Creating Threads

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Creating Threads

The CreateThread function creates a new thread for a process. The creating thread must specify the starting address of the code that the new thread is to execute. Typically, the starting address is the name of a function defined in the program code (for more information, see ThreadProc). This function takes a single parameter and returns a DWORD value. A process can have multiple threads simultaneously executing the same function.

The following is a simple example that demonstrates how to create a new thread that executes the locally defined function, MyThreadFunction.

The calling thread uses the WaitForMultipleObjects function to persist until all worker threads have terminated. The calling thread blocks while it is waiting; to continue processing, a calling thread would use WaitForSingleObject and wait for each worker thread to signal its wait object. Note that if you were to close the handle to a worker thread before it terminated, this does not terminate the worker thread. However, the handle will be unavailable for use in subsequent function calls.

#include <windows.h> #include <tchar.h> #include <strsafe.h> #define MAX_THREADS 3 #define BUF_SIZE 255 DWORD WINAPI MyThreadFunction( LPVOID lpParam ); void ErrorHandler(LPTSTR lpszFunction); // Sample custom data structure for threads to use. // This is passed by a void pointer so it can be any data type // that can be passed using a single void pointer (LPVOID). typedef struct MyData { int val1; int val2; } MYDATA, *PMYDATA; int _tmain() { PMYDATA pDataArray[MAX_THREADS]; DWORD dwThreadIdArray[MAX_THREADS]; HANDLE hThreadArray[MAX_THREADS]; // Create MAX_THREADS worker threads. for( int i=0; i<MAX_THREADS; i++ ) { // Allocate memory for thread data. pDataArray[i] = (PMYDATA) HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof(MYDATA)); if( pDataArray[i] == NULL ) { // If the array allocation fails, the system is out of memory // so there is no point in trying to print an error message. // Just terminate execution. ExitProcess(2); } // Generate unique data for each thread to work with. pDataArray[i]->val1 = i; pDataArray[i]->val2 = i+100; // Create the thread to begin execution on its own. hThreadArray[i] = CreateThread( NULL, // default security attributes 0, // use default stack size MyThreadFunction, // thread function name pDataArray[i], // argument to thread function 0, // use default creation flags &dwThreadIdArray[i]); // returns the thread identifier // Check the return value for success. // If CreateThread fails, terminate execution. // This will automatically clean up threads and memory. if (hThreadArray[i] == NULL) { ErrorHandler(TEXT("CreateThread")); ExitProcess(3); } } // End of main thread creation loop. // Wait until all threads have terminated. WaitForMultipleObjects(MAX_THREADS, hThreadArray, TRUE, INFINITE); // Close all thread handles and free memory allocations. for(int i=0; i<MAX_THREADS; i++) { CloseHandle(hThreadArray[i]); if(pDataArray[i] != NULL) { HeapFree(GetProcessHeap(), 0, pDataArray[i]); pDataArray[i] = NULL; // Ensure address is not reused. } } return 0; } DWORD WINAPI MyThreadFunction( LPVOID lpParam ) { HANDLE hStdout; PMYDATA pDataArray; TCHAR msgBuf[BUF_SIZE]; size_t cchStringSize; DWORD dwChars; // Make sure there is a console to receive output results. hStdout = GetStdHandle(STD_OUTPUT_HANDLE); if( hStdout == INVALID_HANDLE_VALUE ) return 1; // Cast the parameter to the correct data type. // The pointer is known to be valid because // it was checked for NULL before the thread was created. pDataArray = (PMYDATA)lpParam; // Print the parameter values using thread-safe functions. StringCchPrintf(msgBuf, BUF_SIZE, TEXT("Parameters = %d, %d/n"), pDataArray->val1, pDataArray->val2); StringCchLength(msgBuf, BUF_SIZE, &cchStringSize); WriteConsole(hStdout, msgBuf, (DWORD)cchStringSize, &dwChars, NULL); return 0; } void ErrorHandler(LPTSTR lpszFunction) { // Retrieve the system error message for the last-error code. LPVOID lpMsgBuf; LPVOID lpDisplayBuf; DWORD dw = GetLastError(); FormatMessage( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, dw, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPTSTR) &lpMsgBuf, 0, NULL ); // Display the error message. lpDisplayBuf = (LPVOID)LocalAlloc(LMEM_ZEROINIT, (lstrlen((LPCTSTR)lpMsgBuf)+lstrlen((LPCTSTR)lpszFunction)+40)*sizeof(TCHAR)); StringCchPrintf((LPTSTR)lpDisplayBuf, LocalSize(lpDisplayBuf) / sizeof(TCHAR), TEXT("%s failed with error %d: %s"), lpszFunction, dw, lpMsgBuf); MessageBox(NULL, (LPCTSTR)lpDisplayBuf, TEXT("Error"), MB_OK); // Free the error-handling buffer allocations. LocalFree(lpMsgBuf); LocalFree(lpDisplayBuf); }

