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PTHREAD_MUTEX_DESTROY(3P) POSIX Programmer's ManualPTHREAD_MUTEX_DESTROY(3P)
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.
pthread_mutex_destroy, pthread_mutex_init — destroy and initialize a
mutex
#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;
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 is referenced
(for example, while being used in a pthread_cond_timedwait() or
pthread_cond_wait()) 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.
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. 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.
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 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:
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.
None.
None.
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.1‐2008 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.1‐2008 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 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 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 POSIX.1‐2008 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);
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).
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.
None.
pthread_mutex_getprioceiling(3p), pthread_mutexattr_getrobust(3p),
pthread_mutex_lock(3p), pthread_mutex_timedlock(3p),
pthread_mutexattr_getpshared(3p)
The Base Definitions volume of POSIX.1‐2008, pthread.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
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
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PTHREAD_MUTEX_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)