enum
{
  G_THREAD_PRIORITY_LOW,
  G_THREAD_PRIORITY_NORMAL,
  G_THREAD_PRIORITY_HIGH,
  G_THREAD_PRIORITY_URGENT
} GThreadPriority;

Specifies the priority of a thread. 
Note

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.
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
struct GThread {

};

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.
Note

Resources for a joinable thread are not fully released until
g_thread_join() is called for that thread.
g_thread_create ()
GThread*    g_thread_create                 (GThreadFunc func,
                                             gpointer data,
                                             gboolean joinable,
                                             GError **error);

This function creates a new thread with the default priority.

If joinable is TRUE, you can wait for this threads termination
calling g_thread_join(). Otherwise the thread will just disappear, when
ready. 

The new thread executes the function func with the argument
data. If the thread was created successfully, it is returned.

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.
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.

g_thread_create_full ()
GThread*    g_thread_create_full            (GThreadFunc func,
                                             gpointer data,
                                             gulong stack_size,
                                             gboolean joinable,
                                             gboolean bound,
                                             GThreadPriority priority,
                                             GError **error);

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.

If joinable is TRUE, you can wait for this threads termination
calling g_thread_join(). 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.

The new thread executes the function func with the argument
data. If the thread was created successfully, it is returned.

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.
Note

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.
Note

Only use g_thread_create_full(), when 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 for cases, where it is inevitable. 
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.

g_thread_self ()
GThread*    g_thread_self                   (void);

This functions returns the GThread corresponding to the calling thread.
Returns : the current thread.

g_thread_join ()
gpointer    g_thread_join                   (GThread *thread);

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

g_thread_set_priority ()
void        g_thread_set_priority           (GThread *thread,
                                             GThreadPriority priority);

Changes the priority of thread to priority.
Note

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.
thread : a GThread.
priority : a new priority for thread.

g_thread_yield ()
void        g_thread_yield                  ();

Gives way to other threads waiting to be scheduled. 

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.
g_thread_exit ()
void        g_thread_exit                   (gpointer retval);

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 getting retval as the return value of
g_thread_join(). If the current thread is not joinable, retval is
ignored. Calling

g_thread_exit (retval);

is equivalent to calling 

return retval;

in the function func, as given to g_thread_create().
Note

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.
retval : the return value of this thread.

struct GMutex
struct GMutex;

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 1. A function which will not work in a threaded environment
  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;
  }

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:

Example 2. The wrong way to write a thread-safe function
  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;
  }

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:

Example 3. A correct thread-safe function
  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;
  }

GStaticMutex provides a simpler and safer way of doing this.

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.

A GMutex should only be accessed via the following functions.
Note

All of the g_mutex_* functions are actually macros. 
Apart from taking their addresses, you can however use them as if they 
were functions.
g_mutex_new ()
GMutex*     g_mutex_new                     ();

Creates a new GMutex. 
Note

This function will abort, if g_thread_init() has not been called yet.
Returns : a new GMutex.

g_mutex_lock ()
void        g_mutex_lock                    (GMutex *mutex);

Locks mutex. If mutex is already locked by another thread, the
current thread will block until mutex is unlocked by the other
thread.

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

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.
mutex : a GMutex.

g_mutex_trylock ()
gboolean    g_mutex_trylock                 (GMutex *mutex);

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

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

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.
mutex : a GMutex.
Returns : TRUE, if mutex could be locked.

g_mutex_unlock ()
void        g_mutex_unlock                  (GMutex *mutex);

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

This function can also be used, if g_thread_init() has not yet been
called and will do nothing then.
mutex : a GMutex.

g_mutex_free ()
void        g_mutex_free                    (GMutex *mutex);

Destroys mutex.
mutex : a GMutex.

struct GStaticMutex
struct GStaticMutex;

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:

Example 4. Using GStaticMutex to simplify thread-safe programming
  int give_me_next_number ()
  {
    static int current_number = 0;
    int ret_val;
    static GStaticMutex mutex = G_STATIC_MUTEX_INIT;

    g_static_mutex_lock (&mutex);
    ret_val = current_number = calc_next_number (current_number); 
    g_static_mutex_unlock (&mutex);
    return ret_val;
  }

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.

