1PTHREAD_ATTR_DESTROY(P) POSIX Programmer's Manual PTHREAD_ATTR_DESTROY(P)
2
6 pthread_attr_destroy, pthread_attr_init - destroy and initialize the
7 thread attributes object
8
10 #include <pthread.h>
11 int pthread_attr_destroy(pthread_attr_t *attr);
13 int pthread_attr_init(pthread_attr_t *attr);
14
17 The pthread_attr_destroy() function shall destroy a thread attributes
18 object. An implementation may cause pthread_attr_destroy() to set attr
19 to an implementation-defined invalid value. A destroyed attr attributes
20 object can be reinitialized using pthread_attr_init(); the results of
21 otherwise referencing the object after it has been destroyed are unde‐
22 fined.
23 The pthread_attr_init() function shall initialize a thread attributes
25 object attr with the default value for all of the individual attributes
26 used by a given implementation.
27 The resulting attributes object (possibly modified by setting individ‐
29 ual attribute values) when used by pthread_create() defines the
30 attributes of the thread created. A single attributes object can be
31 used in multiple simultaneous calls to pthread_create(). Results are
32 undefined if pthread_attr_init() is called specifying an already ini‐
33 tialized attr attributes object.
34
36 Upon successful completion, pthread_attr_destroy() and
37 pthread_attr_init() shall return a value of 0; otherwise, an error num‐
38 ber shall be returned to indicate the error.
39
41 The pthread_attr_init() function shall fail if:
42 ENOMEM Insufficient memory exists to initialize the thread attributes
44 object.
45 These functions shall not return an error code of [EINTR].
48 The following sections are informative.
50
52 None.
53
55 None.
56
58 Attributes objects are provided for threads, mutexes, and condition
59 variables as a mechanism to support probable future standardization in
60 these areas without requiring that the function itself be changed.
61 Attributes objects provide clean isolation of the configurable aspects
63 of threads. For example, "stack size" is an important attribute of a
64 thread, but it cannot be expressed portably. When porting a threaded
65 program, stack sizes often need to be adjusted. The use of attributes
66 objects can help by allowing the changes to be isolated in a single
67 place, rather than being spread across every instance of thread cre‐
68 ation.
69 Attributes objects can be used to set up "classes' of threads with sim‐
71 ilar attributes; for example, "threads with large stacks and high pri‐
72 ority" or "threads with minimal stacks". These classes can be defined
73 in a single place and then referenced wherever threads need to be cre‐
74 ated. Changes to "class" decisions become straightforward, and detailed
75 analysis of each pthread_create() call is not required.
76 The attributes objects are defined as opaque types as an aid to exten‐
78 sibility. If these objects had been specified as structures, adding
79 new attributes would force recompilation of all multi-threaded programs
80 when the attributes objects are extended; this might not be possible if
81 different program components were supplied by different vendors.
82 Additionally, opaque attributes objects present opportunities for
84 improving performance. Argument validity can be checked once when
85 attributes are set, rather than each time a thread is created. Imple‐
86 mentations often need to cache kernel objects that are expensive to
87 create. Opaque attributes objects provide an efficient mechanism to
88 detect when cached objects become invalid due to attribute changes.
89 Since assignment is not necessarily defined on a given opaque type,
91 implementation-defined default values cannot be defined in a portable
92 way. The solution to this problem is to allow attributes objects to be
93 initialized dynamically by attributes object initialization functions,
94 so that default values can be supplied automatically by the implementa‐
95 tion.
96 The following proposal was provided as a suggested alternative to the
98 supplied attributes:
99 1. Maintain the style of passing a parameter formed by the bitwise-
101 inclusive OR of flags to the initialization routines ( pthread_cre‐
102 ate(), pthread_mutex_init(), pthread_cond_init()). The parameter
103 containing the flags should be an opaque type for extensibility. If
104 no flags are set in the parameter, then the objects are created
105 with default characteristics. An implementation may specify imple‐
106 mentation-defined flag values and associated behavior.
107 2. If further specialization of mutexes and condition variables is
109 necessary, implementations may specify additional procedures that
110 operate on the pthread_mutex_t and pthread_cond_t objects (instead
111 of on attributes objects).
112 The difficulties with this solution are:
114 1. A bitmask is not opaque if bits have to be set into bitvector
116 attributes objects using explicitly-coded bitwise-inclusive OR
117 operations. If the set of options exceeds an int, application pro‐
118 grammers need to know the location of each bit. If bits are set or
119 read by encapsulation (that is, get and set functions), then the
120 bitmask is merely an implementation of attributes objects as cur‐
121 rently defined and should not be exposed to the programmer.
122 2. Many attributes are not Boolean or very small integral values. For
124 example, scheduling policy may be placed in 3-bit or 4-bit, but
125 priority requires 5-bit or more, thereby taking up at least 8 bits
126 out of a possible 16 bits on machines with 16-bit integers.
127 Because of this, the bitmask can only reasonably control whether
128 particular attributes are set or not, and it cannot serve as the
129 repository of the value itself. The value needs to be specified as
130 a function parameter (which is non-extensible), or by setting a
131 structure field (which is non-opaque), or by get and set functions
132 (making the bitmask a redundant addition to the attributes
133 objects).
134 Stack size is defined as an optional attribute because the very notion
136 of a stack is inherently machine-dependent. Some implementations may
137 not be able to change the size of the stack, for example, and others
138 may not need to because stack pages may be discontiguous and can be
139 allocated and released on demand.
140 The attribute mechanism has been designed in large measure for extensi‐
142 bility. Future extensions to the attribute mechanism or to any
143 attributes object defined in this volume of IEEE Std 1003.1-2001 has to
144 be done with care so as not to affect binary-compatibility.
145 Attributes objects, even if allocated by means of dynamic allocation
147 functions such as malloc(), may have their size fixed at compile time.
148 This means, for example, a pthread_create() in an implementation with
149 extensions to pthread_attr_t cannot look beyond the area that the
150 binary application assumes is valid. This suggests that implementa‐
151 tions should maintain a size field in the attributes object, as well as
152 possibly version information, if extensions in different directions
153 (possibly by different vendors) are to be accommodated.
154
156 None.
157
159 pthread_attr_getstackaddr() , pthread_attr_getstacksize() ,
160 pthread_attr_getdetachstate() , pthread_create() , the Base Definitions
161 volume of IEEE Std 1003.1-2001, <pthread.h>
162
164 Portions of this text are reprinted and reproduced in electronic form
165 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
166 -- Portable Operating System Interface (POSIX), The Open Group Base
167 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
168 Electrical and Electronics Engineers, Inc and The Open Group. In the
169 event of any discrepancy between this version and the original IEEE and
170 The Open Group Standard, the original IEEE and The Open Group Standard
171 is the referee document. The original Standard can be obtained online
172 at http://www.opengroup.org/unix/online.html .
173IEEE/The Open Group 2003 PTHREAD_ATTR_DESTROY(P)