The MyThreadFunction function avoids the use of the C run-time library (CRT), as many of its functions are not thread-safe, particularly if you are not using the multithreaded CRT. If you would like to use the CRT in a ThreadProc function, use the_beginthreadex function instead.

It is risky to pass the address of a local variable if the creating thread exits before the new thread, because the pointer becomes invalid. Instead, either pass a pointer to dynamically allocated memory or make the creating thread wait for the new thread to terminate. Data can also be passed from the creating thread to the new thread using global variables. With global variables, it is usually necessary to synchronize access by multiple threads. For more information about synchronization, see Synchronizing Execution of Multiple Threads.

The creating thread can use the arguments to CreateThread to specify the following:

• The security attributes for the handle to the new thread. These security attributes include an inheritance flag that determines whether the handle can be inherited by child processes. The security attributes also include a security descriptor, which the system uses to perform access checks on all subsequent uses of the thread's handle before access is granted.
• The initial stack size of the new thread. The thread's stack is allocated automatically in the memory space of the process; the system increases the stack as needed and frees it when the thread terminates. For more information, see Thread Stack Size.
• A creation flag that enables you to create the thread in a suspended state. When suspended, the thread does not run until the ResumeThread function is called.

You can also create a thread by calling the CreateRemoteThread function. This function is used by debugger processes to create a thread that runs in the address space of the process being debugged.

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Build date: 8/7/2008

CreateThread Function

Creates a thread to execute within the virtual address space of the calling process.

To create a thread that runs in the virtual address space of another process, use the CreateRemoteThread function.

Syntax

HANDLE WINAPI CreateThread( __in_opt LPSECURITY_ATTRIBUTES lpThreadAttributes, __in SIZE_T dwStackSize, __in LPTHREAD_START_ROUTINE lpStartAddress, __in_opt LPVOID lpParameter, __in DWORD dwCreationFlags, __out_opt LPDWORD lpThreadId );

Parameters

lpThreadAttributes [in, optional]

A pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpThreadAttributes is NULL, the handle cannot be inherited.

The lpSecurityDescriptor member of the structure specifies a security descriptor for the new thread. If lpThreadAttributesis NULL, the thread gets a default security descriptor. The ACLs in the default security descriptor for a thread come from the primary token of the creator.

Windows XP/2000:  The ACLs in the default security descriptor for a thread come from the primary or impersonation token of the creator. This behavior changed with Windows XP SP2 and Windows Server 2003. For more information, see Remarks.

dwStackSize [in]

The initial size of the stack, in bytes. The system rounds this value to the nearest page. If this parameter is zero, the new thread uses the default size for the executable. For more information, see Thread Stack Size.

lpStartAddress [in]

A pointer to the application-defined function to be executed by the thread. This pointer represents the starting address of the thread. For more information on the thread function, see ThreadProc.

lpParameter [in, optional]

A pointer to a variable to be passed to the thread.

dwCreationFlags [in]

The flags that control the creation of the thread.

Value	Meaning
0	The thread runs immediately after creation.
CREATE_SUSPENDED 0x00000004	The thread is created in a suspended state, and does not run until theResumeThread function is called.
STACK_SIZE_PARAM_IS_A_RESERVATION 0x00010000	The dwStackSize parameter specifies the initial reserve size of the stack. If this flag is not specified, dwStackSize specifies the commit size. Windows 2000:  The STACK_SIZE_PARAM_IS_A_RESERVATION flag is not supported.

lpThreadId [out, optional]

A pointer to a variable that receives the thread identifier. If this parameter is NULL, the thread identifier is not returned.

