Name

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 a thread implementation to g_thread_init() is provided, GLib will be multi-thread safe. It is not 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_NONE

#define G_THREADS_IMPL_NONE

This macro is defined, if no thread implementation is used. The user 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.

GThreadFunctions

typedef struct {
  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);
} GThreadFunctions;

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 the user calls g_mutex_new() then, mutex_new() from the table provided to g_thread_init() will be called.

g_thread_init ()

void   g_thread_init   (GThreadFunctions *vtable);

The use of GLib from more than one thread, must be followed by an initialization to  the thread system by calling g_thread_init(). Most of the time only a to call g_thread_init (NULL).

Note:   The user should  call only g_thread_init() with a non-NULL parameter.

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. To make sure, that the thread system is initialized, use the following:

    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 the program, the user has 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.

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 its address it can however be used as if it was a function.

GThreadFunc ()

gpointer    (*GThreadFunc)       (gpointer data);

Specifies the type of the func functions passed to g_thread_create() or g_thread_create_full().

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) there are no thread priorities. On other systems (e.g. Solaris) there does not seem to be different scheduling for different priorities. All in all try to avoid being dependent on priorities.

GThread

typedef struct {
} GThread;

The GThread struct represents a running thread. It has three public read-only members, but the underlying struct is bigger, so do 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, the user 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.

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. 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.

If joinable is TRUE, the user 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) there are no thread priorities. On other systems (e.g. Solaris) there does not 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 g_thread_create() cannot be used 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.

 

g_thread_self ()

GThread*    g_thread_self       (void);

This functions returns the GThread corresponding to the calling 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.

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) there are no thread priorities. On other systems (e.g. Solaris) there does not seem to be different scheduling for different priorities. All in all try to avoid being dependent on priorities.

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, the user will encounter better methods to do that. So in general this function should not be used.

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.

 

g_thread_foreach ()

void      g_thread_foreach           (GFunc thread_func, gpointer user_data);

Call thread_func on all existing GThread structures. Note that threads may decide to exit while thread_func is running, so without intimate knowledge about the lifetime of foreign threads, thread_func shouldn't access the GThread* pointer passed in as first argument. However, thread_func will not be called for threads which are known to have exited already.

Due to thread lifetime checks, this function has an execution complexity which is quadratic in the number of existing threads.

GMutex

typedef struct _GMutex 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:

  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 will not work in a multi-threaded application. There current_number must be protected against shared access. A first naive implementation would be:

  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 own programs. One working solution is:

  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 the user wants to use a mutex, the code should also work without calling g_thread_init() first, the user cannot 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, they can be used 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.

 

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 recursive mutexes are needed.

 

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 recursive mutexes are needed.

 

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.

g_mutex_free ()

void    g_mutex_free      (GMutex *mutex);

Destroys mutex.

GStaticMutex

typedef struct _GStaticMutex GStaticMutex;

A GStaticMutex works like a GMutex, but it has one significant advantage. It does not 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 the give_me_next_number() example:

  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;
  }

When the user would like to dynamically create a mutex without a prior call to g_thread_init() as, the code must be usable in non-threaded programs. The function g_mutex_new() cannot be used and thus GMutex also cannot be used as it requires a prior call to g_thread_init(). In such cases, the function GStaticMutex can be used.It must be initialized with g_static_mutex_init() before using it and freed 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, they can be used  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 g_static_mutex_init() must be used.

GStaticMutex my_mutex = G_STATIC_MUTEX_INIT;

g_static_mutex_init ()

void        g_static_mutex_init             (GStaticMutex *mutex);

Initializes mutex. Alternatively, it can be initialized by  G_STATIC_MUTEX_INIT.

g_static_mutex_lock ()

void        g_static_mutex_lock             (GStaticMutex *mutex);

Works like g_mutex_lock(), but for a GStaticMutex.

g_static_mutex_trylock ()

gboolean    g_static_mutex_trylock          (GStaticMutex *mutex);

Works like g_mutex_trylock(), but for a GStaticMutex.

g_static_mutex_unlock ()

void        g_static_mutex_unlock           (GStaticMutex *mutex);

Works like g_mutex_unlock(), but for a GStaticMutex.

g_static_mutex_get_mutex ()

GMutex*     g_static_mutex_get_mutex        (GStaticMutex *mutex);

For some operations (like g_cond_wait()) the user must have a GMutex instead of a GStaticMutex. This function will return the corresponding GMutex for mutex.

g_static_mutex_free ()

void        g_static_mutex_free             (GStaticMutex *mutex);

Releases all resources allocated to mutex.

