path: root/pthreads/README.NONPORTABLE

This file documents non-portable functions and other issues.

Non-portable functions included in pthreads-win32
-------------------------------------------------

BOOL
pthread_win32_test_features_np(int mask)

	This routine allows an application to check which
	run-time auto-detected features are available within
	the library.

	The possible features are:

		PTW32_SYSTEM_INTERLOCKED_COMPARE_EXCHANGE
			Return TRUE if the native version of
			InterlockedCompareExchange() is being used.
			This feature is not meaningful in recent
			library versions as MSVC builds only support
			system implemented ICE. Note that all Mingw
			builds use inlined asm versions of all the
			Interlocked routines.
		PTW32_ALERTABLE_ASYNC_CANCEL
			Return TRUE is the QueueUserAPCEx package
			QUSEREX.DLL is available and the AlertDrv.sys
			driver is loaded into Windows, providing
			alertable (pre-emptive) asyncronous threads
			cancellation. If this feature returns FALSE
			then the default async cancel scheme is in
			use, which cannot cancel blocked threads.

	Features may be Or'ed into the mask parameter, in which case
	the routine returns TRUE if any of the Or'ed features would
	return TRUE. At this stage it doesn't make sense to Or features
	but it may some day.


void *
pthread_timechange_handler_np(void *)

        To improve tolerance against operator or time service
        initiated system clock changes.

        This routine can be called by an application when it
        receives a WM_TIMECHANGE message from the system. At
        present it broadcasts all condition variables so that
        waiting threads can wake up and re-evaluate their
        conditions and restart their timed waits if required.

        It has the same return type and argument type as a
        thread routine so that it may be called directly
        through pthread_create(), i.e. as a separate thread.

        Parameters

        Although a parameter must be supplied, it is ignored.
        The value NULL can be used.

        Return values

        It can return an error EAGAIN to indicate that not
        all condition variables were broadcast for some reason.
        Otherwise, 0 is returned.

        If run as a thread, the return value is returned
        through pthread_join().

        The return value should be cast to an integer.


HANDLE
pthread_getw32threadhandle_np(pthread_t thread);

	Returns the win32 thread handle that the POSIX
	thread "thread" is running as.

	Applications can use the win32 handle to set
	win32 specific attributes of the thread.

DWORD
pthread_getw32threadid_np (pthread_t thread)

	Returns the Windows native thread ID that the POSIX
	thread "thread" is running as.

        Only valid when the library is built where
        ! (defined(__MINGW64__) || defined(__MINGW32__)) || defined (__MSVCRT__) || defined (__DMC__)
        and otherwise returns 0.


int
pthread_mutexattr_setkind_np(pthread_mutexattr_t * attr, int kind)

int
pthread_mutexattr_getkind_np(pthread_mutexattr_t * attr, int *kind)

        These two routines are included for Linux compatibility
        and are direct equivalents to the standard routines
                pthread_mutexattr_settype
                pthread_mutexattr_gettype

        pthread_mutexattr_setkind_np accepts the following
        mutex kinds:
                PTHREAD_MUTEX_FAST_NP
                PTHREAD_MUTEX_ERRORCHECK_NP
                PTHREAD_MUTEX_RECURSIVE_NP

        These are really just equivalent to (respectively):
                PTHREAD_MUTEX_NORMAL
                PTHREAD_MUTEX_ERRORCHECK
                PTHREAD_MUTEX_RECURSIVE

int
pthread_delay_np (const struct timespec *interval)

        This routine causes a thread to delay execution for a specific period of time.
        This period ends at the current time plus the specified interval. The routine
        will not return before the end of the period is reached, but may return an
        arbitrary amount of time after the period has gone by. This can be due to
        system load, thread priorities, and system timer granularity.

        Specifying an interval of zero (0) seconds and zero (0) nanoseconds is
        allowed and can be used to force the thread to give up the processor or to
        deliver a pending cancellation request.

        This routine is a cancellation point.

