GLib.RWLock¶

record (struct)

The GRWLock struct is an opaque data structure to represent a reader-writer lock. It is similar to a Mutex in that it allows multiple threads to coordinate access to a shared resource.

The difference to a mutex is that a reader-writer lock discriminates between read-only ('reader') and full ('writer') access. While only one thread at a time is allowed write access (by holding the 'writer' lock via RWLock.writer_lock), multiple threads can gain simultaneous read-only access (by holding the 'reader' lock via RWLock.reader_lock).

It is unspecified whether readers or writers have priority in acquiring the lock when a reader already holds the lock and a writer is queued to acquire it.

Here is an example for an array with access functions:

GRWLock lock;
  GPtrArray *array;

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

    if (!array)
      return NULL;

    g_rw_lock_reader_lock (&lock);
    if (index < array->len)
      retval = g_ptr_array_index (array, index);
    g_rw_lock_reader_unlock (&lock);

    return retval;
  }

  void
  my_array_set (guint index, gpointer data)
  {
    g_rw_lock_writer_lock (&lock);

    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_rw_lock_writer_unlock (&lock);
  }

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 currently access the array. This is because of the potentially dangerous resizing of the array. Using these functions is fully multi-thread safe now.

If a RWLock is allocated in static storage then it can be used without initialisation. Otherwise, you should call RWLock.init on it and RWLock.clear when done.

A GRWLock should only be accessed with the g_rw_lock_ functions.

Methods¶

clear¶

def clear(self) -> None

Frees the resources allocated to a lock with RWLock.init.

This function should not be used with a RWLock that has been statically allocated.

Calling RWLock.clear when any thread holds the lock leads to undefined behaviour.

init¶

def init(self) -> None

Initializes a RWLock so that it can be used.

This function is useful to initialize a lock that has been allocated on the stack, or as part of a larger structure. It is not necessary to initialise a reader-writer lock that has been statically allocated.

typedef struct {
    GRWLock l;
    ...
  } Blob;

Blob *b;

b = g_new (Blob, 1);
g_rw_lock_init (&b->l);

To undo the effect of RWLock.init when a lock is no longer needed, use RWLock.clear.

Calling RWLock.init on an already initialized RWLock leads to undefined behaviour.

reader_lock¶

def reader_lock(self) -> None

Obtain a read lock on rw_lock. If another thread currently holds the write lock on rw_lock, the current thread will block until the write lock was (held and) released. If another thread does not hold the write lock, but is waiting for it, it is implementation defined whether the reader or writer will block. Read locks can be taken recursively.

Calling RWLock.reader_lock while the current thread already owns a write lock leads to undefined behaviour. Read locks however can be taken recursively, in which case you need to make sure to call RWLock.reader_unlock the same amount of times.

It is implementation-defined how many read locks are allowed to be held on the same lock simultaneously. If the limit is hit, or if a deadlock is detected, a critical warning will be emitted.

reader_trylock¶

def reader_trylock(self) -> bool

Tries to obtain a read lock on rw_lock and returns True if the read lock was successfully obtained. Otherwise it returns False.

reader_unlock¶

def reader_unlock(self) -> None

Release a read lock on rw_lock.

Calling RWLock.reader_unlock on a lock that is not held by the current thread leads to undefined behaviour.

writer_lock¶

def writer_lock(self) -> None

Obtain a write lock on rw_lock. If another thread currently holds a read or write lock on rw_lock, the current thread will block until all other threads have dropped their locks on rw_lock.

Calling RWLock.writer_lock while the current thread already owns a read or write lock on rw_lock leads to undefined behaviour.

writer_trylock¶

def writer_trylock(self) -> bool

Tries to obtain a write lock on rw_lock. If another thread currently holds a read or write lock on rw_lock, it immediately returns False. Otherwise it locks rw_lock and returns True.

writer_unlock¶

def writer_unlock(self) -> None

Release a write lock on rw_lock.

Calling RWLock.writer_unlock on a lock that is not held by the current thread leads to undefined behaviour.

Properties¶

p¶

p: int  # read/write

i¶

i: list[int]  # read/write