_beginthread, _beginthreadex

Creates a thread.

Syntax

uintptr_t _beginthread( // NATIVE CODE
   void( __cdecl *start_address )( void * ),
   unsigned stack_size,
   void *arglist
);
uintptr_t _beginthread( // MANAGED CODE
   void( __clrcall *start_address )( void * ),
   unsigned stack_size,
   void *arglist
);
uintptr_t _beginthreadex( // NATIVE CODE
   void *security,
   unsigned stack_size,
   unsigned ( __stdcall *start_address )( void * ),
   void *arglist,
   unsigned initflag,
   unsigned *thrdaddr
);
uintptr_t _beginthreadex( // MANAGED CODE
   void *security,
   unsigned stack_size,
   unsigned ( __clrcall *start_address )( void * ),
   void *arglist,
   unsigned initflag,
   unsigned *thrdaddr
);

Parameters

start_address
Start address of a routine that begins execution of a new thread. For _beginthread, the calling convention is either __cdecl (for native code) or __clrcall (for managed code). For _beginthreadex, the calling convention is either __stdcall (for native code) or __clrcall (for managed code).

stack_size
Stack size for a new thread, or 0.

arglist
Argument list to be passed to a new thread, or NULL.

Security
Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If Security is NULL, the handle can't be inherited.

initflag
Flags that control the initial state of a new thread. Set initflag to 0 to run immediately, or to CREATE_SUSPENDED to create the thread in a suspended state; use ResumeThread to execute the thread. Set initflag to STACK_SIZE_PARAM_IS_A_RESERVATION flag to use stack_size as the initial reserve size of the stack in bytes; if this flag isn't specified, stack_size specifies the commit size.

thrdaddr
Points to a 32-bit variable that receives the thread identifier. If it's NULL, it's not used.

Return value

If successful, each of these functions returns a handle to the newly created thread; however, if the newly created thread exits too quickly, _beginthread might not return a valid handle. (See the discussion in the Remarks section.) On an error, _beginthread returns -1L, and errno is set to EAGAIN if there are too many threads, to EINVAL if the argument is invalid or the stack size is incorrect, or to EACCES if there are insufficient resources (such as memory). On an error, _beginthreadex returns 0, and errno and _doserrno are set.

If start_address is NULL, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, these functions set errno to EINVAL and return -1.

For more information about these and other return codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

For more information about uintptr_t, see Standard types.

Remarks

The _beginthread function creates a thread that begins execution of a routine at start_address. The routine at start_address must use the __cdecl (for native code) or __clrcall (for managed code) calling convention and should have no return value. When the thread returns from that routine, it's terminated automatically. For more information about threads, see Multithreading support for older code (Visual C++).

_beginthreadex resembles the Win32 CreateThread API more closely than _beginthread does. _beginthreadex differs from _beginthread in the following ways:

• _beginthreadex has three more parameters: initflag, Security, and threadaddr. The new thread can be created in a suspended state, with a specified security, and can be accessed by using thrdaddr, which is the thread identifier.

• The routine at start_address that's passed to _beginthreadex must use the __stdcall (for native code) or __clrcall (for managed code) calling convention and must return a thread exit code.

• _beginthreadex returns 0 on failure, rather than -1L.

• A thread that's created by using _beginthreadex is terminated by a call to _endthreadex.

The _beginthreadex function gives you more control over how the thread is created than _beginthread does. The _endthreadex function is also more flexible. For example, with _beginthreadex, you can use security information, set the initial state of the thread (running or suspended), and get the thread identifier of the newly created thread. You can also use the thread handle that's returned by _beginthreadex with the synchronization APIs, which you can't do with _beginthread.

_beginthreadex is safer to use than _beginthread. If the thread that's generated by _beginthread exits quickly, the handle that's returned to the caller of _beginthread might be invalid or point to another thread. However, the handle that's returned by _beginthreadex has to be closed by the caller of _beginthreadex, so it's guaranteed to be a valid handle if _beginthreadex didn't return an error.