        The timespec structure contains the following two fields:

                tv_sec is an integer number of seconds.
                tv_nsec is an integer number of nanoseconds. 

        Return Values

        If an error condition occurs, this routine returns an integer value
        indicating the type of error. Possible return values are as follows:

        0          Successful completion. 
        [EINVAL]   The value specified by interval is invalid. 

int
pthread_timedjoin_np (pthread_t thread, void **value_ptr, const struct timespec *abstime)

int
pthread_tryjoin_np (pthread_t thread, void **value_ptr)

		These function is added for compatibility with Linux.
		


int
pthread_num_processors_np (void)

        This routine (found on HPUX systems) returns the number of processors
        in the system. This implementation actually returns the number of
        processors available to the process, which can be a lower number
        than the system's number, depending on the process's affinity mask.

BOOL
pthread_win32_process_attach_np (void);

BOOL
pthread_win32_process_detach_np (void);

BOOL
pthread_win32_thread_attach_np (void);

BOOL
pthread_win32_thread_detach_np (void);

	These functions contain the code normally run via DllMain
	when the library is used as a dll. As of version 2.9.0 of the
	library, static builds using either MSC or GCC will call
	pthread_win32_process_* automatically at application startup and
	exit respectively.

	pthread_win32_thread_attach_np() is currently a no-op.
	
	pthread_win32_thread_detach_np() is not a no-op. It cleans up the
	implicit pthread handle that is allocated to any thread not started
	via pthread_create(). Such non-posix threads should call this routine
	when they exit, or call pthread_exit() to both cleanup and exit.

	These functions invariably return TRUE except for
	pthread_win32_process_attach_np() which will return FALSE
	if pthreads-win32 initialisation fails.

int
pthreadCancelableWait (HANDLE waitHandle);

int
pthreadCancelableTimedWait (HANDLE waitHandle, DWORD timeout);

	These two functions provide hooks into the pthread_cancel
	mechanism that will allow you to wait on a Windows handle
	and make it a cancellation point. Both functions block
	until either the given w32 handle is signaled, or
	pthread_cancel has been called. It is implemented using
	WaitForMultipleObjects on 'waitHandle' and a manually
	reset w32 event used to implement pthread_cancel.


Non-portable issues
-------------------

Thread priority

	POSIX defines a single contiguous range of numbers that determine a
	thread's priority. Win32 defines priority classes and priority
	levels relative to these classes. Classes are simply priority base
	levels that the defined priority levels are relative to such that,
	changing a process's priority class will change the priority of all
	of it's threads, while the threads retain the same relativity to each
	other.

	A Win32 system defines a single contiguous monotonic range of values
	that define system priority levels, just like POSIX. However, Win32
	restricts individual threads to a subset of this range on a
	per-process basis.

	The following table shows the base priority levels for combinations
	of priority class and priority value in Win32.
	
