451 lines
17 KiB
Text
451 lines
17 KiB
Text
=========================================
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PTHREADS-WIN32 Frequently Asked Questions
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=========================================
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INDEX
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-----
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Q 1 What is it?
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Q 2 Which of the several dll versions do I use?
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or,
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What are all these pthread*.dll and pthread*.lib files?
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Q 3 What is the library naming convention?
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Q 4 Cleanup code default style or: it used to work when I built
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the library myself, but now it doesn't - why?
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Q 5 Why is the default library version now less exception-friendly?
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Q 6 Should I use Cygwin or Mingw32 as a development environment?
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Q 7 Now that pthreads-win32 builds under Mingw32, why do I get
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memory access violations (segfaults)?
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Q 8 How do I use pthread.dll for Win32 (Visual C++ 5.0)
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Q 9 Cancelation doesn't work for me, why?
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Q 10 How do I generate pthreadGCE.dll and libpthreadw32.a for use
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with Mingw32?
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Q 11 Why isn't pthread_t defined as a scalar (e.g. pointer or int)
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like it is for other POSIX threads implementations?
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=============================================================================
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Q 1 What is it?
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---
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Pthreads-win32 is an Open Source Software implementation of the
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Threads component of the POSIX 1003.1c 1995 Standard for Microsoft's
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Win32 environment. Some functions from POSIX 1003.1b are also
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supported including semaphores. Other related functions include
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the set of read-write lock functions. The library also supports
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some of the functionality of the Open Group's Single Unix
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specification, version 2, namely mutex types.
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See the file "ANNOUNCE" for more information including standards
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conformance details and list of supported routines.
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------------------------------------------------------------------------------
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Q 2 Which of the several dll versions do I use?
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--- or,
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What are all these pthread*.dll and pthread*.lib files?
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Simply, you only use one of them, but you need to choose carefully.
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The most important choice you need to make is whether to use a
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version that uses exceptions internally, or not (there are versions
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of the library that use exceptions as part of the thread
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cancelation and cleanup implementation, and one that uses
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setjmp/longjmp instead).
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There is some contension amongst POSIX threads experts as
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to how POSIX threads cancelation and exit should work
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with languages that include exceptions and handlers, e.g.
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C++ and even C (Microsoft's Structured Exceptions).
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The issue is: should cancelation of a thread in, say,
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a C++ application cause object destructors and C++ exception
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handlers to be invoked as the stack unwinds during thread
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exit, or not?
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There seems to be more opinion in favour of using the
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standard C version of the library (no EH) with C++ applications
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since this appears to be the assumption commercial pthreads
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implementations make. Therefore, if you use an EH version
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of pthreads-win32 then you may be under the illusion that
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your application will be portable, when in fact it is likely to
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behave very differently linked with other pthreads libraries.
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Now you may be asking: why have you kept the EH versions of
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the library?
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There are a couple of reasons:
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- there is division amongst the experts and so the code may
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be needed in the future. (Yes, it's in the repository and we
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can get it out anytime in the future, but ...)
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- pthreads-win32 is one of the few implementations, and possibly
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the only freely available one, that has EH versions. It may be
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useful to people who want to play with or study application
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behaviour under these conditions.
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------------------------------------------------------------------------------
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Q 3 What is the library naming convention?
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---
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Because the library is being built using various exception
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handling schemes and compilers - and because the library
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may not work reliably if these are mixed in an application,
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each different version of the library has it's own name.
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Note 1: the incompatibility is really between EH implementations
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of the different compilers. It should be possible to use the
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standard C version from either compiler with C++ applications
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built with a different compiler. If you use an EH version of
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the library, then you must use the same compiler for the
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application. This is another complication and dependency that
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can be avoided by using only the standard C library version.
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Note 2: if you use a standard C pthread*.dll with a C++
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application, then any functions that you define that are
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intended to be called via pthread_cleanup_push() must be
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__cdecl.
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Note 3: the intention is to also name either the VC or GC
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version (it should be arbitrary) as pthread.dll, including
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pthread.lib and libpthread.a as appropriate.
