1-1 © 2004 JMH Associates. All rights reserved. Windows Application Development Chapter 7 Windows Thread Management

1-2 © 2004 JMH Associates. All rights reserved. Objectives and Benefits Upon completion of this chapter, you will be able to:  Describe Windows thread management  Use threads in Windows applications  Use threads with C library functions  Build and execute threaded applications  Describe scheduling and SMP operation

1-3 © 2004 JMH Associates. All rights reserved. Threads: Benefits and Risks Benefits  Simpler program models  Faster code – in many cases  Exploit multiple processors  Exploit inherent application parallelism  Reliable, understandable, maintainable code Risks  Slower performance – in some cases  Potential defects

1-4 © 2004 JMH Associates. All rights reserved. ContentsContents 1.Process and Thread Overview 2.Thread Management 3.Waiting for Thread Termination 4.The C Library and Threads 5.Demonstration: Building a Threaded Application 6.Thread Priority, Scheduling, SMP Impact 7.Lab Exercise 7-1

1-5 © 2004 JMH Associates. All rights reserved. 1. Process and Thread Overview Threads in a process share data and code  Each thread has its own stack for function calls  Calling thread can pass an argument to a thread at creation time  This argument is on the stack  Each thread can allocate its own Thread Local Storage (TLS) indices and set TLS values

1-6 © 2004 JMH Associates. All rights reserved. Process and Thread Overview Threads are scheduled and run independently  The executive schedules threads  Threads run asynchronously  Threads can be preempted  Or restarted at any time

1-7 © 2004 JMH Associates. All rights reserved. Processes and Threads Process Code Global Variables Process Heap Process Resources Open Files Heaps … Environment Block... Thread 1 Thread Local Storage Stack Thread N Thread Local Storage Stack

1-8 © 2004 JMH Associates. All rights reserved. Read File B Single-Threaded ProgramMultithreaded Program Read File A Read File B Reading File B before File A would give the same results Read File A Wait for Thread 1 and Thread 2 to finish Merge data from both files Thread 1 Thread 3 Thread 2 Thread 3 Merge data from both files Threads Performing Parallel Tasks

1-9 © 2004 JMH Associates. All rights reserved. 2. Thread Management Creating a Thread The Thread Function Thread Termination Thread Exit Codes Thread Identities Suspending and Resuming Threads

1-10 © 2004 JMH Associates. All rights reserved. Creating a Thread (1 of 6) Specify the thread’s start address within the process’ code Specify the stack size, and the stack consumes space within the process’ address space  The stack cannot be expanded

1-11 © 2004 JMH Associates. All rights reserved. Creating a Thread (2 of 6) Specify a pointer to an argument for the thread  Can be nearly anything  Interpreted by the thread itself CreateThread returns a thread’s ID value and its handle  A NULL handle value indicates failure

1-12 © 2004 JMH Associates. All rights reserved. Creating a Thread (3 of 6) HANDLE CreateThread ( LPSECURITY_ATTRIBUTES lpsa, DWORD cbStack, LPTHREAD_START_ROUTINE lpStartAddr, LPVOID lpvThreadParm, DWORD dwCreate, LPDWORD lpIDThread )

1-13 © 2004 JMH Associates. All rights reserved. Creating a Thread (4 of 6) Parameters lpsa  Security attributes structure (use NULL ) cbStack  Byte size for the new thread’s stack  Use 0 to default to the primary thread’s stack size (1 MB)

1-14 © 2004 JMH Associates. All rights reserved. Creating a Thread (5 of 6) lpStartAddr  Points to the function (within the calling process) to be executed  Accepts a single pointer argument and returns a 32-bit DWORD exit code  The thread can interpret the argument as a DWORD or a pointer lpThreadParm  The pointer passed as the thread argument

1-15 © 2004 JMH Associates. All rights reserved. Creating a Thread (6 of 6) dwCreate  If zero, the thread is immediately ready to run  If CREATE_SUSPENDED, the new thread will be in the suspended state, requiring a ResumeThread function call to move the thread to the ready state lpIDThread  Points to a DWORD that receives the new thread’s identifier; NULL OK on W2000/NT