You can call _endthread or _endthreadex explicitly to terminate a thread; however, _endthread or _endthreadex is called automatically when the thread returns from the routine that's passed as a parameter. Terminating a thread with a call to _endthread or _endthreadex helps ensure correct recovery of resources that are allocated for the thread.

_endthread automatically closes the thread handle, whereas _endthreadex doesn't. Therefore, when you use _beginthread and _endthread, don't explicitly close the thread handle by calling the Win32 CloseHandle API. This behavior differs from the Win32 ExitThread API.

The operating system handles the allocation of the stack when either _beginthread or _beginthreadex is called; you don't have to pass the address of the thread stack to either of these functions. In addition, the stack_size argument can be 0, in which case the operating system uses the same value as the stack that's specified for the main thread.

arglist is a parameter to be passed to the newly created thread. Typically, it's the address of a data item, such as a character string. arglist can be NULL if it isn't needed, but _beginthread and _beginthreadex must be given some value to pass to the new thread. All threads are terminated if any thread calls abort, exit, _exit, or ExitProcess.

The locale of the new thread is initialized by using the per-process global current locale info. If per-thread locale is enabled by a call to _configthreadlocale (either globally or for new threads only), the thread can change its locale independently from other threads by calling setlocale or _wsetlocale. Threads that don't have the per-thread locale flag set can affect the locale info in all other threads that also don't have the per-thread locale flag set, and also all newly created threads. For more information, see Locale.

For /clr code, _beginthread and _beginthreadex each have two overloads. One takes a native calling-convention function pointer, and the other takes a __clrcall function pointer. The first overload isn't application domain-safe and never will be. If you're writing /clr code, you must ensure that the new thread enters the correct application domain before it accesses managed resources. You can do so, for example, by using call_in_appdomain. The second overload is application domain-safe; the newly created thread will always end up in the application domain of the caller of _beginthread or _beginthreadex.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

For more compatibility information, see Compatibility.

Libraries

Multithreaded versions of the C run-time libraries only.

To use _beginthread or _beginthreadex, the application must link with one of the multithreaded C run-time libraries.

Examples

The following example uses _beginthread and _endthread.

// crt_BEGTHRD.C
// compile with: /MT /D "_X86_" /c
// processor: x86
#include <windows.h>
#include <process.h>    /* _beginthread, _endthread */
#include <stddef.h>
#include <stdlib.h>
#include <conio.h>

void Bounce( void * );
void CheckKey( void * );

// GetRandom returns a random integer between min and max.
#define GetRandom( min, max ) ((rand() % (int)(((max) + 1) - (min))) + (min))
// GetGlyph returns a printable ASCII character value
#define GetGlyph( val ) ((char)((val + 32) % 93 + 33))

BOOL repeat = TRUE;                 // Global repeat flag
HANDLE hStdOut;                     // Handle for console window
CONSOLE_SCREEN_BUFFER_INFO csbi;    // Console information structure

int main()
{
    int param = 0;
    int * pparam = &param;

    // Get display screen's text row and column information.
    hStdOut = GetStdHandle( STD_OUTPUT_HANDLE );
    GetConsoleScreenBufferInfo( hStdOut, &csbi );

    // Launch CheckKey thread to check for terminating keystroke.
    _beginthread( CheckKey, 0, NULL );

    // Loop until CheckKey terminates program or 1000 threads created.
    while( repeat && param < 1000 )
    {
        // launch another character thread.
        _beginthread( Bounce, 0, (void *) pparam );

        // increment the thread parameter
        param++;

        // Wait one second between loops.
        Sleep( 1000L );
    }
}

// CheckKey - Thread to wait for a keystroke, then clear repeat flag.
void CheckKey( void * ignored )
{
    _getch();
    repeat = 0;    // _endthread implied
}

