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 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.

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.

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 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 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 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.

FUTURE DIRECTIONS

None.

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 .