1-16 © 2004 JMH Associates. All rights reserved. The Thread Function DWORD WINAPI MyThreadFunc ( PVOID pThParam ) {... ExitThread (ExitCode); /* OR */ return ExitCode; }

1-17 © 2004 JMH Associates. All rights reserved. Thread Termination (1 of 3) Threads are terminated by ExitProcess  The process and all its threads terminate  The exit code returned by the thread start function same as the process exit code  Or a thread can simply return with its exit code

1-18 © 2004 JMH Associates. All rights reserved. Thread Termination (2 of 3) ExitThread is the preferred technique  The thread’s stack is deallocated on termination VOID ExitThread (DWORD (dwExitCode) When the last thread in a process terminates, so does the process itself

1-19 © 2004 JMH Associates. All rights reserved. Thread Termination (3 of 3) You can terminate a different thread with TerminateThread  Dangerous: The thread’s stack and other resources will not be deallocated  Better to let the thread terminate itself A thread will remain in the system until the last handle to it is closed (using CloseHandle )  Then the thread will be deleted Any other thread can retrieve the exit code

1-20 © 2004 JMH Associates. All rights reserved. Thread Exit Codes BOOL GetExitCodeThread ( HANDLE hThread, LPDWORD lpdwExitCode ) lpdwExitCode  Contains the thread’s exit code  It could be STILL_ACTIVE

1-21 © 2004 JMH Associates. All rights reserved. Thread Identities (1 of 2) A thread has a permanent “ ThreadId ” A thread is usually accessed by HANDLE An ID can be converted to a HANDLE

1-22 © 2004 JMH Associates. All rights reserved. Thread Identities (2 of 2) HANDLE GetCurrentThread (VOID); DWORD GetCurrentThreadId (VOID); HANDLE OpenThread ( DWORD dwDesiredAccess, BOOL InheritableHandle, DWORD ThreadId ); /* >= Windows 2000 only */

1-23 © 2004 JMH Associates. All rights reserved. Suspend & Resume Threads (1 of 2) Every thread has a suspend count  A thread can execute only if this count is zero A thread can be created in the suspended state One thread can increment or decrement the suspend count of another: DWORD ResumeThread (HANDLE hThread)

1-24 © 2004 JMH Associates. All rights reserved. Suspend & Resume Threads (2 of 2) DWORD SuspendThread (HANDLE hThread) Both functions return previous suspend count 0xFFFFFFFF indicates failure Useful in preventing “race conditions”  Do not allow threads to start until initialization is complete Unsafe for general synchronization

1-25 © 2004 JMH Associates. All rights reserved. 3. Waiting for Thread Termination Wait for a thread to terminate using general purpose wait functions WaitForSingleObject or WaitForMultipleObjects  Using thread handles The wait functions wait for the thread handle to become signaled  Thread handle is signaled when thread terminates

1-26 © 2004 JMH Associates. All rights reserved. Waiting for Thread Termination (2 of 2) ExitThread and TerminateThread set the object to the signaled state  Releasing all other threads waiting on the object ExitProcess sets the process’ state and all its threads’ states to signaled

1-27 © 2004 JMH Associates. All rights reserved. The Wait Functions (1 of 2) DWORD WaitForSingleObject ( HANDLE hObject, DWORD dwTimeOut )

1-28 © 2004 JMH Associates. All rights reserved. The Wait Functions (2 of 2) DWORD WaitForMultipleObjects ( DWORD cObjects, LPHANDLE lphObjects, BOOL fWaitAll, DWORD dwTimeOut ) Return: The cause of the wait completion

1-29 © 2004 JMH Associates. All rights reserved. Wait Options (1 of 2) Specify either a single handle hObject Or an array of cObjects referenced by lphObjects cObjects should not exceed MAXIMUM_WAIT_OBJECTS - 64