	 Process Priority Class               Thread Priority Level
	 -----------------------------------------------------------------
	 1 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_IDLE
	 1 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_IDLE
	 1 NORMAL_PRIORITY_CLASS              THREAD_PRIORITY_IDLE
	 1 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_IDLE
	 1 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_IDLE
	 2 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_LOWEST
	 3 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_BELOW_NORMAL
	 4 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_NORMAL
	 4 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_LOWEST
	 5 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_ABOVE_NORMAL
	 5 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_BELOW_NORMAL
	 5 Background NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_LOWEST
	 6 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_HIGHEST
	 6 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_NORMAL
	 6 Background NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_BELOW_NORMAL
	 7 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_ABOVE_NORMAL
	 7 Background NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_NORMAL
	 7 Foreground NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_LOWEST
 	 8 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_HIGHEST
	 8 NORMAL_PRIORITY_CLASS              THREAD_PRIORITY_ABOVE_NORMAL
	 8 Foreground NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_BELOW_NORMAL
	 8 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_LOWEST
	 9 NORMAL_PRIORITY_CLASS              THREAD_PRIORITY_HIGHEST
	 9 Foreground NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_NORMAL
	 9 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_BELOW_NORMAL
	10 Foreground NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_ABOVE_NORMAL
	10 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_NORMAL
	11 Foreground NORMAL_PRIORITY_CLASS   THREAD_PRIORITY_HIGHEST
	11 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_ABOVE_NORMAL
	11 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_LOWEST
	12 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_HIGHEST
	12 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_BELOW_NORMAL
	13 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_NORMAL
	14 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_ABOVE_NORMAL
	15 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_HIGHEST
	15 HIGH_PRIORITY_CLASS                THREAD_PRIORITY_TIME_CRITICAL
	15 IDLE_PRIORITY_CLASS                THREAD_PRIORITY_TIME_CRITICAL
	15 BELOW_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_TIME_CRITICAL
	15 NORMAL_PRIORITY_CLASS              THREAD_PRIORITY_TIME_CRITICAL
	15 ABOVE_NORMAL_PRIORITY_CLASS        THREAD_PRIORITY_TIME_CRITICAL
	16 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_IDLE
	17 REALTIME_PRIORITY_CLASS            -7
	18 REALTIME_PRIORITY_CLASS            -6
	19 REALTIME_PRIORITY_CLASS            -5
	20 REALTIME_PRIORITY_CLASS            -4
	21 REALTIME_PRIORITY_CLASS            -3
	22 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_LOWEST
	23 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_BELOW_NORMAL
	24 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_NORMAL
	25 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_ABOVE_NORMAL
	26 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_HIGHEST
	27 REALTIME_PRIORITY_CLASS             3
	28 REALTIME_PRIORITY_CLASS             4
	29 REALTIME_PRIORITY_CLASS             5
	30 REALTIME_PRIORITY_CLASS             6
	31 REALTIME_PRIORITY_CLASS            THREAD_PRIORITY_TIME_CRITICAL
	
	Windows NT:  Values -7, -6, -5, -4, -3, 3, 4, 5, and 6 are not supported.


	As you can see, the real priority levels available to any individual
	Win32 thread are non-contiguous.

	An application using pthreads-win32 should not make assumptions about
	the numbers used to represent thread priority levels, except that they
	are monotonic between the values returned by sched_get_priority_min()
	and sched_get_priority_max(). E.g. Windows 95, 98, NT, 2000, XP make
	available a non-contiguous range of numbers between -15 and 15, while
	at least one version of WinCE (3.0) defines the minimum priority
	(THREAD_PRIORITY_LOWEST) as 5, and the maximum priority
	(THREAD_PRIORITY_HIGHEST) as 1.

	Internally, pthreads-win32 maps any priority levels between
	THREAD_PRIORITY_IDLE and THREAD_PRIORITY_LOWEST to THREAD_PRIORITY_LOWEST,
	or between THREAD_PRIORITY_TIME_CRITICAL and THREAD_PRIORITY_HIGHEST to
	THREAD_PRIORITY_HIGHEST. Currently, this also applies to
	REALTIME_PRIORITY_CLASSi even if levels -7, -6, -5, -4, -3, 3, 4, 5, and 6
	are supported.

	If it wishes, a Win32 application using pthreads-win32 can use the Win32
	defined priority macros THREAD_PRIORITY_IDLE through
	THREAD_PRIORITY_TIME_CRITICAL.


The opacity of the pthread_t datatype
-------------------------------------
and possible solutions for portable null/compare/hash, etc
----------------------------------------------------------

Because pthread_t is an opague datatype an implementation is permitted to define
pthread_t in any way it wishes. That includes defining some bits, if it is
scalar, or members, if it is an aggregate, to store information that may be
extra to the unique identifying value of the ID. As a result, pthread_t values
may not be directly comparable.

If you want your code to be portable you must adhere to the following contraints:

1) Don't assume it is a scalar data type, e.g. an integer or pointer value. There
are several other implementations where pthread_t is also a struct. See our FAQ
Question 11 for our reasons for defining pthread_t as a struct.