// Bounce - Thread to create and control a colored letter that moves
// around on the screen.
//
// Params: parg - the value to create the character from
void Bounce( void * parg )
{
    char       blankcell = 0x20;
    CHAR_INFO  ci;
    COORD      oldcoord, cellsize, origin;
    DWORD      result;
    SMALL_RECT region;

    cellsize.X = cellsize.Y = 1;
    origin.X = origin.Y = 0;

    // Generate location, letter and color attribute from thread argument.
    srand( _threadid );
    oldcoord.X = region.Left = region.Right =
        GetRandom(csbi.srWindow.Left, csbi.srWindow.Right - 1);
    oldcoord.Y = region.Top = region.Bottom =
        GetRandom(csbi.srWindow.Top, csbi.srWindow.Bottom - 1);
    ci.Char.AsciiChar = GetGlyph(*((int *)parg));
    ci.Attributes = GetRandom(1, 15);

    while (repeat)
    {
        // Pause between loops.
        Sleep( 100L );

        // Blank out our old position on the screen, and draw new letter.
        WriteConsoleOutputCharacterA(hStdOut, &blankcell, 1, oldcoord, &result);
        WriteConsoleOutputA(hStdOut, &ci, cellsize, origin, &region);

        // Increment the coordinate for next placement of the block.
        oldcoord.X = region.Left;
        oldcoord.Y = region.Top;
        region.Left = region.Right += GetRandom(-1, 1);
        region.Top = region.Bottom += GetRandom(-1, 1);

        // Correct placement (and beep) if about to go off the screen.
        if (region.Left < csbi.srWindow.Left)
            region.Left = region.Right = csbi.srWindow.Left + 1;
        else if (region.Right >= csbi.srWindow.Right)
            region.Left = region.Right = csbi.srWindow.Right - 2;
        else if (region.Top < csbi.srWindow.Top)
            region.Top = region.Bottom = csbi.srWindow.Top + 1;
        else if (region.Bottom >= csbi.srWindow.Bottom)
            region.Top = region.Bottom = csbi.srWindow.Bottom - 2;

        // If not at a screen border, continue, otherwise beep.
        else
            continue;
        Beep((ci.Char.AsciiChar - 'A') * 100, 175);
    }
    // _endthread given to terminate
    _endthread();
}

Press any key to end the sample application.

The following sample code demonstrates how you can use the thread handle that's returned by _beginthreadex with the synchronization API WaitForSingleObject. The main thread waits for the second thread to terminate before it continues. When the second thread calls _endthreadex, it causes its thread object to go to the signaled state, which allows the primary thread to continue running. It can't be done with _beginthread and _endthread, because _endthread calls CloseHandle, which destroys the thread object before it can be set to the signaled state.

// crt_begthrdex.cpp
// compile with: /MT
#include <windows.h>
#include <stdio.h>
#include <process.h>

unsigned Counter;
unsigned __stdcall SecondThreadFunc( void* pArguments )
{
    printf( "In second thread...\n" );

    while ( Counter < 1000000 )
        Counter++;

    _endthreadex( 0 );
    return 0;
}

int main()
{
    HANDLE hThread;
    unsigned threadID;

    printf( "Creating second thread...\n" );

    // Create the second thread.
    hThread = (HANDLE)_beginthreadex( NULL, 0, &SecondThreadFunc, NULL, 0, &threadID );

    // Wait until second thread terminates. If you comment out the line
    // below, Counter will not be correct because the thread has not
    // terminated, and Counter most likely has not been incremented to
    // 1000000 yet.
    WaitForSingleObject( hThread, INFINITE );
    printf( "Counter should be 1000000; it is-> %d\n", Counter );
    // Destroy the thread object.
    CloseHandle( hThread );
}

Creating second thread...
In second thread...
Counter should be 1000000; it is-> 1000000

See also

• Process and environment control
• _endthread, _endthreadex
• abort
• exit, _Exit, _exit
• GetExitCodeThread