1-30 © 2004 JMH Associates. All rights reserved. Wait Options (2 of 2) dwTimeOut is in milliseconds  0 means the function returns immediately after testing the state of the specified objects  Use INFINITE for no timeout  Wait forever for a thread to terminate GetExitCodeThread  Returns the thread exit code

1-31 © 2004 JMH Associates. All rights reserved. Wait Function Return Values (1 of 3) fWaitAll  If TRUE, wait for all threads to terminate Possible return values are:  WAIT_OBJECT_0  The thread terminated (if calling WaitForMultipleObjects ; fWaitAll set)

1-32 © 2004 JMH Associates. All rights reserved. Wait Function Return Values (2 of 3)  WAIT_OBJECT_0 + n where 0 <= n < cObjects  Subtract WAIT_OBJECT_0 from the return value to determine which thread terminated when calling WaitForMultipleObjects with fWaitAll set  WAIT_TIMEOUT  Timeout period elapsed

1-33 © 2004 JMH Associates. All rights reserved. Wait Function Return Values (3 of 3)  WAIT_ABANDONED  Not possible with thread handles  WAIT_FAILED  Call GetLastError for thread-specific error code

1-34 © 2004 JMH Associates. All rights reserved. 4. The C Library and Threads Nearly all programs (and thread functions) use the C library But the normal C library is not “thread safe” The C function _beginthreadex has exactly the same parameters as CreateThread

1-35 © 2004 JMH Associates. All rights reserved. Using _beginthreadex (1 of 3) Cast the _beginthreadex return value to (HANDLE) Use _endthreadex in place of ExitThread #include

1-36 © 2004 JMH Associates. All rights reserved. Using _beginthreadex (2 of 3) Set the multithreaded environment as follows:  #define _MT in every source file before  Link with LIBCMT.LIB  Override the default library

1-37 © 2004 JMH Associates. All rights reserved. Using _beginthreadex (3 of 3) Preferred method using Visual C++ From the menu bar:  Build Settings — C/C++ Tab  Code Generation category  Select a multithreaded run-time library

1-38 © 2004 JMH Associates. All rights reserved. Ex: A Simple Boss Thread HANDLE hWork[K]; volatile LONGLONG WorkDone[K], iTh; /* !! */... for (iTh = 0; iTh < K; iTh++) { WorkDone[ith] = 0; hWork[iTh] = _beginthreadex (NULL, 0, WorkTh, (PVOID)&iTh, 0, NULL); /* BUG! */ } WaitForMultipleObjects (K, hWork, TRUE, INFINITE); for (iTh = 0; iTh < K; iTh++) printf (“Thread %d did %d workunits\n”, iTh, WorkDone[iTh]);

1-39 © 2004 JMH Associates. All rights reserved. Ex: A Simple Worker Thread DWORD WINAPI WorkTh (PVOID pThNum) { DWORD ThNum = (DWORD)(*pThNum); while (...) { /* Perform work */ WorkDone[ThNum]++; } _endthreadex (0); }

1-40 © 2004 JMH Associates. All rights reserved. 5. Demonstration: Building a Threaded Application Using Visual C++ Ver. 6.0, 7.0

1-41 © 2004 JMH Associates. All rights reserved. Demonstration (1 of 2) Implement multithreaded word count program wcMT  Include the source code of wc with your application and execute it as a thread  Create one thread for each file to analyze  Compare performance of single process, multi- ple process, & multiple thread implementations  If you use other libraries, build thread-safe versions now!

1-42 © 2004 JMH Associates. All rights reserved. Demonstration (2 of 2) THIS IS A BOSS/WORKER SYSTEM  Shows common thread management techniques  See picture on the next overhead CHALLENGE:  Also try at wcMTx.c and wcMTxx.c  These contains some common bugs  What are the bug symptoms?