Return Value

If the function succeeds, the return value is a handle to the new thread.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Note that CreateThread may succeed even if lpStartAddress points to data, code, or is not accessible. If the start address is invalid when the thread runs, an exception occurs, and the thread terminates. Thread termination due to a invalid start address is handled as an error exit for the thread's process. This behavior is similar to the asynchronous nature of CreateProcess, where the process is created even if it refers to invalid or missing dynamic-link libraries (DLLs).

Remarks

The number of threads a process can create is limited by the available virtual memory. By default, every thread has one megabyte of stack space. Therefore, you can create at most 2,028 threads. If you reduce the default stack size, you can create more threads. However, your application will have better performance if you create one thread per processor and build queues of requests for which the application maintains the context information. A thread would process all requests in a queue before processing requests in the next queue.

The new thread handle is created with the THREAD_ALL_ACCESS access right. If a security descriptor is not provided when the thread is created, a default security descriptor is constructed for the new thread using the primary token of the process that is creating the thread. When a caller attempts to access the thread with the OpenThread function, the effective token of the caller is evaluated against this security descriptor to grant or deny access.

Windows XP/2000:  If a security descriptor is not provided when the thread is created, a default security descriptor is constructed using the effective token of the thread. If the thread is impersonating another user, the thread's effective token is the impersonation token and the default security descriptor allows access only to the impersonation token's TokenDefaultDacl owner or members. If the thread is not impersonating another user, the thread's effective token is its primary token. This behavior changed starting with Windows XP SP2 and Windows Server 2003. For more information, see  Thread Security and Access Rights.

The newly created thread has full access rights to itself when calling the GetCurrentThread function.

Windows Server 2003 and Windows XP/2000:  The thread's access rights to itself are computed by evaluating the primary token of the process in which the thread was created against the default security descriptor constructed for the thread. If the thread is created in a remote process, the primary token of the remote process is used. As a result, the newly created thread may have reduced access rights to itself when calling  GetCurrentThread. Some access rights including THREAD_SET_THREAD_TOKEN and THREAD_GET_CONTEXT may not be present, leading to unexpected failures. For this reason, creating a thread while impersonating another user is not recommended.

The thread execution begins at the function specified by the lpStartAddress parameter. If this function returns, the DWORDreturn value is used to terminate the thread in an implicit call to the ExitThread function. Use the GetExitCodeThread function to get the thread's return value.

The thread is created with a thread priority of THREAD_PRIORITY_NORMAL. Use the GetThreadPriority and SetThreadPriorityfunctions to get and set the priority value of a thread.

When a thread terminates, the thread object attains a signaled state, satisfying any threads that were waiting on the object.

The thread object remains in the system until the thread has terminated and all handles to it have been closed through a call toCloseHandle.

The ExitProcess, ExitThread, CreateThread, CreateRemoteThread functions, and a process that is starting (as the result of a call by CreateProcess) are serialized between each other within a process. Only one of these events can happen in an address space at a time. This means that the following restrictions hold:

• During process startup and DLL initialization routines, new threads can be created, but they do not begin execution until DLL initialization is done for the process.
• Only one thread in a process can be in a DLL initialization or detach routine at a time.
• ExitProcess does not return until no threads are in their DLL initialization or detach routines.

A thread in an executable that calls the C run-time library (CRT) should use the _beginthreadex and _endthreadex functions for thread management rather than CreateThread and ExitThread; this requires the use of the multi-threaded version of the CRT. If a thread created using CreateThread calls the CRT, the CRT may terminate the process in low-memory conditions.

Examples

For an example, see Creating Threads.

Requirements

Client	Requires Windows Vista, Windows XP, or Windows 2000 Professional.
Server	Requires Windows Server 2008, Windows Server 2003, or Windows 2000 Server.
Header	Declared in Winbase.h; include Windows.h.
Library	Use Kernel32.lib.
DLL	Requires Kernel32.dll.

See Also

CloseHandle
CreateProcess
CreateRemoteThread
ExitProcess
ExitThread
GetExitCodeThread
GetThreadPriority
Process and Thread Functions
ResumeThread
SetThreadPriority
SECURITY_ATTRIBUTES
ThreadProc
Threads