Using Thread Local Storage in a Dynamic-Link Library
This section shows the use of a DLL entry-point function to set up a thread local storage (TLS) index to provide private storage for each thread of a multithreaded process.
The TLS index is stored in a global variable, making it available to all of the DLL functions. This example assumes that the DLL's global data is not shared, because the TLS index is not necessarily the same for each process that loads the DLL.
The entry-point function uses the TlsAlloc function to allocate a TLS index whenever a process loads the DLL. Each thread can then use this index to store a pointer to its own block of memory.
When the entry-point function is called with the DLL_PROCESS_ATTACH value, the code performs the following actions:
- Uses the TlsAlloc function to allocate a TLS index.
- Allocates a block of memory to be used exclusively by the initial thread of the process.
- Uses the TLS index in a call to the TlsSetValue function to store the address of the memory block in the TLS slot associated with the index.
Each time the process creates a new thread, the entry-point function is called with the DLL_THREAD_ATTACH value. The entry-point function then allocates a block of memory for the new thread and stores a pointer to it by using the TLS index.
When a function requires access to the data associated with a TLS index, specify the index in a call to the TlsGetValue function. This retrieves the contents of the TLS slot for the calling thread, which in this case is a pointer to the memory block for the data. When a process uses load-time linking with this DLL, the entry-point function is sufficient to manage the thread local storage. Problems can occur with a process that uses run-time linking because the entry-point function is not called for threads that exist before the LoadLibrary function is called, so TLS memory is not allocated for these threads. This example solves this problem by checking the value returned by the TlsGetValue function and allocating memory if the value indicates that the TLS slot for this thread is not set.
When each thread no longer needs to use a TLS index, it must free the memory whose pointer is stored in the TLS slot. When all threads have finished using a TLS index, use the TlsFree function to release the index.
When a thread terminates, the entry-point function is called with the DLL_THREAD_DETACH value and the memory for that thread is freed. When a process terminates, the entry-point function is called with the DLL_PROCESS_DETACH value and the memory referenced by the pointer in the TLS index is freed.
// The DLL code
#include <windows.h>
static DWORD dwTlsIndex; // address of shared memory
// DllMain() is the entry-point function for this DLL.
BOOL WINAPI DllMain(HINSTANCE hinstDLL, // DLL module handle
DWORD fdwReason, // reason called
LPVOID lpvReserved) // reserved
{
LPVOID lpvData;
BOOL fIgnore;
switch (fdwReason)
{
// The DLL is loading due to process
// initialization or a call to LoadLibrary.
case DLL_PROCESS_ATTACH:
// Allocate a TLS index.
if ((dwTlsIndex = TlsAlloc()) == TLS_OUT_OF_INDEXES)
return FALSE;
// No break: Initialize the index for first thread.
// The attached process creates a new thread.
case DLL_THREAD_ATTACH:
// Initialize the TLS index for this thread.
lpvData = (LPVOID) LocalAlloc(LPTR, 256);
if (lpvData != NULL)
fIgnore = TlsSetValue(dwTlsIndex, lpvData);
break;
// The thread of the attached process terminates.
case DLL_THREAD_DETACH:
// Release the allocated memory for this thread.
lpvData = TlsGetValue(dwTlsIndex);
if (lpvData != NULL)
LocalFree((HLOCAL) lpvData);
break;
// DLL unload due to process termination or FreeLibrary.
case DLL_PROCESS_DETACH:
// Release the allocated memory for this thread.
lpvData = TlsGetValue(dwTlsIndex);
if (lpvData != NULL)
LocalFree((HLOCAL) lpvData);
// Release the TLS index.
TlsFree(dwTlsIndex);
break;
default:
break;
}
return TRUE;
UNREFERENCED_PARAMETER(hinstDLL);
UNREFERENCED_PARAMETER(lpvReserved);
}
// The export mechanism used here is the __declspec(export)
// method supported by Microsoft Visual Studio, but any
// other export method supported by your development
// environment may be substituted.
#ifdef __cplusplus // If used by C++ code,
extern "C" { // we need to export the C interface
#endif
__declspec(dllexport)
BOOL WINAPI StoreData(DWORD dw)
{
LPVOID lpvData;
DWORD * pData; // The stored memory pointer
lpvData = TlsGetValue(dwTlsIndex);
if (lpvData == NULL)
{
lpvData = (LPVOID) LocalAlloc(LPTR, 256);
if (lpvData == NULL)
return FALSE;
if (!TlsSetValue(dwTlsIndex, lpvData))
return FALSE;
}
pData = (DWORD *) lpvData; // Cast to my data type.
// In this example, it is only a pointer to a DWORD
// but it can be a structure pointer to contain more complicated data.
(*pData) = dw;
return TRUE;
}
__declspec(dllexport)
BOOL WINAPI GetData(DWORD *pdw)
{
LPVOID lpvData;
DWORD * pData; // The stored memory pointer
lpvData = TlsGetValue(dwTlsIndex);
if (lpvData == NULL)
return FALSE;
pData = (DWORD *) lpvData;
(*pdw) = (*pData);
return TRUE;
}
#ifdef __cplusplus
}
#endif
The following code demonstrates the use of the DLL functions defined in the previous example.
#include <windows.h>
#include <stdio.h>
#define THREADCOUNT 4
#define DLL_NAME TEXT("testdll")
VOID ErrorExit(LPSTR);
extern "C" BOOL WINAPI StoreData(DWORD dw);
extern "C" BOOL WINAPI GetData(DWORD *pdw);
DWORD WINAPI ThreadFunc(VOID)
{
int i;
if(!StoreData(GetCurrentThreadId()))
ErrorExit("StoreData error");
for(i=0; i<THREADCOUNT; i++)
{
DWORD dwOut;
if(!GetData(&dwOut))
ErrorExit("GetData error");
if( dwOut != GetCurrentThreadId())
printf("thread %d: data is incorrect (%d)\n", GetCurrentThreadId(), dwOut);
else printf("thread %d: data is correct\n", GetCurrentThreadId());
Sleep(0);
}
return 0;
}
int main(VOID)
{
DWORD IDThread;
HANDLE hThread[THREADCOUNT];
int i;
HMODULE hm;
// Load the DLL
hm = LoadLibrary(DLL_NAME);
if(!hm)
{
ErrorExit("DLL failed to load");
}
// Create multiple threads.
for (i = 0; i < THREADCOUNT; i++)
{
hThread[i] = CreateThread(NULL, // default security attributes
0, // use default stack size
(LPTHREAD_START_ROUTINE) ThreadFunc, // thread function
NULL, // no thread function argument
0, // use default creation flags
&IDThread); // returns thread identifier
// Check the return value for success.
if (hThread[i] == NULL)
ErrorExit("CreateThread error\n");
}
WaitForMultipleObjects(THREADCOUNT, hThread, TRUE, INFINITE);
FreeLibrary(hm);
return 0;
}
VOID ErrorExit (LPSTR lpszMessage)
{
fprintf(stderr, "%s\n", lpszMessage);
ExitProcess(0);
}
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