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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, 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
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.
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 specified 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 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 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 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 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);
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
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 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(P)