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.