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PTHREAD_MUTEX_DESTROY(3P)  POSIX Programmer's Manual PTHREAD_MUTEX_DESTROY(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       pthread_mutex_destroy, pthread_mutex_init - destroy  and  initialize  a
       mutex
SYNOPSIS
       #include <pthread.h>
       int pthread_mutex_destroy(pthread_mutex_t *mutex);
       int pthread_mutex_init(pthread_mutex_t *restrict mutex,
              const pthread_mutexattr_t *restrict attr);
       pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

DESCRIPTION
       The  pthread_mutex_destroy()  function  shall  destroy the mutex object
       referenced by mutex; the mutex object becomes,  in  effect,  uninitial-
       ized.  An  implementation  may cause pthread_mutex_destroy() to set the
       object referenced by mutex to  an  invalid  value.  A  destroyed  mutex
       object  can be reinitialized using pthread_mutex_init(); the results of
       otherwise referencing the object after it has been destroyed are  unde-
       fined.
       It  shall  be  safe  to  destroy an initialized mutex that is unlocked.
       Attempting to destroy a locked mutex results in undefined behavior.
       The pthread_mutex_init() function shall initialize the mutex referenced
       by  mutex  with  attributes  specified  by  attr.  If attr is NULL, the
       default mutex attributes are used; the effect  shall  be  the  same  as
       passing the address of a default mutex attributes object. Upon success-
       ful initialization, the state of  the  mutex  becomes  initialized  and
       unlocked.
       Only  mutex  itself  may  be  used for performing synchronization.  The
       result   of   referring   to   copies   of   mutex    in    calls    to
       pthread_mutex_lock(),  pthread_mutex_trylock(), pthread_mutex_unlock(),
       and pthread_mutex_destroy() is undefined.
       Attempting to initialize an already initialized mutex results in  unde-
       fined behavior.
       In  cases  where  default  mutex  attributes are appropriate, the macro
       PTHREAD_MUTEX_INITIALIZER can be used to initialize  mutexes  that  are
       statically  allocated.  The  effect shall be equivalent to dynamic ini-
       tialization by a call to pthread_mutex_init() with parameter attr spec-
       ified as NULL, except that no error checks are performed.
RETURN VALUE
       If  successful,  the  pthread_mutex_destroy()  and pthread_mutex_init()
       functions shall return  zero;  otherwise,  an  error  number  shall  be
       returned to indicate the error.
       The  [EBUSY]  and [EINVAL] error checks, if implemented, act as if they
       were performed immediately at the beginning of processing for the func-
       tion  and  shall  cause an error return prior to modifying the state of
       the mutex specified by mutex.
ERRORS
       The pthread_mutex_destroy() function may fail if:
       EBUSY  The implementation has detected an attempt to destroy the object
              referenced  by mutex while it is locked or referenced (for exam-
              ple,  while  being  used  in   a   pthread_cond_timedwait()   or
              pthread_cond_wait()) by another thread.
       EINVAL The value specified by mutex is invalid.

       The pthread_mutex_init() function shall fail if:
       EAGAIN The system lacked the necessary resources (other than memory) to
              initialize another mutex.
       ENOMEM Insufficient memory exists to initialize the mutex.
       EPERM  The caller does not have the privilege to perform the operation.

       The pthread_mutex_init() function may fail if:
       EBUSY  The implementation has detected an attempt to  reinitialize  the
              object  referenced  by  mutex, a previously initialized, but not
              yet destroyed, mutex.
       EINVAL The value specified by attr is invalid.

