Tag: TLS

Posted on

Using C/C++ TLS callbacks in Visual Studio with your 32 or 64bits programs

In the following article, I share with you how to use TLS callbacks in your C/C++ program compiled with Visual Studio.

Background

TLS (thread local storage) callbacks are a mechanism provided by the Windows loader to give your program a chance to do initialization/deinitialization tasks when the process starts, terminates, a thread is created or terminated.

A TLS callback has the following prototype:

Plain text
Copy to clipboard
Open code in new window
EnlighterJS 3 Syntax Highlighter
typedef VOID (NTAPI *PIMAGE_TLS_CALLBACK) (
PVOID DllHandle,
DWORD Reason,
PVOID Reserved);
typedef VOID (NTAPI *PIMAGE_TLS_CALLBACK) ( PVOID DllHandle, DWORD Reason, PVOID Reserved);
typedef VOID (NTAPI *PIMAGE_TLS_CALLBACK) (
    PVOID DllHandle,
    DWORD Reason,
    PVOID Reserved);

The Reason argument can be any of the following constants:

  • DLL_PROCESS_ATTACH   = 1
  • DLL_PROCESS_DETACH  = 0
  • DLL_THREAD_ATTACH  = 2
  • DLL_THREAD_DETACH  = 3

The TLS callbacks are encoded inside the compiled program’s TLS data directory(IMAGE_DIRECTORY_ENTRY_TLS). Please refer to the PE file structure. Continue reading “Using C/C++ TLS callbacks in Visual Studio with your 32 or 64bits programs”