#include <pthread.h> int pthread_attr_destroy(pthread_attr_t *attr); int pthread_attr_init(pthread_attr_t *attr);
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.
These functions shall not return an error code of [EINTR].
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:
The difficulties with this solution are:
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.1-2008 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.
The Base Definitions volume of POSIX.1-2008, <pthread.h>
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