Using Thread Local Storage

Thread local storage (TLS) enables multiple threads of the same process to use an index allocated by the TlsAlloc function to store and retrieve a value that is local to the thread. In this example, an index is allocated when the process starts. When each thread starts, it allocates a block of dynamic memory and stores a pointer to this memory in the TLS slot using the TlsSetValue function. The CommonFunc function uses the TlsGetValue function to access the data associated with the index that is local to the calling thread. Before each thread terminates, it releases its dynamic memory. Before the process terminates, it calls TlsFree to release the index.

#include <windows.h> 
#include <stdio.h> 
#define THREADCOUNT 4 
DWORD dwTlsIndex; 
VOID ErrorExit (LPCSTR message);
VOID CommonFunc(VOID) 
   LPVOID lpvData; 
// Retrieve a data pointer for the current thread. 
   lpvData = TlsGetValue(dwTlsIndex); 
   if ((lpvData == 0) && (GetLastError() != ERROR_SUCCESS)) 
      ErrorExit("TlsGetValue error"); 
// Use the data stored for the current thread. 
   printf("common: thread %d: lpvData=%lx\n", 
      GetCurrentThreadId(), lpvData); 
   LPVOID lpvData; 
// Initialize the TLS index for this thread. 
   lpvData = (LPVOID) LocalAlloc(LPTR, 256); 
   if (! TlsSetValue(dwTlsIndex, lpvData)) 
      ErrorExit("TlsSetValue error"); 
   printf("thread %d: lpvData=%lx\n", GetCurrentThreadId(), lpvData); 
// Release the dynamic memory before the thread returns. 
   lpvData = TlsGetValue(dwTlsIndex); 
   if (lpvData != 0) 
      LocalFree((HLOCAL) lpvData); 
   return 0; 
int main(VOID) 
   DWORD IDThread; 
   int i; 
// Allocate a TLS index. 
   if ((dwTlsIndex = TlsAlloc()) == TLS_OUT_OF_INDEXES) 
      ErrorExit("TlsAlloc failed"); 
// 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"); 
   for (i = 0; i < THREADCOUNT; i++) 
      WaitForSingleObject(hThread[i], INFINITE); 

   return 0; 
VOID ErrorExit (LPCSTR message)
   fprintf(stderr, "%s\n", message); 

Using Thread Local Storage in a Dynamic-Link Library