2) You must not compare them using relational or equality operators. You must use
the API function pthread_equal() to test for equality.

3) Never attempt to reference individual members.


The problem

Certain applications would like to be able to access only the 'pure' pthread_t
id values, primarily to use as keys into data structures to manage threads or
thread-related data, but this is not possible in a maximally portable and
standards compliant way for current POSIX threads implementations.

For implementations that define pthread_t as a scalar, programmers often employ
direct relational and equality operators on pthread_t. This code will break when
ported to an implementation that defines pthread_t as an aggregate type.

For implementations that define pthread_t as an aggregate, e.g. a struct,
programmers can use memcmp etc., but then face the prospect that the struct may
include alignment padding bytes or bits as well as extra implementation-specific
members that are not part of the unique identifying value.

[While this is not currently the case for pthreads-win32, opacity also
means that an implementation is free to change the definition, which should
generally only require that applications be recompiled and relinked, not
rewritten.]


Doesn't the compiler take care of padding?

The C89 and later standards only effectively guarrantee element-by-element
equivalence following an assignment or pass by value of a struct or union,
therefore undefined areas of any two otherwise equivalent pthread_t instances
can still compare differently, e.g. attempting to compare two such pthread_t
variables byte-by-byte, e.g. memcmp(&t1, &t2, sizeof(pthread_t) may give an
incorrect result. In practice I'm reasonably confident that compilers routinely
also copy the padding bytes, mainly because assignment of unions would be far
too complicated otherwise. But it just isn't guarranteed by the standard.

Illustration:

We have two thread IDs t1 and t2

pthread_t t1, t2;

In an application we create the threads and intend to store the thread IDs in an
ordered data structure (linked list, tree, etc) so we need to be able to compare
them in order to insert them initially and also to traverse.

Suppose pthread_t contains undefined padding bits and our compiler copies our
pthread_t [struct] element-by-element, then for the assignment:

pthread_t temp = t1;

temp and t1 will be equivalent and correct but a byte-for-byte comparison such as
memcmp(&temp, &t1, sizeof(pthread_t)) == 0 may not return true as we expect because
the undefined bits may not have the same values in the two variable instances.

Similarly if passing by value under the same conditions.

If, on the other hand, the undefined bits are at least constant through every
assignment and pass-by-value then the byte-for-byte comparison
memcmp(&temp, &t1, sizeof(pthread_t)) == 0 will always return the expected result.
How can we force the behaviour we need?


Solutions

Adding new functions to the standard API or as non-portable extentions is
the only reliable and portable way to provide the necessary operations.
Remember also that POSIX is not tied to the C language. The most common
functions that have been suggested are:

pthread_null()
pthread_compare()
pthread_hash()

A single more general purpose function could also be defined as a
basis for at least the last two of the above functions.

First we need to list the freedoms and constraints with respect
to pthread_t so that we can be sure our solution is compatible with the
standard.

What is known or may be deduced from the standard:
1) pthread_t must be able to be passed by value, so it must be a single object.
2) from (1) it must be copyable so cannot embed thread-state information, locks
or other volatile objects required to manage the thread it associates with.
3) pthread_t may carry additional information, e.g. for debugging or to manage
itself.
4) there is an implicit requirement that the size of pthread_t is determinable
at compile-time and size-invariant, because it must be able to copy the object
(i.e. through assignment and pass-by-value). Such copies must be genuine
duplicates, not merely a copy of a pointer to a common instance such as
would be the case if pthread_t were defined as an array.


Suppose we define the following function:

/* This function shall return it's argument */
pthread_t* pthread_normalize(pthread_t* thread);

For scalar or aggregate pthread_t types this function would simply zero any bits
within the pthread_t that don't uniquely identify the thread, including padding,
such that client code can return consistent results from operations done on the
result. If the additional bits are a pointer to an associate structure then
this function would ensure that the memory used to store that associate
structure does not leak. After normalization the following compare would be
valid and repeatable:

memcmp(pthread_normalize(&t1),pthread_normalize(&t2),sizeof(pthread_t))

Note 1: such comparisons are intended merely to order and sort pthread_t values
and allow them to index various data structures. They are not intended to reveal
anything about the relationships between threads, like startup order.

