pthread_mutex_destroy(3p) — Linux manual page



PROLOG         top

       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         top

       pthread_mutex_destroy, pthread_mutex_init — destroy and
       initialize a mutex

SYNOPSIS         top

       #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         top

       The pthread_mutex_destroy() function shall destroy the mutex
       object referenced by mutex; the mutex object becomes, in effect,
       uninitialized. 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 undefined.

       It shall be safe to destroy an initialized mutex that is
       unlocked.  Attempting to destroy a locked mutex, or a mutex that
       another thread is attempting to lock, or a mutex that is being
       used in a pthread_cond_timedwait() or pthread_cond_wait() call by
       another thread, 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 successful initialization, the state of the mutex
       becomes initialized and unlocked.

       See Section 2.9.9, Synchronization Object Copies and Alternative
       Mappings for further requirements.

       Attempting to initialize an already initialized mutex results in
       undefined behavior.

       In cases where default mutex attributes are appropriate, the
       macro PTHREAD_MUTEX_INITIALIZER can be used to initialize
       mutexes. The effect shall be equivalent to dynamic initialization
       by a call to pthread_mutex_init() with parameter attr specified
       as NULL, except that no error checks are performed.

       The behavior is undefined if the value specified by the mutex
       argument to pthread_mutex_destroy() does not refer to an
       initialized mutex.

       The behavior is undefined if the value specified by the attr
       argument to pthread_mutex_init() does not refer to an initialized
       mutex attributes object.

RETURN VALUE         top

       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.

ERRORS         top

       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

       The pthread_mutex_init() function may fail if:

       EINVAL The attributes object referenced by attr has the robust
              mutex attribute set without the process-shared attribute
              being set.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES         top




RATIONALE         top

       If an implementation detects that the value specified by the
       mutex argument to pthread_mutex_destroy() does not refer to an
       initialized mutex, it is recommended that the function should
       fail and report an [EINVAL] error.

       If an implementation detects that the value specified by the
       mutex argument to pthread_mutex_destroy() or pthread_mutex_init()
       refers to a locked mutex or a mutex that is referenced (for
       example, while being used in a pthread_cond_timedwait() or
       pthread_cond_wait()) by another thread, or detects that the value
       specified by the mutex argument to pthread_mutex_init() refers to
       an already initialized mutex, it is recommended that the function
       should fail and report an [EBUSY] error.

       If an implementation detects that the value specified by the attr
       argument to pthread_mutex_init() does not refer to an initialized
       mutex attributes object, it is recommended that the function
       should fail and report an [EINVAL] error.

   Alternate Implementations Possible
       This volume of POSIX.1‐2017 supports several alternative
       implementations 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 POSIX.1‐2017
       does not define assignment or equality for this type, and it uses
       the term ``initialize'' to reinforce the (more restrictive)
       notion that the lock may actually reside in the mutex object

       Note that this precludes an over-specification of the type of the
       mutex or condition variable and motivates the opaqueness of the

       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 reference) a mutex that has already been destroyed.

   Tradeoff Between Error Checks and Performance Supported
       Many error conditions that can occur are not required to be
       detected by the implementation in order to let implementations
       trade off performance versus degree of error checking according
       to the needs of their specific applications and execution
       environment. As a general rule, conditions caused by the system
       (such as insufficient memory) are required to be detected, but
       conditions caused by an erroneously coded application (such as
       failing to provide adequate synchronization to prevent a mutex
       from being deleted while in use) are specified to result in
       undefined behavior.

       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 correct application under very tight
       performance constraints in an embedded 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

       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 POSIX.1‐2017 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. When the
       behavior is undefined, no error number is specified to be
       returned on implementations that do detect the condition. This is
       because undefined behavior means anything can happen, which
       includes returning with any value (which might happen to be a
       valid, but different, error number). However, since the error
       number might be useful to application developers when diagnosing
       problems during application development, a recommendation is made
       in rationale that implementors should return a particular error
       number if their implementation does detect the condition.

   Why No Limits are Defined
       Defining symbols for the maximum number of mutexes and condition
       variables 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
       synchronization objects allows modules with private static
       synchronization variables to avoid runtime initialization tests
       and overhead. Furthermore, it simplifies the coding of self-
       initializing modules. Such modules 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
       initialization 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);
              /* Do work. */

       With static initialization, the same routine could be coded as

           static pthread_mutex_t foo_mutex = PTHREAD_MUTEX_INITIALIZER;

           void foo()
              /* Do work. */

       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 allocated. 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 initialization.

       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: initialization. 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 significantly 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 statically 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 altogether 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 particular

   Destroying Mutexes
       A mutex can be destroyed immediately after it is unlocked.
       However, since attempting to destroy a locked mutex, or a mutex
       that another thread is attempting to lock, or a mutex that is
       being used in a pthread_cond_timedwait() or pthread_cond_wait()
       call by another thread, results in undefined behavior, care must
       be taken to ensure that no other thread may be referencing the

   Robust Mutexes
       Implementations are required to provide robust mutexes for
       mutexes with the process-shared attribute set to
       PTHREAD_PROCESS_SHARED. Implementations are allowed, but not
       required, to provide robust mutexes when the process-shared
       attribute is set to PTHREAD_PROCESS_PRIVATE.



SEE ALSO         top

       pthread_mutexattr_getrobust(3p), pthread_mutex_lock(3p),
       pthread_mutex_timedlock(3p), pthread_mutexattr_getpshared(3p)

       The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017            PTHREAD..._DESTROY(3P)

Pages that refer to this page: pthread.h(0p)pthread_condattr_destroy(3p)pthread_condattr_getclock(3p)pthread_condattr_getpshared(3p)pthread_cond_destroy(3p)pthread_mutexattr_destroy(3p)pthread_mutexattr_getprioceiling(3p)pthread_mutexattr_getprotocol(3p)pthread_mutexattr_getpshared(3p)pthread_mutexattr_getrobust(3p)pthread_mutex_getprioceiling(3p)pthread_mutex_init(3p)pthread_mutex_lock(3p)pthread_mutex_timedlock(3p)