This function does not have to be called for a GStaticMutex with an unbounded lifetime, i.e. objects declared 'static', but if GStaticMutex is a member of a structure and the structure is freed,  GStaticMutex must also 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 names of existing variables as the parameter can be used, e.g. the name of the variable that the user intents to protect with the lock. Look at the give_me_next_number() example using the G_LOCK_* 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;
  }

G_LOCK_DEFINE_STATIC()

#define     G_LOCK_DEFINE_STATIC(name)

This works like G_LOCK_DEFINE, but it creates a static object.

G_LOCK_EXTERN()

#define     G_LOCK_EXTERN(name)

This declares a lock, that is defined with G_LOCK_DEFINE in another module.

G_LOCK()

#define     G_LOCK(name)

Works like g_mutex_lock(), but for a lock defined with G_LOCK_DEFINE.

G_TRYLOCK()

#define     G_TRYLOCK(name)

Works like g_mutex_trylock(), but for a lock defined with G_LOCK_DEFINE.

G_UNLOCK()

#define     G_UNLOCK(name)

Works like g_mutex_unlock(), but for a lock defined with G_LOCK_DEFINE.

GStaticRecMutex

typedef struct {
} GStaticRecMutex;

A GStaticRecMutex works like a GStaticMutex, but it can be locked multiple times by one thread. If entered n times, however, it must be unlocked n times again to let other threads lock it. An exception is the function g_static_rec_mutex_unlock_full(), that allows 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 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, it can be initialized by  G_STATIC_REC_MUTEX_INIT.

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.

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.

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.

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.

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(),  a call to  g_static_rec_mutex_lock_full() with the depth returned by this function ca be done.

g_static_rec_mutex_free ()

void        g_static_rec_mutex_free         (GStaticRecMutex *mutex);

Releases all resources allocated to a GStaticRecMutex.

A call to this function is not needed for a GStaticRecMutex with an unbounded lifetime, i.e. objects declared 'static', but if  GStaticRecMutex is a member of a structure and the structure is freed, then GStaticRecMutex also needs to be freed.

GStaticRWLock

typedef 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:

  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 one at a time and only if no readers are currently accessing 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 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,it can be initialized by G_STATIC_RW_LOCK_INIT.

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.

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().

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.

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().

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().

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.

g_static_rw_lock_free ()

void        g_static_rw_lock_free           (GStaticRWLock *lock);

Releases all resources allocated to lock.

A call to this function is not needed for a GStaticRWLock with an unbounded lifetime, i.e. objects declared 'static', but if  GStaticRWLock is a member of a structure and the structure is freed, then  GStaticRWLock must also be freed.

GCond

typedef struct _GCond 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.

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;
}

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,  they can be used 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.

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.

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.

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.

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.

g_cond_free ()

void        g_cond_free                     (GCond *cond);

Destroys the GCond.

GPrivate

typedef struct _GPrivate 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. Use the  give_me_next_number() example from above. If the user does not want current_number to be shared between the threads, but to be private to each thread. This can be done as follows:

  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. The integer value thus obtained, 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,  they can be used 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: A GPrivate can not be freed. Reuse it instead, to avoid shortage or use GStaticPrivate. This function will abort, if g_thread_init() has not been called yet.

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() has not 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.

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.

GStaticPrivate

typedef struct {
} GStaticPrivate;

A GStaticPrivate works almost like a GPrivate, but it has one significant advantage. It does not 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 the give_me_next_number() example with GStaticPrivate:

  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, it can be initialized by  G_STATIC_PRIVATE_INIT.

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.

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().

 

g_static_private_free ()

void        g_static_private_free           (GStaticPrivate *private_key);

Releases all resources allocated to private_key.

A call to this function is not needed for a GStaticPrivate with an unbounded lifetime, i.e. objects declared 'static', but if  GStaticPrivate is a member of a structure and the structure is freed, then GStaticPrivate must be freed.

GOnce

typedef struct {
  volatile GOnceStatus status;
  volatile gpointer retval;
} GOnce;

A GOnce struct controls a one-time initialization function. Any one-time initialization function must have its own unique GOnce struct.

enum GOnceStatus

typedef enum
{
  G_ONCE_STATUS_NOTCALLED,
  G_ONCE_STATUS_PROGRESS,
  G_ONCE_STATUS_READY  
} GOnceStatus;

The possible stati of a one-time initialization function controlled by a GOnce struct.

G_ONCE_INIT

#define G_ONCE_INIT { G_ONCE_STATUS_NOTCALLED, NULL }

A GOnce must be initialized with this macro, before it can be used.

GOnce my_once = G_ONCE_INIT;