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

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

All of the g_static_mutex_* functions are actually 
macros. Apart from taking their addresses, you can however use them 
as if they were functions.
G_STATIC_MUTEX_INIT
#define G_STATIC_MUTEX_INIT

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().

GStaticMutex my_mutex = G_STATIC_MUTEX_INIT;

g_static_mutex_init ()
void        g_static_mutex_init             (GStaticMutex *mutex);

Initializes mutex. Alternatively you can initialize it with
G_STATIC_MUTEX_INIT.
mutex : a GStaticMutex to be initialized.

g_static_mutex_lock ()
void        g_static_mutex_lock             (GStaticMutex *mutex);

Works like g_mutex_lock(), but for a GStaticMutex.
mutex : a GStaticMutex.

g_static_mutex_trylock ()
gboolean    g_static_mutex_trylock          (GStaticMutex *mutex);

Works like g_mutex_trylock(), but for a GStaticMutex.
mutex : a GStaticMutex.
Returns : TRUE, if the GStaticMutex could be locked.

g_static_mutex_unlock ()
void        g_static_mutex_unlock           (GStaticMutex *mutex);

Works like g_mutex_unlock(), but for a GStaticMutex.
mutex : a GStaticMutex.

g_static_mutex_get_mutex ()
GMutex*     g_static_mutex_get_mutex        (GStaticMutex *mutex);

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.
mutex : a GStaticMutex.
Returns : the GMutex corresponding to mutex.

g_static_mutex_free ()
void        g_static_mutex_free             (GStaticMutex *mutex);

Releases all resources allocated to mutex. 

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.
mutex : a GStaticMutex to be freed.

G_LOCK_DEFINE()
#define     G_LOCK_DEFINE(name)

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:

Example 5. Using the G_LOCK_* convenience macros
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;
  }

name : the name of the lock.

G_LOCK_DEFINE_STATIC()
#define     G_LOCK_DEFINE_STATIC(name)

This works like G_LOCK_DEFINE, but it creates a static object.
name : the name of the lock.

G_LOCK_EXTERN()
#define     G_LOCK_EXTERN(name)

This declares a lock, that is defined with G_LOCK_DEFINE in another module.
name : the name of the lock.

G_LOCK()
#define     G_LOCK(name)

Works like g_mutex_lock(), but for a lock defined with G_LOCK_DEFINE.
name : the name of the lock.

G_TRYLOCK()
#define     G_TRYLOCK(name)

Works like g_mutex_trylock(), but for a lock defined with G_LOCK_DEFINE.
name : the name of the lock.
Returns : TRUE, if the lock could be locked.

G_UNLOCK()
#define     G_UNLOCK(name)

Works like g_mutex_unlock(), but for a lock defined with G_LOCK_DEFINE.
name : the name of the lock.

struct GStaticRecMutex
struct GStaticRecMutex {

};

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().

Even though GStaticRecMutex is not opaque, it should only be used with
the following functions.

All of the g_static_rec_mutex_* functions can also 
be used, if g_thread_init() has not been called.
G_STATIC_REC_MUTEX_INIT
#define G_STATIC_REC_MUTEX_INIT { G_STATIC_MUTEX_INIT }

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().

GStaticRecMutex my_mutex = G_STATIC_REC_MUTEX_INIT;

g_static_rec_mutex_init ()
void        g_static_rec_mutex_init         (GStaticRecMutex *mutex);

A GStaticRecMutex must be initialized with this function, before it
can be used. Alternatively you can initialize it with
G_STATIC_REC_MUTEX_INIT.
mutex : a GStaticRecMutex to be initialized.

g_static_rec_mutex_lock ()
void        g_static_rec_mutex_lock         (GStaticRecMutex *mutex);

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.
mutex : a GStaticRecMutex to lock.

g_static_rec_mutex_trylock ()
gboolean    g_static_rec_mutex_trylock      (GStaticRecMutex *mutex);

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

g_static_rec_mutex_unlock ()
void        g_static_rec_mutex_unlock       (GStaticRecMutex *mutex);

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.
mutex : a GStaticRecMutex to unlock.

g_static_rec_mutex_lock_full ()
void        g_static_rec_mutex_lock_full    (GStaticRecMutex *mutex,
                                             guint depth);

Works like calling g_static_rec_mutex_lock() for mutex depth times.
mutex : a GStaticRecMutex to lock.
depth : number of times this mutex has to be unlocked to be completely unlocked.

g_static_rec_mutex_unlock_full ()
guint       g_static_rec_mutex_unlock_full  (GStaticRecMutex *mutex);

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

g_static_rec_mutex_free ()
void        g_static_rec_mutex_free         (GStaticRecMutex *mutex);

Releases all resources allocated to a GStaticRecMutex.