Note 2: the normalized pthread_t is also a valid pthread_t that uniquely
identifies the same thread.

Advantages:
1) In most existing implementations this function would reduce to a no-op that
emits no additional instructions, i.e after in-lining or optimisation, or if
defined as a macro:
#define pthread_normalise(tptr) (tptr)

2) This single function allows an application to portably derive
application-level versions of any of the other required functions.

3) It is a generic function that could enable unanticipated uses.

Disadvantages:
1) Less efficient than dedicated compare or hash functions for implementations
that include significant extra non-id elements in pthread_t.

2) Still need to be concerned about padding if copying normalized pthread_t.
See the later section on defining pthread_t to neutralise padding issues.

Generally a pthread_t may need to be normalized every time it is used,
which could have a significant impact. However, this is a design decision
for the implementor in a competitive environment. An implementation is free
to define a pthread_t in a way that minimises or eliminates padding or
renders this function a no-op.

Hazards:
1) Pass-by-reference directly modifies 'thread' so the application must
synchronise access or ensure that the pointer refers to a copy. The alternative
of pass-by-value/return-by-value was considered but then this requires two copy
operations, disadvantaging implementations where this function is not a no-op
in terms of speed of execution. This function is intended to be used in high
frequency situations and needs to be efficient, or at least not unnecessarily
inefficient. The alternative also sits awkwardly with functions like memcmp.

2) [Non-compliant] code that uses relational and equality operators on
arithmetic or pointer style pthread_t types would need to be rewritten, but it
should be rewritten anyway.


C implementation of null/compare/hash functions using pthread_normalize():

/* In pthread.h */
pthread_t* pthread_normalize(pthread_t* thread);

/* In user code */
/* User-level bitclear function - clear bits in loc corresponding to mask */
void* bitclear (void* loc, void* mask, size_t count);

typedef unsigned int hash_t;

/* User-level hash function */
hash_t hash(void* ptr, size_t count);

/*
 * User-level pthr_null function - modifies the origin thread handle.
 * The concept of a null pthread_t is highly implementation dependent
 * and this design may be far from the mark. For example, in an
 * implementation "null" may mean setting a special value inside one
 * element of pthread_t to mean "INVALID". However, if that value was zero and
 * formed part of the id component then we may get away with this design.
 */
pthread_t* pthr_null(pthread_t* tp)
{
  /* 
   * This should have the same effect as memset(tp, 0, sizeof(pthread_t))
   * We're just showing that we can do it.
   */
  void* p = (void*) pthread_normalize(tp);
  return (pthread_t*) bitclear(p, p, sizeof(pthread_t));
}

/*
 * Safe user-level pthr_compare function - modifies temporary thread handle copies
 */
int pthr_compare_safe(pthread_t thread1, pthread_t thread2)
{
  return memcmp(pthread_normalize(&thread1), pthread_normalize(&thread2), sizeof(pthread_t));
}

/*
 * Fast user-level pthr_compare function - modifies origin thread handles
 */
int pthr_compare_fast(pthread_t* thread1, pthread_t* thread2)
{
  return memcmp(pthread_normalize(&thread1), pthread_normalize(&thread2), sizeof(pthread_t));
}

/*
 * Safe user-level pthr_hash function - modifies temporary thread handle copy
 */
hash_t pthr_hash_safe(pthread_t thread)
{
  return hash((void *) pthread_normalize(&thread), sizeof(pthread_t));
}

/*
 * Fast user-level pthr_hash function - modifies origin thread handle
 */
hash_t pthr_hash_fast(pthread_t thread)
{
  return hash((void *) pthread_normalize(&thread), sizeof(pthread_t));
}

