уничтожить и инициализировать объект атрибутов потока (destroy and initialize the thread attributes object)
Пролог (Prolog)
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.
Имя (Name)
pthread_attr_destroy, pthread_attr_init — destroy and initialize
the thread attributes object
Синопсис (Synopsis)
#include <pthread.h>
int pthread_attr_destroy(pthread_attr_t *attr);
int pthread_attr_init(pthread_attr_t *attr);
Описание (Description)
The pthread_attr_destroy() function shall destroy a thread
attributes object. An implementation may cause
pthread_attr_destroy() to set attr to an implementation-defined
invalid value. A destroyed attr attributes object can be
reinitialized using pthread_attr_init(); the results of otherwise
referencing the object after it has been destroyed are undefined.
The pthread_attr_init() function shall initialize a thread
attributes object attr with the default value for all of the
individual attributes used by a given implementation.
The resulting attributes object (possibly modified by setting
individual attribute values) when used by pthread_create()
defines the attributes of the thread created. A single attributes
object can be used in multiple simultaneous calls to
pthread_create(). Results are undefined if pthread_attr_init()
is called specifying an already initialized attr attributes
object.
The behavior is undefined if the value specified by the attr
argument to pthread_attr_destroy() does not refer to an
initialized thread attributes object.
Возвращаемое значение (Return value)
Upon successful completion, pthread_attr_destroy() and
pthread_attr_init() shall return a value of 0; otherwise, an
error number shall be returned to indicate the error.
Ошибки (Error)
The pthread_attr_init() function shall fail if:
ENOMEM Insufficient memory exists to initialize the thread
attributes object.
These functions shall not return an error code of [EINTR].
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
Attributes objects are provided for threads, mutexes, and
condition variables as a mechanism to support probable future
standardization in these areas without requiring that the
function itself be changed.
Attributes objects provide clean isolation of the configurable
aspects of threads. For example, ``stack size'' is an important
attribute of a thread, but it cannot be expressed portably. When
porting a threaded program, stack sizes often need to be
adjusted. The use of attributes objects can help by allowing the
changes to be isolated in a single place, rather than being
spread across every instance of thread creation.
Attributes objects can be used to set up ``classes' of threads
with similar attributes; for example, ``threads with large stacks
and high priority'' or ``threads with minimal stacks''. These
classes can be defined in a single place and then referenced
wherever threads need to be created. Changes to ``class''
decisions become straightforward, and detailed analysis of each
pthread_create() call is not required.
The attributes objects are defined as opaque types as an aid to
extensibility. If these objects had been specified as structures,
adding new attributes would force recompilation of all multi-
threaded programs when the attributes objects are extended; this
might not be possible if different program components were
supplied by different vendors.
Additionally, opaque attributes objects present opportunities for
improving performance. Argument validity can be checked once when
attributes are set, rather than each time a thread is created.
Implementations often need to cache kernel objects that are
expensive to create. Opaque attributes objects provide an
efficient mechanism to detect when cached objects become invalid
due to attribute changes.
Since assignment is not necessarily defined on a given opaque
type, implementation-defined default values cannot be defined in
a portable way. The solution to this problem is to allow
attributes objects to be initialized dynamically by attributes
object initialization functions, so that default values can be
supplied automatically by the implementation.
The following proposal was provided as a suggested alternative to
the supplied attributes:
1. Maintain the style of passing a parameter formed by the
bitwise-inclusive OR of flags to the initialization routines
(pthread_create(), pthread_mutex_init(),
pthread_cond_init()). The parameter containing the flags
should be an opaque type for extensibility. If no flags are
set in the parameter, then the objects are created with
default characteristics. An implementation may specify
implementation-defined flag values and associated behavior.
2. If further specialization of mutexes and condition variables
is necessary, implementations may specify additional
procedures that operate on the pthread_mutex_t and
pthread_cond_t objects (instead of on attributes objects).
The difficulties with this solution are:
1. A bitmask is not opaque if bits have to be set into bitvector
attributes objects using explicitly-coded bitwise-inclusive
OR operations. If the set of options exceeds an int,
application programmers need to know the location of each
bit. If bits are set or read by encapsulation (that is, get
and set functions), then the bitmask is merely an
implementation of attributes objects as currently defined and
should not be exposed to the programmer.
2. Many attributes are not Boolean or very small integral
values. For example, scheduling policy may be placed in 3-bit
or 4-bit, but priority requires 5-bit or more, thereby taking
up at least 8 bits out of a possible 16 bits on machines with
16-bit integers. Because of this, the bitmask can only
reasonably control whether particular attributes are set or
not, and it cannot serve as the repository of the value
itself. The value needs to be specified as a function
parameter (which is non-extensible), or by setting a
structure field (which is non-opaque), or by get and set
functions (making the bitmask a redundant addition to the
attributes objects).
Stack size is defined as an optional attribute because the very
notion of a stack is inherently machine-dependent. Some
implementations may not be able to change the size of the stack,
for example, and others may not need to because stack pages may
be discontiguous and can be allocated and released on demand.
The attribute mechanism has been designed in large measure for
extensibility. Future extensions to the attribute mechanism or to
any attributes object defined in this volume of POSIX.1‐2017 has
to be done with care so as not to affect binary-compatibility.
Attributes objects, even if allocated by means of dynamic
allocation functions such as malloc(), may have their size fixed
at compile time. This means, for example, a pthread_create() in
an implementation with extensions to pthread_attr_t cannot look
beyond the area that the binary application assumes is valid.
This suggests that implementations should maintain a size field
in the attributes object, as well as possibly version
information, if extensions in different directions (possibly by
different vendors) are to be accommodated.
If an implementation detects that the value specified by the attr
argument to pthread_attr_destroy() does not refer to an
initialized thread attributes object, it is recommended that the
function should fail and report an [EINVAL] error.
If an implementation detects that the value specified by the attr
argument to pthread_attr_init() refers to an already initialized
thread attributes object, it is recommended that the function
should fail and report an [EBUSY] error.
Будущие направления (Future directions)
None.
Смотри также (See also)
pthread_attr_getstacksize(3p), pthread_attr_getdetachstate(3p),
pthread_create(3p)
The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)
