Updated: 11 December 1998

Guide to DECthreads

Obtains the object name from a mutex object.

Syntax

pthread_mutex_getname_np(
mutex ,
name ,
len );

Argument	Data Type	Access
mutex	opaque pthread_mutex_t	read
name	char	write
len	opaque size_t	read

C Binding

Arguments

mutex

Address of the mutex object whose object name is to be obtained.

name

Location to store the obtained object name.

len

Length in bytes of buffer at the location specified by name.

Description

This routine copies the object name from the mutex object specified by the mutex argument to the buffer at the location specified by the name argument. Before calling this routine, your program must allocate the buffer indicated by name.

The object name is a C language string and provides an identifier that is meaningful to a person debugging a multithreaded application based on DECthreads. The maximum number of characters in the object name is 31.

If the specified condition variable object has not been previously set with an object name, this routine copies a C language null string into the buffer at location name.

Return Values

Associated Routines

• pthread_mutex_setname_np()

Initializes a mutex with attributes specified by the attr argument.

Syntax

pthread_mutex_init(
mutex ,
attr );

Argument	Data Type	Access
mutex	opaque pthread_mutex_t	write
attr	opaque pthread_mutexattr_t	read

C Binding

Arguments

mutex

Mutex created.

attr

Mutex attributes object to be used to initialize the characteristics of the created mutex.

Description

This routine initializes a mutex with the attributes specified by the mutex attributes object specified in the attr argument. A mutex is a synchronization object that allows multiple threads to serialize their access to shared data.

The mutex is initialized and set to the unlocked state. If attr is set to NULL, the default mutex attributes are used. The pthread_mutexattr_settype() routine can be used to specify the type of mutex that is created (normal, recursive, or errorcheck).

See Chapter 2 for more information about mutex usage.

A mutex is a resource of the process, not part of any particular thread. A mutex is neither destroyed nor unlocked automatically when any thread exits. Because mutexes are shared, they may be allocated in heap or static memory, but not on a stack.

Use the PTHREAD_MUTEX_INITIALIZER macro to statically initialize a mutex without calling this routine. Statically initialized mutexes need not be destroyed using pthread_mutex_destroy(). Use this macro as follows:
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER

Only normal mutexes can be statically initialized.

Return Values

Associated Routines

• pthread_mutexattr_init()
• pthread_mutexattr_gettype()
• pthread_mutexattr_settype()
• pthread_mutex_lock()
• pthread_mutex_trylock()
• pthread_mutex_unlock()

Locks an unlocked mutex. If the mutex is already locked, the calling thread blocks until the mutex becomes available.

Syntax

pthread_mutex_lock(
mutex );

Argument	Data Type	Access
mutex	opaque pthread_mutex_t	read

C Binding

Arguments

mutex

Mutex to be locked.

Description

This routine locks a mutex with behavior that depends upon the type of mutex, as follows:

• If a normal or default mutex is specified, a deadlock can result if the current owner of the mutex calls this routine in an attempt to lock the mutex a second time. (The deadlock is not detected or reported.)
• If a recursive mutex is specified, the current owner of the mutex can relock the same mutex without blocking. The lock count is incremented for each recursive lock within the thread.
• If an errorcheck mutex is specified and the current owner tries to lock the mutex a second time, this routine reports the [EDEADLK] error. If the mutex is locked by another thread, the calling thread waits for the mutex to become available.

Use the pthread_mutexattr_settype() routine to set the type of the mutex to normal, default, recursive, or errorcheck. For more information about mutexes, see Chapter 2.

The thread that has locked a mutex becomes its current owner and remains the owner until the same thread has unlocked it. This routine returns with the mutex in the locked state and with the calling thread as the mutex's current owner.

A recursive or errorcheck mutex records the identity of the thread that locks it, allowing debuggers to display this information. In most cases, normal and default mutexes do not record the owning thread's identity.

Return Values

Associated Routines

• pthread_mutexattr_settype()
• pthread_mutex_destroy()
• pthread_mutex_init()
• pthread_mutex_trylock()
• pthread_mutex_unlock()

Changes the object name in a mutex object.

Syntax

pthread_mutex_setname_np(
mutex ,
name ,
mbz );

Argument	Data Type	Access
mutex	opaque pthread_mutex_t	write
name	char	read
mbz	void	read

C Binding

Arguments

mutex

Address of the mutex object whose object name is to be changed.

name

Object name value to copy into the mutex object.

mbz

(Must be zero) Argument for use by DECthreads.

Description

This routine changes the object name in the mutex object specified by the mutex argument to the value specified by the name argument. To set a new mutex object's object name, call this routine immediately after initializing the mutex object.

The object name is a C language string and provides an identifier that is meaningful to a person debugging a multithreaded application based on DECthreads. The maximum number of characters in the object name is 31.

Return Values

Associated Routines

• pthread_mutex_getname_np()

Attempts to lock the specified mutex. If the mutex is already locked, the calling thread does not wait for the mutex to become available.

Syntax

pthread_mutex_trylock(
mutex );

Argument	Data Type	Access
mutex	opaque pthread_mutex_t	read

C Binding

Arguments

mutex

Mutex to be locked.

