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.
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.
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;
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
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
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