       These functions shall not return an error code of [EINTR].
       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       None.
RATIONALE
   Alternate Implementations Possible
       This volume of IEEE Std 1003.1-2001 supports several alternative imple-
       mentations of mutexes. An implementation may store the lock directly in
       the object of type pthread_mutex_t.  Alternatively,  an  implementation
       may  store  the lock in the heap and merely store a pointer, handle, or
       unique ID in the mutex object. Either implementation has advantages  or
       may  be  required  on certain hardware configurations. So that portable
       code can be written that is invariant to this choice,  this  volume  of
       IEEE Std 1003.1-2001  does  not  define assignment or equality for this
       type, and it uses the term "initialize" to reinforce the (more restric-
       tive)  notion  that  the  lock  may actually reside in the mutex object
       itself.
       Note that this precludes an over-specification of the type of the mutex
       or condition variable and motivates the opaqueness of the type.
       An   implementation   is   permitted,   but   not   required,  to  have
       pthread_mutex_destroy() store an illegal value into  the  mutex.   This
       may  help detect erroneous programs that try to lock (or otherwise ref-
       erence) a mutex that has already been destroyed.
   Tradeoff Between Error Checks and Performance Supported
       Many of the error checks were made optional in order to let implementa-
       tions  trade  off performance versus degree of error checking according
       to the needs of their specific applications and execution  environment.
       As  a  general rule, errors or conditions caused by the system (such as
       insufficient memory) always need to be reported, but errors due  to  an
       erroneously coded application (such as failing to provide adequate syn-
       chronization to prevent a mutex from being deleted while  in  use)  are
       made optional.
       A  wide range of implementations is thus made possible. For example, an
       implementation intended for application debugging may implement all  of
       the error checks, but an implementation running a single, provably cor-
       rect application under very tight performance constraints in an  embed-
       ded  computer  might  implement minimal checks. An implementation might
       even be provided in two versions, similar to the options that compilers
       provide:  a  full-checking, but slower version; and a limited-checking,
       but faster version. To forbid this optionality would be a disservice to
       users.
       By  carefully  limiting  the use of "undefined behavior" only to things
       that an erroneous (badly coded) application might do, and  by  defining
       that  resource-not-available  errors  are  mandatory,  this  volume  of
       IEEE Std 1003.1-2001 ensures that  a  fully-conforming  application  is
       portable  across  the  full range of implementations, while not forcing
       all implementations to add overhead to check for numerous things that a
       correct program never does.
   Why No Limits are Defined
       Defining  symbols for the maximum number of mutexes and condition vari-
       ables was considered but rejected because the number of  these  objects
       may  change  dynamically. Furthermore, many implementations place these
       objects into application memory; thus, there is no explicit maximum.
   Static Initializers for Mutexes and Condition Variables
       Providing for static initialization of  statically  allocated  synchro-
       nization  objects  allows  modules  with private static synchronization
       variables to avoid runtime initialization tests and overhead.  Further-
       more,  it simplifies the coding of self-initializing modules. Such mod-
       ules are common in C libraries, where for various  reasons  the  design
       calls  for  self-initialization instead of requiring an explicit module
       initialization function to be called. An example use of static initial-
       ization follows.
       Without  static initialization, a self-initializing routine foo() might
       look as follows:

              static pthread_once_t foo_once = PTHREAD_ONCE_INIT;
              static pthread_mutex_t foo_mutex;

              void foo_init()
              {
                  pthread_mutex_init(&foo_mutex, NULL);
              }

              void foo()
              {
                  pthread_once(&foo_once, foo_init);
                  pthread_mutex_lock(&foo_mutex);
                 /* Do work. */
                  pthread_mutex_unlock(&foo_mutex);
              }
       With static initialization, the same routine could be coded as follows:

              static pthread_mutex_t foo_mutex = PTHREAD_MUTEX_INITIALIZER;