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.
mutex : a GStaticRecMutex to be freed.

struct GStaticRWLock
struct GStaticRWLock {

};

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 6. An array with access functions
  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 (&rwlock);

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

    g_static_rw_lock_reader_unlock (&rwlock);

    return retval;
  }

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

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) 
will only 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 these functions is fully multi-thread safe now. 

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.

Even though GStaticRWLock is not opaque, it should only be used with
the following functions.

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

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()
have 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.
G_STATIC_RW_LOCK_INIT
#define G_STATIC_RW_LOCK_INIT { G_STATIC_MUTEX_INIT, NULL, NULL, 0, FALSE, 0, 0 }

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().

GStaticRWLock my_lock = G_STATIC_RW_LOCK_INIT;

g_static_rw_lock_init ()
void        g_static_rw_lock_init           (GStaticRWLock *lock);

A GStaticRWLock must be initialized with this function, before it can
be used. Alternatively you can initialize it with
G_STATIC_RW_LOCK_INIT.
lock : a GStaticRWLock to be initialized.

g_static_rw_lock_reader_lock ()
void        g_static_rw_lock_reader_lock    (GStaticRWLock *lock);

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().

GStaticRWLock is not recursive. It might seem to be possible to
recursively lock for reading, but that can result in a deadlock as
well, due to writer preference.
lock : a GStaticRWLock to lock for reading.

g_static_rw_lock_reader_trylock ()
gboolean    g_static_rw_lock_reader_trylock (GStaticRWLock *lock);

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().
lock : a GStaticRWLock to lock for reading.
Returns : TRUE, if lock could be locked for reading.

g_static_rw_lock_reader_unlock ()
void        g_static_rw_lock_reader_unlock  (GStaticRWLock *lock);

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.
lock : a GStaticRWLock to unlock after reading.

g_static_rw_lock_writer_lock ()
void        g_static_rw_lock_writer_lock    (GStaticRWLock *lock);

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().
lock : a GStaticRWLock to lock for writing.

g_static_rw_lock_writer_trylock ()
gboolean    g_static_rw_lock_writer_trylock (GStaticRWLock *lock);

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().
lock : a GStaticRWLock to lock for writing.
Returns : TRUE, if lock could be locked for writing.

g_static_rw_lock_writer_unlock ()
void        g_static_rw_lock_writer_unlock  (GStaticRWLock *lock);

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.
lock : a GStaticRWLock to unlock after writing.

g_static_rw_lock_free ()
void        g_static_rw_lock_free           (GStaticRWLock *lock);

Releases all resources allocated to lock. 

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.
lock : a GStaticRWLock to be freed.

struct GCond
struct GCond;

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. 

Example 7. Using GCond to block a thread until a condition is satisfied
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 = NULL;
  g_mutex_unlock (data_mutex);
  return data;
}

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().
Note

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.

A GCond should only be accessed via the following functions.
Note

All of the g_cond_* functions are actually macros. 
Apart from taking their addresses, you can however use them as if they 
were functions.
g_cond_new ()
GCond*      g_cond_new                      ();

Creates a new GCond. This function will abort, if g_thread_init()
has not been called yet.
Returns : a new GCond.

g_cond_signal ()
void        g_cond_signal                   (GCond *cond);

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.

This function can also be used, if g_thread_init() has
not yet been called and will do nothing then.
cond : a GCond.

g_cond_broadcast ()
void        g_cond_broadcast                (GCond *cond);

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.