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In general:
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pthread[VG]{SE,CE,C}.dll
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pthread[VG]{SE,CE,C}.lib
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where:
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[VG] indicates the compiler
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V - MS VC
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G - GNU C
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{SE,CE,C} indicates the exception handling scheme
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SE - Structured EH
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CE - C++ EH
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C - no exceptions - uses setjmp/longjmp
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For example:
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pthreadVSE.dll (MSVC/SEH)
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pthreadGCE.dll (GNUC/C++ EH)
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pthreadGC.dll (GNUC/not dependent on exceptions)
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The GNU library archive file names have changed to:
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libpthreadGCE.a
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libpthreadGC.a
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------------------------------------------------------------------------------
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Q 4 Cleanup code default style or: it used to work when I built
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--- the library myself, but now it doesn't - why?
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Up to and including snapshot 2001-07-12, if not defined, the cleanup
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style was determined automatically from the compiler used, and one
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of the following was defined accordingly:
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__CLEANUP_SEH MSVC only
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__CLEANUP_CXX C++, including MSVC++, GNU G++
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__CLEANUP_C C, including GNU GCC, not MSVC
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These defines determine the style of cleanup (see pthread.h) and,
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most importantly, the way that cancelation and thread exit (via
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pthread_exit) is performed (see the routine ptw32_throw() in private.c).
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In short, the exceptions versions of the library throw an exception
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when a thread is canceled or exits (via pthread_exit()), which is
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caught by a handler in the thread startup routine, so that the
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the correct stack unwinding occurs regardless of where the thread
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is when it's canceled or exits via pthread_exit().
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After snapshot 2001-07-12, unless your build explicitly defines (e.g.
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via a compiler option) __CLEANUP_SEH, __CLEANUP_CXX, or __CLEANUP_C, then
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the build now ALWAYS defaults to __CLEANUP_C style cleanup. This style
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uses setjmp/longjmp in the cancelation and pthread_exit implementations,
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and therefore won't do stack unwinding even when linked to applications
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that have it (e.g. C++ apps). This is for consistency with most/all
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commercial Unix POSIX threads implementations.
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Although it was not clearly documented before, it is still necessary to
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build your application using the same __CLEANUP_* define as was
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used for the version of the library that you link with, so that the
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correct parts of pthread.h are included. That is, the possible
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defines require the following library versions:
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__CLEANUP_SEH pthreadVSE.dll
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__CLEANUP_CXX pthreadVCE.dll or pthreadGCE.dll
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__CLEANUP_C pthreadVC.dll or pthreadGC.dll
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THE POINT OF ALL THIS IS: if you have not been defining one of these
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explicitly, then the defaults have been set according to the compiler
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and language you are using, as described at the top of this
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section.
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THIS NOW CHANGES, as has been explained above. For example:
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If you were building your application with MSVC++ i.e. using C++
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exceptions (rather than SEH) and not explicitly defining one of
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__CLEANUP_*, then __CLEANUP_C++ was defined for you in pthread.h.
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You should have been linking with pthreadVCE.dll, which does
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stack unwinding.
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If you now build your application as you had before, pthread.h will now
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set __CLEANUP_C as the default style, and you will need to link
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with pthreadVC.dll. Stack unwinding will now NOT occur when a
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thread is canceled, nor when the thread calls pthread_exit().
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Your application will now most likely behave differently to previous
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versions, and in non-obvious ways. Most likely is that local
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objects may not be destroyed or cleaned up after a thread
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is canceled.
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If you want the same behaviour as before, then you must now define
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__CLEANUP_C++ explicitly using a compiler option and link with
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pthreadVCE.dll as you did before.
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------------------------------------------------------------------------------
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Q 5 Why is the default library version now less exception-friendly?
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---
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Because most commercial Unix POSIX threads implementations don't allow you to
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choose to have stack unwinding. (Compaq's TRU64 Unix is possibly an exception.)
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Therefore, providing it in pthread-win32 as a default could be dangerous
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and non-portable. We still provide the choice but you must now consciously
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make it.
