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.
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