This function can also be used, if g_thread_init() has
not yet been called and will do nothing then.
cond : a GCond.

g_cond_wait ()
void        g_cond_wait                     (GCond *cond,
                                             GMutex *mutex);

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

This function can also be used, if g_thread_init() has not yet been
called and will immediately return then.
cond : a GCond.
mutex : a GMutex, that is currently locked.

g_cond_timed_wait ()
gboolean    g_cond_timed_wait               (GCond *cond,
                                             GMutex *mutex,
                                             GTimeVal *abs_time);

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.

If abs_time is NULL, g_cond_timed_wait() acts like g_cond_wait().

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

To easily calculate abs_time a combination of g_get_current_time()
and g_time_val_add() can be used.
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.

g_cond_free ()
void        g_cond_free                     (GCond *cond);

Destroys the GCond.
cond : a GCond.

struct GPrivate
struct GPrivate;

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 8. Using GPrivate for per-thread data
  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;
  }

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.

The GPrivate struct should only be accessed via the following functions.
Note

All of the g_private_* functions are actually macros. 
Apart from taking their addresses, you can however use them as if they were 
functions.
g_private_new ()
GPrivate*   g_private_new                   (GDestroyNotify destructor);

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.
Note

destructor is working quite differently from notify in
g_static_private_set().
Note

A GPrivate can not be freed. Reuse it instead, if you can to avoid
shortage or use GStaticPrivate.
Note

This function will abort, if g_thread_init() has not been called yet.
destructor : a function to handle the data keyed to GPrivate, when a
thread ends.
Returns : a new GPrivate.

g_private_get ()
gpointer    g_private_get                   (GPrivate *private_key);

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.

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.
private_key : a GPrivate.
Returns : the corresponding pointer.

g_private_set ()
void        g_private_set                   (GPrivate *private_key,
                                             gpointer data);

Sets the pointer keyed to private_key for the current thread.

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.
private_key : a GPrivate.
data : the new pointer.

struct GStaticPrivate
struct GStaticPrivate {

};

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:

Example 9. Using GStaticPrivate for per-thread data
  int give_me_next_number ()
  {
    static GStaticPrivate current_number_key = G_STATIC_PRIVATE_INIT;
    int *current_number = g_static_private_get (&current_number_key);

    if (!current_number)
    {
      current_number = g_new (int,1);
      *current_number = 0;
      g_static_private_set (&current_number_key, current_number, g_free);
    }
    *current_number = calc_next_number (*current_number); 
    return *current_number;
  }

G_STATIC_PRIVATE_INIT
#define G_STATIC_PRIVATE_INIT 

Every GStaticPrivate must be initialized with this macro, before it can
be used.

GStaticPrivate my_private = G_STATIC_PRIVATE_INIT;

g_static_private_init ()
void        g_static_private_init           (GStaticPrivate *private_key);

Initializes private_key. Alternatively you can initialize it with
G_STATIC_PRIVATE_INIT.
private_key : a GStaticPrivate to be initialized.

g_static_private_get ()
gpointer    g_static_private_get            (GStaticPrivate *private_key);

Works like g_private_get() only for a GStaticPrivate.

This function also works, if g_thread_init() has not yet been called.
private_key : a GStaticPrivate.
Returns : the corresponding pointer.

g_static_private_set ()
void        g_static_private_set            (GStaticPrivate *private_key,
                                             gpointer data,
                                             GDestroyNotify notify);

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.

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().
Note

notify is working quite differently from destructor in
g_private_new().
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.

g_static_private_free ()
void        g_static_private_free           (GStaticPrivate *private_key);

Releases all resources allocated to private_key. 

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.
private_key : a GStaticPrivate to be freed.

See Also

GThreadPool Thread pools.
GAsyncQueue Send asynchronous messages between threads.

<< The Main Event Loop Thread Pools >>
Title: 
Threads

GLib Reference Manual 

Threads

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

#include <glib.h>

#define     G_THREADS_ENABLED
#define     G_THREADS_IMPL_POSIX
#define     G_THREADS_IMPL_SOLARIS
#define     G_THREADS_IMPL_NONE

