Tru64 UNIX
Guide to the POSIX Threads Library


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pthread_mutex_getname_np

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 #include <pthread.h>

int
pthread_mutex_getname_np (
pthread_mutex_t *mutex,
char *name,
size_t len);


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. 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 If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified by mutex is not a valid mutex.

Associated Routines

pthread_mutex_setname_np()

pthread_mutex_init

Initializes a mutex.

Syntax

pthread_mutex_init(
mutex ,
attr );

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

C Binding #include <pthread.h>

int
pthread_mutex_init (
pthread_mutex_t *mutex,
const pthread_mutexattr_t *attr);


Arguments

mutex

Mutex to be initialized.

attr

Mutex attributes object that defines the characteristics of the mutex to be initialized.

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.

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.

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. If a mutex is allocated on a stack, static initializers cannot be used on the mutex.

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error, the mutex is not initialized, and the contents of mutex are undefined. Possible return values are as follows:
Return Description
0 Successful completion.
[EAGAIN] The system lacks the necessary resources to initialize the mutex.
[EBUSY] The implementation has detected an attempt to reinitialize the mutex (a previously initialized, but not yet destroyed mutex).
[EINVAL] The value specified by mutex is not a valid mutex.
[ENOMEM] Insufficient memory exists to initialize the mutex.
[EPERM] The caller does not have privileges to perform this operation.

Associated Routines

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

pthread_mutex_lock

Locks an unlocked mutex.

Syntax

pthread_mutex_lock(
mutex );

Argument Data Type Access
mutex opaque pthread_mutex_t read

C Binding #include <pthread.h>

int
pthread_mutex_lock (
pthread_mutex_t *mutex);


Arguments

mutex

Mutex to be locked.

Description

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

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 its 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 If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EDEADLK] A deadlock condition is detected.
[EINVAL] The value specified by mutex is not a valid mutex.

Associated Routines

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

pthread_mutex_setname_np

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 #include <pthread.h>

int
pthread_mutex_setname_np (
pthread_mutex_t *mutex,
const char *name,
void *mbz);


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

Reserved for future use. The value must be zero (0).

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. The maximum number of characters in the object name is 31.

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified by mutex is not a valid mutex, or the length in characters of name exceeds 31.
[ENOMEM] Insufficient memory to create a copy of the object name string.

Associated Routines

pthread_mutex_getname_np()

pthread_mutex_trylock

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 #include <pthread.h>

int
pthread_mutex_trylock (
pthread_mutex_t *mutex);


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:

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 If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EBUSY] The mutex is already locked; therefore, it was not acquired.
[EINVAL] The value specified by mutex is not a valid mutex.

Associated Routines

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

pthread_mutex_unlock

Unlocks the specified mutex.

Syntax

pthread_mutex_unlock(
mutex );

Argument Data Type Access
mutex opaque pthread_mutex_t read

C Binding #include <pthread.h>

int
pthread_mutex_unlock (
pthread_mutex_t *mutex);


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:

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 Tru64 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 If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified for mutex is not a valid mutex.
[EPERM] The calling thread does not own the mutex.

Associated Routines

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

pthread_once

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

Syntax

pthread_once(
once _control,
routine );

Argument Data Type Access
once_control opaque pthread_once_t modify
routine procedure read

C Binding #include <pthread.h>

int
pthread_once (
pthread_once_t *once_control,
void (*routine) (void));


Arguments

once_control

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

routine

Address of a procedure to be executed once. This routine is called only once, regardless of the number of times it and its associated once_control block 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 routine with no arguments. Subsequent calls to pthread_once() with the same once_control will not call the routine. On return from pthread_once() , it is guaranteed that the 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 a routine that directly or indirectly results in a recursive call to pthread_once() and that specifies the same 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 If an error occurs, this routine returns an integer indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion
[EINVAL] Invalid argument


pthread_rwlockattr_destroy

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 #include <pthread.h>

int
pthread_rwlockattr_destroy (
pthread_rwlockattr_t *attr);


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 If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified by attr is not a valid attributes block.

Associated Routines

pthread_rwlockattr_init()
pthread_rwlock_init()


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