Description

This routine attempts to lock the mutex specified in the mutex argument. When a thread calls this routine, an attempt is made to immediately lock the mutex. If the mutex is successfully locked, this routine returns zero (0) and the calling thread becomes the mutex's current owner. If the specified mutex is locked when a thread calls this routine, the calling thread does not wait for the mutex to become available.

The behavior of this routine is as follows:

• For a normal, default, or errorcheck mutex: if the mutex is locked by any thread (including the calling thread) when this routine is called, this routine returns [EBUSY] and the calling thread does not wait to acquire the lock.
• For a normal or errorcheck mutex: if the mutex is not owned, this routine returns zero (0) and the mutex becomes locked.
• For a recursive mutex: if the mutex is owned by the current thread, this routine returns zero (0) and the mutex lock count is incremented. (To unlock a recursive mutex, each call to pthread_mutex_trylock() must be matched by a call to pthread_mutex_unlock().)

Use the pthread_mutexattr_settype() routine to set the mutex type attribute (normal, default, recursive, or errorcheck). For information about mutex types and their usage, see Chapter 2.

Return Values

Associated Routines

• pthread_mutexattr_settype()
• pthread_mutex_destroy()
• pthread_mutex_init()
• pthread_mutex_lock()
• pthread_mutex_unlock()

Unlocks the specified mutex.

Syntax

pthread_mutex_unlock(
mutex );

Argument	Data Type	Access
mutex	opaque pthread_mutex_t	read

C Binding

Arguments

mutex

Mutex to be unlocked.

Description

This routine unlocks the mutex specified by the mutex argument.

This routine behaves as follows, based on the type of the specified mutex:

• For a normal, default, or errorcheck mutex: if the mutex is owned by the calling thread, it is unlocked with no current owner. Further, for a normal or default mutex: if the mutex is not locked or is locked by another thread, this routine can also return [EPERM], but this is not guaranteed. For an errorcheck mutex: if the mutex is not locked or is locked by another thread, this routine returns [EPERM].
• For a recursive mutex: if the mutex is owned by the calling thread, the lock count is decremented. The mutex remains locked and owned until the lock count reaches zero (0). When the lock count reaches zero, the mutex becomes unlocked with no current owner.

If one or more threads are waiting to lock the specified mutex, and the mutex becomes unlocked, this routine causes one thread to unblock and to try to acquire the mutex. The scheduling policy is used to determine which thread to unblock. For the SCHED_FIFO and SCHED_RR policies, a blocked thread is chosen in priority order, using first-in/first-out within priorities. Note that the mutex might not be acquired by the awakened thread, if any other running thread attempts to lock the mutex first.

On DIGITAL UNIX, if a signal is delivered to a thread waiting for a mutex, upon return from the signal handler, the thread resumes waiting for the mutex as if it was not interrupted.

Return Values

Associated Routines

• pthread_mutexattr_settype()
• pthread_mutex_destroy()
• pthread_mutex_init()
• pthread_mutex_lock()
• pthread_mutex_trylock()

Calls an initialization routine that is executed by a single thread, once.

Syntax

pthread_once(
once _control,
init _routine );

Argument	Data Type	Access
once_control	opaque pthread_once_t	modify
init_routine	procedure	read

C Binding

Arguments

once_control

Address of a record that defines the one-time initialization code. Each one-time initialization routine must have its own unique pthread_once_t record.

init_routine

Address of a procedure that performs the initialization. This routine is called only once, regardless of the number of times it and its associated once_control are passed to pthread_once().

Description

The first call to this routine by any thread in a process with a given once_control will call the specified init_routine with no arguments. Subsequent calls to pthread_once() with the same once_control will not call the init_routine. On return from pthread_once(), it is guaranteed that the initialization routine has completed.

For example, a mutex or a per-thread context key must be created exactly once. Calling pthread_once() ensures that the initialization is serialized across multiple threads. Other threads that reach the same point in the code would be delayed until the first thread is finished.

Note If you specify an init_routine that directly or indirectly results in a recursive call to pthread_once() and that specifies the same init_routine argument, the recursive call can result in a deadlock.

To initialize the once_control record, your program can zero out the entire structure, or you can use the PTHREAD_ONCE_INIT macro, which is defined in the pthread.h header file, to statically initialize that structure. If using PTHREAD_ONCE_INIT, declare the once_control record as follows:

pthread_once_t once_control = PTHREAD_ONCE_INIT;

Note that it is often easier to simply lock a statically initialized mutex, check a control flag, and perform necessary initialization (in-line) rather than using pthread_once(). For example, you can code an initialization routine that begins with the following basic logic:

init() { static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; static int flag = FALSE; pthread_mutex_lock(&mutex); if(!flag) { /* initialization code goes here */ flag = TRUE; } pthread_mutex_unlock(&mutex); }

Return Values

Destroys a previously initialized read-write lock attributes object.

Syntax

pthread_rwlockattr_destroy(
attr );

Argument	Data Type	Access
attr	opaque pthread_rwlockattr_t	write

C Binding

Arguments

attr

Address of the read-write lock attributes object to be destroyed.

Description

This routine destroys the read-write lock attributes object referenced by attr; that is, the object becomes uninitialized.

After successful completion of this routine, the results of using attr in a call to any routine (other than pthread_rwlockattr_init()) are unpredictable.

Return Values

Associated Routines

• pthread_rwlockattr_init()
• pthread_rwlock_init()

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