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_NONE

#define             G_THREAD_ERROR
enum                GThreadError;

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

gpointer            (*GThreadFunc)                      (gpointer data);
enum                GThreadPriority;
                    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);
void                g_thread_foreach                    (GFunc thread_func,
                                                         gpointer user_data);

                    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);

                    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)

                    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);

                    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);

                    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);

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

                    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);

                    GOnce;
enum                GOnceStatus;
#define             G_ONCE_INIT
#define             g_once                              (once, func, arg)
gboolean            g_once_init_enter                   (volatile gsize *value_location);
void                g_once_init_leave                   (volatile gsize *value_location,
                                                         gsize initialization_value);

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 if the program is not carefully designed. In particular, due to the concurrent nature of threads, no assumptions on the order of execution of code running in different threads can be made, unless order is explicitly forced by the programmer through synchronization primitives.

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

You must call g_thread_init() before executing any other GLib functions in a threaded GLib program. After that, GLib is completely thread safe (all global data is automatically locked), but individual data structure instances are not automatically locked for performance reasons. So, for example you must coordinate accesses to the same GHashTable from multiple threads. The two notable exceptions from this rule are GMainLoop and GAsyncQueue, which are threadsafe and needs no further application-level locking to be accessed from multiple threads.

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

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


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. 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.

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 the table are directly used by their g_* prepended counterparts (described in this document). For example, if you call g_mutex_new() then mutex_new() from the table provided to g_thread_init() will be called.

Note

Do not use this struct unless you know what you are doing.


g_thread_init ()

void                g_thread_init                       (GThreadFunctions *vtable);

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

Note

Do not call g_thread_init() with a non-NULL parameter unless 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 this:

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

After that line, either the thread system is initialized or, if no thread system is available in GLib (i.e. either G_THREADS_ENABLED is not defined or G_THREADS_IMPL_NONE is defined), the program will abort.

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 TRUE if the thread system is initialized, and FALSE if it is 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) there are no thread priorities. On other systems (e.g. Solaris) there doesn't seem to be different scheduling for different priorities. All in all try to avoid being dependent on priorities.

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

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 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 it terminates.

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

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 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() if you really can't use g_thread_create() instead. g_thread_create() does not take stack_size, bound, and priority as arguments, as they should only be used in cases in which it is unavoidable.

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

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 this 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 get 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.

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.

thread_func :

function to call for all GThread structures

user_data :

second argument to thread_func

Since 2.10


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:

Example 2. 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 3. 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. Please do not use such constructs in your own programs! One working solution is:

Example 4. 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, and your code should also work without calling g_thread_init() first, then you can not use a GMutex, as g_mutex_new() requires that the thread system be initialized. Use a GStaticMutex instead.

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 be used even if g_thread_init() has not yet been called, and, in that case, will do nothing.

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 be used even if g_thread_init() has not yet been called, and, in that case, will immediately return TRUE.

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 be used even if g_thread_init() has not yet been called, and, in that case, will do nothing.

mutex :

a GMutex.

g_mutex_free ()

void                g_mutex_free                        (GMutex *mutex);

Destroys mutex.

mutex :

a GMutex.

GStaticMutex

typedef struct _GStaticMutex 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 5. 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 apart from g_static_mutex_get_mutex can also be used even if g_thread_init() has not yet been called. Then they do nothing, apart from g_static_mutex_trylock, which does nothing but returning TRUE.

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 anywhere variable definitions may appear in programs, i.e. in the first block of a function or outside of functions. The name parameter will be mangled to get the name of the GStaticMutex. This means that you can use names of existing variables as the parameter - e.g. the name of the variable you intent to protect with the lock. Look at our give_me_next_number() example using the G_LOCK_* macros:

Example 6. 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.

GStaticRecMutex

typedef struct {
} GStaticRecMutex;

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

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

All of the g_static_rec_mutex_* functions can be used even if g_thread_init() has not been called. Then they do nothing, apart from g_static_rec_mutex_trylock, which does nothing but returning TRUE.


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 thread will be allowed to lock mutex only when it has been unlocked as many times as it had been locked before. If mutex is completely unlocked and another thread is blocked in a g_static_rec_mutex_lock() call for mutex, it will be woken and can lock mutex itself.

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.

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:

Example 7. 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 at a time and only if no readers currently access the array. This is because of the potentially dangerous resizing of the array. Using these functions is fully multi-thread safe now.

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

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

All of the g_static_rw_lock_* functions can be used even if g_thread_init() has not been called. Then they do nothing, apart from g_static_rw_lock_*_trylock, which does nothing but returning TRUE.