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WHY NOT REMOVE THE EXCEPTIONS VERSIONS OF THE LIBRARY ALTOGETHER?
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There are a few reasons:
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- because there are well respected POSIX threads people who believe
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that POSIX threads implementations should be exceptions-aware and
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do the expected thing in that context. (There are equally respected
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people who believe it should not be easily accessible, if it's there
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at all.)
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- because pthreads-win32 is one of the few implementations that has
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the choice, perhaps the only freely available one, and so offers
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a laboratory to people who may want to explore the effects;
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- although the code will always be around somewhere for anyone who
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wants it, once it's removed from the current version it will not be
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nearly as visible to people who may have a use for it.
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------------------------------------------------------------------------------
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Q 6 Should I use Cygwin or Mingw32 as a development environment?
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---
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Important: see Q7 also.
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Use Mingw32 with the MSVCRT library to build applications that use
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the pthreads DLL.
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Cygwin's own internal support for POSIX threads is growing.
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Consult that project's documentation for more information.
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------------------------------------------------------------------------------
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Q 7 Now that pthreads-win32 builds under Mingw32, why do I get
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--- memory access violations (segfaults)?
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The latest Mingw32 package has thread-safe exception handling (see Q10).
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Also, see Q6 above.
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------------------------------------------------------------------------------
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Q 8 How do I use pthread.dll for Win32 (Visual C++ 5.0)
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---
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>
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> I'm a "rookie" when it comes to your pthread implementation. I'm currently
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> desperately trying to install the prebuilt .dll file into my MSVC compiler.
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> Could you please provide me with explicit instructions on how to do this (or
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> direct me to a resource(s) where I can acquire such information)?
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>
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> Thank you,
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>
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You should have a .dll, .lib, .def, and three .h files. It is recommended
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that you use pthreadVC.dll, rather than pthreadVCE.dll or pthreadVSE.dll
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(see Q2 above).
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The .dll can go in any directory listed in your PATH environment
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variable, so putting it into C:\WINDOWS should work.
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The .lib file can go in any directory listed in your LIB environment
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variable.
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The .h files can go in any directory listed in your INCLUDE
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environment variable.
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Or you might prefer to put the .lib and .h files into a new directory
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and add its path to LIB and INCLUDE. You can probably do this easiest
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by editing the file:-
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C:\Program Files\DevStudio\vc\bin\vcvars32.bat
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The .def file isn't used by anything in the pre-compiled version but
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is included for information.
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Cheers.
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Ross
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------------------------------------------------------------------------------
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Q 9 Cancelation doesn't work for me, why?
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---
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> I'm investigating a problem regarding thread cancelation. The thread I want
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> to cancel has PTHREAD_CANCEL_ASYNCHRONOUS, however, this piece of code
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> blocks on the join():
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>
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> if ((retv = Pthread_cancel( recvThread )) == 0)
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> {
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> retv = Pthread_join( recvThread, 0 );
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> }
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>
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> Pthread_* are just macro's; they call pthread_*.
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>
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> The thread recvThread seems to block on a select() call. It doesn't get
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> cancelled.
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>
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> Two questions:
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>
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> 1) is this normal behaviour?
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>
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> 2) if not, how does the cancel mechanism work? I'm not very familliar to
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> win32 programming, so I don't really understand how the *Event() family of
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> calls work.
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The answer to your first question is, normal POSIX behaviour would
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be to asynchronously cancel the thread. However, even that doesn't
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guarantee cancelation as the standard only says it should be
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cancelled as soon as possible.
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Snapshot 99-11-02 or earlier only partially supports asynchronous cancellation.
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Snapshots since then simulate async cancelation by poking the address of
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a cancelation routine into the PC of the threads context. This requires
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the thread to be resumed in some way for the cancelation to actually
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proceed. This is not true async cancelation, but it is as close as we've
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been able to get to it.
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If the thread you're trying to cancel is blocked (for instance, it could be
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waiting for data from the network), it will only get cancelled when it unblocks
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(when the data arrives). For true pre-emptive cancelation in these cases,
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pthreads-win32 from snapshot 2004-05-16 can automatically recognise and use the
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QueueUserAPCEx package by Panagiotis E. Hadjidoukas. This package is available
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from the pthreads-win32 ftp site and is included in the pthreads-win32
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self-unpacking zip from 2004-05-16 onwards.
