CreateThread

The 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.

HANDLE CreateThread(
  LPSECURITY_ATTRIBUTES lpThreadAttributes, // SD
  DWORD dwStackSize,                        // initial stack size
  LPTHREAD_START_ROUTINE lpStartAddress,    // thread function
  LPVOID lpParameter,                       // thread argument
  DWORD dwCreationFlags,                    // creation option
  LPDWORD lpThreadId                        // thread identifier
);

Parameters

lpThreadAttributes

[in] 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.

Windows NT/2000: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new thread. If lpThreadAttributes is NULL, the thread gets a default security descriptor.

dwStackSize

[in] Specifies the initial commit size of the stack, in bytes. The system rounds this value to the nearest page. If this value is zero, or is smaller than the default commit size, the default is to use the same size as the calling thread. For more information, see Thread Stack Size.

lpStartAddress

[in] Pointer to the application-defined function of type LPTHREAD_START_ROUTINE to be executed by the thread and represents the starting address of the thread. For more information on the thread function, see ThreadProc.

lpParameter

[in] Specifies a single parameter value passed to the thread.

dwCreationFlags

[in] Specifies additional flags that control the creation of the thread. If the CREATE_SUSPENDED flag is specified, the thread is created in a suspended state, and will not run until the ResumeThread function is called. If this value is zero, the thread runs immediately after creation. At this time, no other values are supported.

lpThreadId

[out] Pointer to a variable that receives the thread identifier.

Windows NT/2000: If this parameter is NULL, the thread identifier is not returned.

Windows 95/98: This parameter may not be NULL.

Return Values

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).

Windows 95/98: CreateThread succeeds only when it is called in the context of a 32-bit program. A 32-bit DLL cannot create an additional thread when that DLL is being called by a 16-bit program.

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 2028 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 THREAD_ALL_ACCESS to the new thread. If a security descriptor is not provided, the handle can be used in any function that requires a thread object handle. When a security descriptor is provided, an access check is performed on all subsequent uses of the handle before access is granted. If the access check denies access, the requesting process cannot use the handle to gain access to the thread. If the thread impersonates a client, then calls CreateThread with a NULL security descriptor, the thread object created has a default security descriptor which allows access only to the impersonation token's TokenDefaultDacl owner or members. For more information, see Thread Security and Access Rights.

The thread execution begins at the function specified by the lpStartAddress parameter. If this function returns, the DWORD return 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 SetThreadPriority functions 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 to CloseHandle.

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 that uses functions from the C run-time libraries should use the beginthread and endthread C run-time functions for thread management rather than CreateThread and ExitThread. Failure to do so results in small memory leaks when ExitThread is called.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

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