              void foo()
              {
                  pthread_mutex_lock(&foo_mutex);
                 /* Do work. */
                  pthread_mutex_unlock(&foo_mutex);
              }
       Note that the static initialization both eliminates the  need  for  the
       initialization  test  inside pthread_once() and the fetch of &foo_mutex
       to  learn  the  address  to  be  passed  to   pthread_mutex_lock()   or
       pthread_mutex_unlock().
       Thus, the C code written to initialize static objects is simpler on all
       systems and is also faster on a large class of systems; those where the
       (entire) synchronization object can be stored in application memory.
       Yet  the  locking  performance  question  is  likely  to  be raised for
       machines that require mutexes to be allocated out  of  special  memory.
       Such  machines  actually  have  to  have mutexes and possibly condition
       variables contain pointers to the actual hardware  locks.   For  static
       initialization  to work on such machines, pthread_mutex_lock() also has
       to test whether or not the pointer to the actual lock  has  been  allo-
       cated.  If it has not, pthread_mutex_lock() has to initialize it before
       use. The reservation of such resources can be made when the program  is
       loaded, and hence return codes have not been added to mutex locking and
       condition variable waiting to indicate failure to complete  initializa-
       tion.
       This  runtime  test  in  pthread_mutex_lock() would at first seem to be
       extra work; an extra test is required to see whether  the  pointer  has
       been  initialized.  On most machines this would actually be implemented
       as a fetch of the pointer, testing the pointer against zero,  and  then
       using  the  pointer  if it has already been initialized. While the test
       might seem to add extra work, the extra effort of testing a register is
       usually  negligible since no extra memory references are actually done.
       As more and more machines provide caches, the real expenses are  memory
       references, not instructions executed.
       Alternatively,  depending  on the machine architecture, there are often
       ways to eliminate all overhead in the most important case: on the  lock
       operations  that occur after the lock has been initialized. This can be
       done by shifting more overhead to the less frequent operation: initial-
       ization.  Since out-of-line mutex allocation also means that an address
       has to be dereferenced to find the actual lock, one technique  that  is
       widely  applicable is to have static initialization store a bogus value
       for that address; in particular, an address that causes a machine fault
       to  occur. When such a fault occurs upon the first attempt to lock such
       a mutex, validity checks can be done, and then the correct address  for
       the  actual  lock can be filled in. Subsequent lock operations incur no
       extra overhead since they do not "fault".  This is merely one technique
       that  can be used to support static initialization, while not adversely
       affecting the performance of lock acquisition. No doubt there are other
       techniques that are highly machine-dependent.
       The locking overhead for machines doing out-of-line mutex allocation is
       thus similar for modules being  implicitly  initialized,  where  it  is
       improved  for those doing mutex allocation entirely inline.  The inline
       case is thus made much faster, and the out-of-line case is not signifi-
       cantly worse.
       Besides  the  issue of locking performance for such machines, a concern
       is raised that it is possible that threads would  serialize  contending
       for  initialization locks when attempting to finish initializing stati-
       cally allocated mutexes. (Such finishing would typically involve taking
       an  internal  lock,  allocating  a  structure, storing a pointer to the
       structure in the mutex, and releasing the internal lock.)  First,  many
       implementations would reduce such serialization by hashing on the mutex
       address. Second, such serialization can only occur a bounded number  of
       times.  In particular, it can happen at most as many times as there are
       statically allocated  synchronization  objects.  Dynamically  allocated
       objects   would   still  be  initialized  via  pthread_mutex_init()  or
       pthread_cond_init().
       Finally, if none of the above optimization techniques  for  out-of-line
       allocation  yields  sufficient  performance  for an application on some
       implementation, the application can avoid static  initialization  alto-
       gether  by explicitly initializing all synchronization objects with the
       corresponding pthread_*_init() functions, which are  supported  by  all
       implementations.  An implementation can also document the tradeoffs and
       advise which initialization technique is more efficient for  that  par-
       ticular implementation.
   Destroying Mutexes
       A mutex can be destroyed immediately after it is unlocked. For example,
       consider the following code:

              struct obj {
              pthread_mutex_t om;
                  int refcnt;
                  ...
              };

              obj_done(struct obj *op)
              {
                  pthread_mutex_lock(&op->om);
                  if (--op->refcnt == 0) {
                      pthread_mutex_unlock(&op->om);
              (A)     pthread_mutex_destroy(&op->om);
              (B)     free(op);
                  } else
              (C)     pthread_mutex_unlock(&op->om);
              }
       In this case obj is reference counted and obj_done() is called whenever
       a  reference to the object is dropped.  Implementations are required to
       allow an object to be destroyed and freed and potentially unmapped (for
       example,  lines A and B) immediately after the object is unlocked (line
       C).
FUTURE DIRECTIONS
       None.
SEE ALSO
       pthread_mutex_getprioceiling(),                   pthread_mutex_lock(),
       pthread_mutex_timedlock(),   pthread_mutexattr_getpshared(),  the  Base
       Definitions volume of IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003            PTHREAD_MUTEX_DESTROY(3P)