PTHREAD_ATTR_DESTROY(P) POSIX Programmer's Manual PTHREAD_ATTR_DESTROY(P)NAME
pthread_attr_destroy, pthread_attr_init - destroy and initialize the
thread attributes object
SYNOPSIS
#include <pthread.h>
int pthread_attr_destroy(pthread_attr_t *attr);
int pthread_attr_init(pthread_attr_t *attr);
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 unde‐
fined.
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 individ‐
ual 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 ini‐
tialized attr attributes object.
RETURN VALUE
Upon successful completion, pthread_attr_destroy() and
pthread_attr_init() shall return a value of 0; otherwise, an error num‐
ber shall be returned to indicate the error.
ERRORS
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.
EXAMPLES
None.
APPLICATION USAGE
None.
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 cre‐
ation.
Attributes objects can be used to set up "classes' of threads with sim‐
ilar attributes; for example, "threads with large stacks and high pri‐
ority" or "threads with minimal stacks". These classes can be defined
in a single place and then referenced wherever threads need to be cre‐
ated. 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 exten‐
sibility. 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. Imple‐
mentations 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 implementa‐
tion.
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_cre‐
ate(), 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 imple‐
mentation-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 pro‐
grammers 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 cur‐
rently 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 extensi‐
bility. Future extensions to the attribute mechanism or to any
attributes object defined in this volume of IEEE Std 1003.1-2001 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 implementa‐
tions 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.
FUTURE DIRECTIONS
None.
SEE ALSOpthread_attr_getstackaddr() , pthread_attr_getstacksize() ,
pthread_attr_getdetachstate() , pthread_create() , 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_ATTR_DESTROY(P)