1-43 © 2004 JMH Associates. All rights reserved. Boss/Worker Model for wcMT ProgramResources Files Workers Thread K Thread 1 Thread 0 main () Boss

1-44 © 2004 JMH Associates. All rights reserved. 6.Thread Priority and Scheduling Windows kernels run the highest-priority thread that is ready for execution 4 priority classes set by CreateProcess  IDLE_PRIORITY_CLASS (base priority 4)  NORMAL_PRIORITY_CLASS (9 or 7)  HIGH_PRIORITY_CLASS (13)  REALTIME_PRIORITY_CLASS (24) Windows XP extensions

1-45 © 2004 JMH Associates. All rights reserved. Thread Priority (1 of 2) Change or determine a thread’s priority  For itself  For another process, security permitting DWORD SetPriorityClass ( HANDLE hProcess, DWORD dwPriority) DWORD GetPriorityClass ( HANDLE hProcess)

1-46 © 2004 JMH Associates. All rights reserved. Thread Priority (2 of 2) Thread priorities set relative to base priority  THREAD_PRIORITY_LOWEST  THREAD_PRIORITY_BELOW_NORMAL  THREAD_PRIORITY_NORMAL  THREAD_PRIORITY_ABOVE_NORMAL  THREAD_PRIORITY_HIGHEST

1-47 © 2004 JMH Associates. All rights reserved. Process Priority Cautions Use high thread priorities with caution Avoid real time priorities for user processes  User threads may preempt executive threads Assure fairness  All threads should run eventually  Real time priorities may prevent fairness  “Priority inversion”  “Thread starvation”

1-48 © 2004 JMH Associates. All rights reserved. Thread States and Transitions (1 of 2) A thread is running when it is on a processor  SMP systems have multiple processors  Intel Xeon provides single processor multiprocessing - Hyperthreading The executive can place a running thread in the wait state  I/O operations wait wait for data transfer  The thread is blocked or sleeping A thread is ready if it could be running

1-49 © 2004 JMH Associates. All rights reserved. Terminated Running Initialized Waiting Ready Create a new thread Time slice is over Goes to ready queue Wait, Sleep Or any blocking call Object is signaled Thread Id is reused Scheduled on a free processor ExitThread TerminateThread

1-50 © 2004 JMH Associates. All rights reserved. Thread States and Transitions (2 of 3) The scheduler can place a ready thread on any available processor The executive moves a running thread to the ready state if the thread’s time slice expires  Sleep(0) moves a running thread to ready The executive makes a waiting thread ready as soon as appropriate handles are signaled  The thread wakes up

1-51 © 2004 JMH Associates. All rights reserved. Thread States and Transitions (3 of 3) A thread can be suspended or resumed A thread is in the terminated state after it has been terminated but there still open handles  Other threads may interrogate the thread’s state and exit code

1-52 © 2004 JMH Associates. All rights reserved. SummarySummary Threads allow concurrent processing  Reducing overall system overhead  Potentially simplifying programs Single-threaded programs can be inefficient  Managing concurrent and interacting tasks Threads can streamline I/O-bound programs There are reliability and performance risks  Design and code carefully

1-53 © 2004 JMH Associates. All rights reserved. 8. Lab Exercise 7-1 Implement a multithreaded sort command, sortMT nTh FileName  Use the scheme shown on the next overhead  nTh : number of threads  If properly implemented, it will transparently take advantage of SMP systems

1-54 © 2004 JMH Associates. All rights reserved. sortMT Design Array qsort merge Thread0 Thread1 Thread2 Thread3 for (i = 0; i < 4; i++) CreateThread ( ) Wait (Thread0) /* Array is Sorted */ wait (Thread3) wait (Thread2) wait (Thread1)

1-55 © 2004 JMH Associates. All rights reserved. Lab Exercise (cont’d) CAUTION:  The lab solution requires that the number of records in the file to be sorted be a multiple of the number of threads  The number of threads must be a power of two SUGGESTION:  Start with sortMTx.c, a buggy solution  The correct solution is also provided

1-56 © 2004 JMH Associates. All rights reserved. Extra Credit — Additional Exercise Remove the restrictions:  The command form is sortMT nTh file  nTh must be a power of two  The number of records in file must be a multiple of nTh What is the effect of multiple threads on per- formance, even on a single processor system?  Hint: Test with large files