Note

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


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, 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, immediately returns FALSE. Otherwise 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 is waiting to lock lock for writing and all locks for reading have been unlocked, the waiting thread is woken up and can lock lock for writing. If no thread is waiting to lock lock for writing, and some thread or threads are waiting to lock lock for reading, the waiting threads are woken up and can lock lock for reading.

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.

GCond

typedef struct _GCond GCond;

The GCond struct is an opaque data structure that represents a condition. Threads can block on a GCond if they find a certain condition to be false. If other threads change the state of this condition they signal the GCond, and that causes the waiting threads to be woken up.

Example 8. 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;
  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. It is not guaranteed that the waiting thread will find the condition fulfilled after it wakes up, even if the signaling thread left the condition in that state: another thread may have altered the condition before the waiting thread got the chance to be woken up, even if the condition itself is protected by a GMutex, like above.

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 be used even if g_thread_init() has not yet been called, and, in that case, will do nothing.

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 be used even if g_thread_init() has not yet been called, and, in that case, will do nothing.

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 be used even if g_thread_init() has not yet been called, and, in that case, will immediately return.

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 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 be used even if g_thread_init() has not yet been called, and, in that case, will immediately return TRUE.

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 cond was signalled, or FALSE on timeout.

g_cond_free ()

void                g_cond_free                         (GCond *cond);

Destroys the GCond.

cond :

a 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. Take our give_me_next_number() example from above. Suppose we don't want current_number to be shared between the threads, but instead to be private to each thread. This can be done as follows:

Example 9. 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 used 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 destroy 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. If g_private_set() hasn't been called for the current private_key and thread yet, this pointer will be NULL.

This function can be used even if g_thread_init() has not yet been called, and, in that case, will return the value of private_key casted to gpointer. Note however, that private data set before g_thread_init() will not be retained after the call. Instead, NULL will be returned in all threads directly after g_thread_init(), regardless of any g_private_set() calls issued before threading system intialization.

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 be used even if g_thread_init() has not yet been called, and, in that case, will set private_key to data casted to GPrivate*. See g_private_get() for resulting caveats.

private_key :

a GPrivate.

data :

the new pointer.

GStaticPrivate

typedef 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 ""

Example 10. 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 works even 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 works even if g_thread_init() has not yet been called. If g_thread_init() is called later, the data keyed to private_key will be inherited only by the main thread, i.e. the one that called g_thread_init().

Note

notify is used 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.

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.

volatile GOnceStatus status;

the status of the GOnce

volatile gpointer retval;

the value returned by the call to the function, if status is G_ONCE_STATUS_READY

Since 2.4


enum GOnceStatus

typedef enum
{
  G_ONCE_STATUS_NOTCALLED,
  G_ONCE_STATUS_PROGRESS,
  G_ONCE_STATUS_READY  
} GOnceStatus;

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

G_ONCE_STATUS_NOTCALLED

the function has not been called yet.

G_ONCE_STATUS_PROGRESS

the function call is currently in progress.

G_ONCE_STATUS_READY

the function has been called.

Since 2.4


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;

Since 2.4


g_once()

#define             g_once(once, func, arg)

The first call to this routine by a process with a given GOnce struct calls func with the given argument. Thereafter, subsequent calls to g_once() with the same GOnce struct do not call func again, but return the stored result of the first call. On return from g_once(), the status of once will be G_ONCE_STATUS_READY.

For example, a mutex or a thread-specific data key must be created exactly once. In a threaded environment, calling g_once() ensures that the initialization is serialized across multiple threads.

Note

Calling g_once() recursively on the same GOnce struct in func will lead to a deadlock.

gpointer 
get_debug_flags ()
{
  static GOnce my_once = G_ONCE_INIT;
  
  g_once (&my_once, parse_debug_flags, NULL);

  return my_once.retval;
}

once :

a GOnce structure

func :

the GThreadFunc function associated to once. This function is called only once, regardless of the number of times it and its associated GOnce struct are passed to g_once() .

arg :

data to be passed to func

Since 2.4


g_once_init_enter ()

gboolean            g_once_init_enter                   (volatile gsize *value_location);

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

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

value_location :

location of a static initializable variable containing 0.

Returns :

TRUE if the initialization section should be entered, FALSE and blocks otherwise

Since 2.14


g_once_init_leave ()

void                g_once_init_leave                   (volatile gsize *value_location,
                                                         gsize initialization_value);

Counterpart to g_once_init_enter(). Expects a location of a static 0-initialized initialization variable, and an initialization value other than 0. Sets the variable to the initialization value, and releases concurrent threads blocking in g_once_init_enter() on this initialization variable.

value_location :

location of a static initializable variable containing 0.

initialization_value :

new non-0 value for *value_location.

Since 2.14

See Also

GThreadPool

Thread pools.

GAsyncQueue

Send asynchronous messages between threads.