/* User-level bitclear function - modifies the origin array */
void* bitclear(void* loc, void* mask, size_t count)
{
  int i;
  for (i=0; i < count; i++) {
    (unsigned char) *loc++ &= ~((unsigned char) *mask++);
  }
}

/* Donald Knuth hash */
hash_t hash(void* str, size_t count)
{
   hash_t hash = (hash_t) count;
   unsigned int i = 0;

   for(i = 0; i < len; str++, i++)
   {
      hash = ((hash << 5) ^ (hash >> 27)) ^ (*str);
   }
   return hash;
}

/* Example of advantage point (3) - split a thread handle into its id and non-id values */
pthread_t id = thread, non-id = thread;
bitclear((void*) &non-id, (void*) pthread_normalize(&id), sizeof(pthread_t));


A pthread_t type change proposal to neutralise the effects of padding

Even if pthread_nornalize() is available, padding is still a problem because
the standard only garrantees element-by-element equivalence through
copy operations (assignment and pass-by-value). So padding bit values can
still change randomly after calls to pthread_normalize().

[I suspect that most compilers take the easy path and always byte-copy anyway,
partly because it becomes too complex to do (e.g. unions that contain sub-aggregates)
but also because programmers can easily design their aggregates to minimise and
often eliminate padding].

How can we eliminate the problem of padding bytes in structs? Could
defining pthread_t as a union rather than a struct provide a solution?

In fact, the Linux pthread.h defines most of it's pthread_*_t objects (but not
pthread_t itself) as unions, possibly for this and/or other reasons. We'll
borrow some element naming from there but the ideas themselves are well known
- the __align element used to force alignment of the union comes from K&R's
storage allocator example.

/* Essentially our current pthread_t renamed */
typedef struct {
  struct thread_state_t * __p;
  long __x; /* sequence counter */
} thread_id_t;

Ensuring that the last element in the above struct is a long ensures that the
overall struct size is a multiple of sizeof(long), so there should be no trailing
padding in this struct or the union we define below.
(Later we'll see that we can handle internal but not trailing padding.)

/* New pthread_t */
typedef union {
  char __size[sizeof(thread_id_t)]; /* array as the first element */
  thread_id_t __tid;
  long __align;  /* Ensure that the union starts on long boundary */
} pthread_t;

This guarrantees that, during an assignment or pass-by-value, the compiler copies
every byte in our thread_id_t because the compiler guarrantees that the __size
array, which we have ensured is the equal-largest element in the union, retains
equivalence.

This means that pthread_t values stored, assigned and passed by value will at least
carry the value of any undefined padding bytes along and therefore ensure that
those values remain consistent. Our comparisons will return consistent results and
our hashes of [zero initialised] pthread_t values will also return consistent
results.

We have also removed the need for a pthread_null() function; we can initialise
at declaration time or easily create our own const pthread_t to use in assignments
later:

const pthread_t null_tid = {0}; /* braces are required */

pthread_t t;
...
t = null_tid;


Note that we don't have to explicitly make use of the __size array at all. It's
there just to force the compiler behaviour we want.


Partial solutions without a pthread_normalize function


An application-level pthread_null and pthread_compare proposal
(and pthread_hash proposal by extention)

In order to deal with the problem of scalar/aggregate pthread_t type disparity in
portable code I suggest using an old-fashioned union, e.g.:

Contraints:
- there is no padding, or padding values are preserved through assignment and
  pass-by-value (see above);
- there are no extra non-id values in the pthread_t.