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Using deferred cancelation would normally be the way to go, however,
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even though the POSIX threads standard lists a number of C library
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functions that are defined as deferred cancelation points, there is
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no hookup between those which are provided by Windows and the
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pthreads-win32 library.
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Incidently, it's worth noting for code portability that the older POSIX
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threads standards cancelation point lists didn't include "select" because
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(as I read in Butenhof) it wasn't part of POSIX. However, it does appear in
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the SUSV3.
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Effectively, the only mandatory cancelation points that pthreads-win32
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recognises are those the library implements itself, ie.
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pthread_testcancel
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pthread_cond_wait
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pthread_cond_timedwait
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pthread_join
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sem_wait
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sem_timedwait
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pthread_delay_np
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The following routines from the non-mandatory list in SUSV3 are
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cancelation points in pthreads-win32:
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pthread_rwlock_wrlock
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pthread_rwlock_timedwrlock
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The following routines from the non-mandatory list in SUSV3 are not
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cancelation points in pthreads-win32:
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pthread_rwlock_rdlock
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pthread_rwlock_timedrdlock
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Pthreads-win32 also provides two functions that allow you to create
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cancelation points within your application, but only for cases where
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a thread is going to block on a Win32 handle. These are:
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pthreadCancelableWait(HANDLE waitHandle) /* Infinite wait */
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pthreadCancelableTimedWait(HANDLE waitHandle, DWORD timeout)
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------------------------------------------------------------------------------
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Q 10 How do I create thread-safe applications using
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---- pthreadGCE.dll, libpthreadw32.a and Mingw32?
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This should not be a problem with recent versions of MinGW32.
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For early versions, see Thomas Pfaff's email at:
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http://sources.redhat.com/ml/pthreads-win32/2002/msg00000.html
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------------------------------------------------------------------------------
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Q 11 Why isn't pthread_t defined as a scalar (e.g. pointer or int)
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like it is for other POSIX threads implementations?
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----
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Originally pthread_t was defined as a pointer (to the opaque pthread_t_
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struct) and later it was changed to a struct containing the original
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pointer plus a sequence counter. This is allowed under both the original
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POSIX Threads Standard and the current Single Unix Specification.
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When pthread_t is a simple pointer to a struct some very difficult to
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debug problems arise from the process of freeing and later allocing
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thread structs because new pthread_t handles can acquire the identity of
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previously detached threads. The change to a struct was made, along with
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some changes to their internal managment, in order to guarantee (for
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practical applications) that the pthread_t handle will be unique over the
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life of the running process.
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Where application code attempts to compare one pthread_t against another
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directly, a compiler error will be emitted because structs can't be
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compared at that level. This should signal a potentially serious problem
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in the code design, which would go undetected if pthread_t was a scalar.
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The POSIX Threading API provides a function named pthread_equal() to
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compare pthread_t thread handles.
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Other pthreads implementations, such as Sun's, use an int as the handle
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but do guarantee uniqueness within the process scope. Win32 scalar typed
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thread handles also guarantee uniqueness in system scope. It wasn't clear
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how well the internal management of these handles would scale as the
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number of threads and the fragmentation of the sequence numbering
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increased for applications where thousands or millions of threads are
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created and detached over time. The current management of threads within
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pthreads-win32 using structs for pthread_t, and reusing without ever
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freeing them, reduces the management time overheads to a constant, which
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could be important given that pthreads-win32 threads are built on top of
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Win32 threads and will therefore include that management overhead on top
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of their own. The cost is that the memory resources used for thread
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handles will remain at the peak level until the process exits.
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While it may be inconvenient for developers to be forced away from making
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assumptions about the internals of pthread_t, the advantage for the
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future development of pthread-win32, as well as those applications that
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use it and other pthread implementations, is that the library is free to
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change pthread_t internals and management as better methods arise.
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