#define     G_THREAD_ERROR
enum        GThreadError;

struct      GThreadFunctions;
void        g_thread_init                   (GThreadFunctions *vtable);
gboolean    g_thread_supported              ();

gpointer    (*GThreadFunc)                  (gpointer data);
enum        GThreadPriority;
struct      GThread;
GThread*    g_thread_create                 (GThreadFunc func,
                                             gpointer data,
                                             gboolean joinable,
                                             GError **error);
GThread*    g_thread_create_full            (GThreadFunc func,
                                             gpointer data,
                                             gulong stack_size,
                                             gboolean joinable,
                                             gboolean bound,
                                             GThreadPriority priority,
                                             GError **error);
GThread*    g_thread_self                   (void);
gpointer    g_thread_join                   (GThread *thread);
void        g_thread_set_priority           (GThread *thread,
                                             GThreadPriority priority);
void        g_thread_yield                  ();
void        g_thread_exit                   (gpointer retval);

struct      GMutex;
GMutex*     g_mutex_new                     ();
void        g_mutex_lock                    (GMutex *mutex);
gboolean    g_mutex_trylock                 (GMutex *mutex);
void        g_mutex_unlock                  (GMutex *mutex);
void        g_mutex_free                    (GMutex *mutex);

struct      GStaticMutex;
#define     G_STATIC_MUTEX_INIT
void        g_static_mutex_init             (GStaticMutex *mutex);
void        g_static_mutex_lock             (GStaticMutex *mutex);
gboolean    g_static_mutex_trylock          (GStaticMutex *mutex);
void        g_static_mutex_unlock           (GStaticMutex *mutex);
GMutex*     g_static_mutex_get_mutex        (GStaticMutex *mutex);
void        g_static_mutex_free             (GStaticMutex *mutex);

#define     G_LOCK_DEFINE                   (name)
#define     G_LOCK_DEFINE_STATIC            (name)
#define     G_LOCK_EXTERN                   (name)
#define     G_LOCK                          (name)
#define     G_TRYLOCK                       (name)
#define     G_UNLOCK                        (name)

struct      GStaticRecMutex;
#define     G_STATIC_REC_MUTEX_INIT
void        g_static_rec_mutex_init         (GStaticRecMutex *mutex);
void        g_static_rec_mutex_lock         (GStaticRecMutex *mutex);
gboolean    g_static_rec_mutex_trylock      (GStaticRecMutex *mutex);
void        g_static_rec_mutex_unlock       (GStaticRecMutex *mutex);
void        g_static_rec_mutex_lock_full    (GStaticRecMutex *mutex,
                                             guint depth);
guint       g_static_rec_mutex_unlock_full  (GStaticRecMutex *mutex);
void        g_static_rec_mutex_free         (GStaticRecMutex *mutex);

struct      GStaticRWLock;
#define     G_STATIC_RW_LOCK_INIT
void        g_static_rw_lock_init           (GStaticRWLock *lock);
void        g_static_rw_lock_reader_lock    (GStaticRWLock *lock);
gboolean    g_static_rw_lock_reader_trylock (GStaticRWLock *lock);
void        g_static_rw_lock_reader_unlock  (GStaticRWLock *lock);
void        g_static_rw_lock_writer_lock    (GStaticRWLock *lock);
gboolean    g_static_rw_lock_writer_trylock (GStaticRWLock *lock);
void        g_static_rw_lock_writer_unlock  (GStaticRWLock *lock);
void        g_static_rw_lock_free           (GStaticRWLock *lock);

struct      GCond;
GCond*      g_cond_new                      ();
void        g_cond_signal                   (GCond *cond);
void        g_cond_broadcast                (GCond *cond);
void        g_cond_wait                     (GCond *cond,
                                             GMutex *mutex);
gboolean    g_cond_timed_wait               (GCond *cond,
                                             GMutex *mutex,
                                             GTimeVal *abs_time);
void        g_cond_free                     (GCond *cond);

struct      GPrivate;
GPrivate*   g_private_new                   (GDestroyNotify destructor);
gpointer    g_private_get                   (GPrivate *private_key);
void        g_private_set                   (GPrivate *private_key,
                                             gpointer data);

struct      GStaticPrivate;
#define     G_STATIC_PRIVATE_INIT
void        g_static_private_init           (GStaticPrivate *private_key);
gpointer    g_static_private_get            (GStaticPrivate *private_key);
void        g_static_private_set            (GStaticPrivate *private_key,
                                             gpointer data,
                                             GDestroyNotify notify);
void        g_static_private_free           (GStaticPrivate *private_key);

Description