PTHREAD_MUTEX_DESTROY(P) POSIX Programmer's Manual PTHREAD_MUTEX_DESTROY(P)
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
undefined.
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 undefined 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 initialization by a call to pthread_mutex_init() with parameter attr speci-
fied 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
function 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 example,
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 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 por-
table 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 restrictive) 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 reference) 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 implementations 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
synchronization 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 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 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 porta-
ble 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 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 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 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);
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 initial-
ization 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 initial-
ization.
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 acquisi-
tion. 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 sig-
nificantly 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 tak-
ing 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 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 Technol-
ogy -- 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(P)