Example 1: A null initialiser for pthread_t variables...

typedef union {
    unsigned char b[sizeof(pthread_t)];
    pthread_t t;
} init_t;

const init_t initial = {0};

pthread_t tid = initial.t; /* init tid to all zeroes */


Example 2: A comparison function for pthread_t values

typedef union {
   unsigned char b[sizeof(pthread_t)];
   pthread_t t;
} pthcmp_t;

int pthcmp(pthread_t left, pthread_t right)
{
  /*
  * Compare two pthread handles in a way that imposes a repeatable but arbitrary
  * ordering on them.
  * I.e. given the same set of pthread_t handles the ordering should be the same
  * each time but the order has no particular meaning other than that. E.g.
  * the ordering does not imply the thread start sequence, or any other
  * relationship between threads.
  *
  * Return values are:
  * 1 : left is greater than right
  * 0 : left is equal to right
  * -1 : left is less than right
  */
  int i;
  pthcmp_t L, R;
  L.t = left;
  R.t = right;
  for (i = 0; i < sizeof(pthread_t); i++)
  {
    if (L.b[i] > R.b[i])
      return 1;
    else if (L.b[i] < R.b[i])
      return -1;
  }
  return 0;
}

It has been pointed out that the C99 standard allows for the possibility that
integer types also may include padding bits, which could invalidate the above
method. This addition to C99 was specifically included after it was pointed
out that there was one, presumably not particularly well known, architecture
that included a padding bit in it's 32 bit integer type. See section 6.2.6.2
of both the standard and the rationale, specifically the paragraph starting at
line 16 on page 43 of the rationale.


An aside

Certain compilers, e.g. gcc and one of the IBM compilers, include a feature
extention: provided the union contains a member of the same type as the
object then the object may be cast to the union itself.

We could use this feature to speed up the pthrcmp() function from example 2
above by casting rather than assigning the pthread_t arguments to the union, e.g.:

int pthcmp(pthread_t left, pthread_t right)
{
  /*
  * Compare two pthread handles in a way that imposes a repeatable but arbitrary
  * ordering on them.
  * I.e. given the same set of pthread_t handles the ordering should be the same
  * each time but the order has no particular meaning other than that. E.g.
  * the ordering does not imply the thread start sequence, or any other
  * relationship between threads.
  *
  * Return values are:
  * 1 : left is greater than right
  * 0 : left is equal to right
  * -1 : left is less than right
  */
  int i;
  for (i = 0; i < sizeof(pthread_t); i++)
  {
    if (((pthcmp_t)left).b[i] > ((pthcmp_t)right).b[i])
      return 1;
    else if (((pthcmp_t)left).b[i] < ((pthcmp_t)right).b[i])
      return -1;
  }
  return 0;
}


Result thus far

We can't remove undefined bits if they are there in pthread_t already, but we have
attempted to render them inert for comparison and hashing functions by making them
consistent through assignment, copy and pass-by-value.

Note: Hashing pthread_t values requires that all pthread_t variables be initialised
to the same value (usually all zeros) before being assigned a proper thread ID, i.e.
to ensure that any padding bits are zero, or at least the same value for all
pthread_t. Since all pthread_t values are generated by the library in the first
instance this need not be an application-level operation.


Conclusion

I've attempted to resolve the multiple issues of type opacity and the possible
presence of undefined bits and bytes in pthread_t values, which prevent
applications from comparing or hashing pthread handles.

Two complimentary partial solutions have been proposed, one an application-level
scheme to handle both scalar and aggregate pthread_t types equally, plus a
definition of pthread_t itself that neutralises padding bits and bytes by
coercing semantics out of the compiler to eliminate variations in the values of
padding bits.

I have not provided any solution to the problem of handling extra values embedded
in pthread_t, e.g. debugging or trap information that an implementation is entitled
to include. Therefore none of this replaces the portability and flexibility of API
functions but what functions are needed? The threads standard is unlikely to
include new functions that can be implemented by a combination of existing features
and more generic functions (several references in the threads rationale suggest this).
Therefore I propose that the following function could replace the several functions
that have been suggested in conversations:

pthread_t * pthread_normalize(pthread_t * handle);

For most existing pthreads implementations this function, or macro, would reduce to
a no-op with zero call overhead. Most of the other desired operations on pthread_t
values (null, compare, hash, etc.) can be trivially derived from this and other
standard functions.