GI.GLib.Structs.RWLock

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 rWLockWriterLock), multiple threads can gain simultaneous read-only access (by holding the 'reader' lock via rWLockReaderLock).

Here is an example for an array with access functions:

C code

 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 rWLockInit on it and rWLockClear when done.

A GRWLock should only be accessed with the g_rw_lock_ functions.

Exported types

newtype RWLock

Constructors

RWLock (ManagedPtr RWLock)

Instances

WrappedPtr RWLock

Methods

wrappedPtrCalloc :: IO (Ptr RWLock)

wrappedPtrCopy :: RWLock -> IO RWLock

wrappedPtrFree :: Maybe (FunPtr (Ptr RWLock -> IO ()))

(~) AttrOpTag tag AttrSet => Constructible RWLock tag

Methods

new :: MonadIO m => (ManagedPtr RWLock -> RWLock) -> [AttrOp RWLock tag] -> m RWLock

newZeroRWLock :: MonadIO m => m RWLock

Construct a RWLock struct initialized to zero.

noRWLock :: Maybe RWLock

Methods

clear

rWLockClear

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: an initialized RWLock

-> m ()

Frees the resources allocated to a lock with rWLockInit.

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

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

Sine: 2.32

init

rWLockInit

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: an uninitialized RWLock

-> m ()

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.

C code

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

Blob *b;

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

To undo the effect of rWLockInit when a lock is no longer needed, use rWLockClear.

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

Since: 2.32

readerLock

rWLockReaderLock

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: a RWLock

-> m ()

Obtain a read lock on rwLock. If another thread currently holds the write lock on rwLock or blocks waiting for it, the current thread will block. Read locks can be taken recursively.

It is implementation-defined how many threads are allowed to hold read locks on the same lock simultaneously.

Since: 2.32

readerTrylock

rWLockReaderTrylock

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: a RWLock

-> m Bool

Returns: True if rwLock could be locked

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

Since: 2.32

readerUnlock

rWLockReaderUnlock

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: a RWLock

-> m ()

Release a read lock on rwLock.

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

Since: 2.32

writerLock

rWLockWriterLock

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: a RWLock

-> m ()

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

Since: 2.32

writerTrylock

rWLockWriterTrylock

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: a RWLock

-> m Bool

Returns: True if rwLock could be locked

Tries to obtain a write lock on rwLock. If any other thread holds a read or write lock on rwLock, it immediately returns False. Otherwise it locks rwLock and returns True.

Since: 2.32

writerUnlock

rWLockWriterUnlock

Arguments

:: (HasCallStack, MonadIO m)

=> RWLock

rwLock: a RWLock

-> m ()

Release a write lock on rwLock.

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

Since: 2.32