TLS callback minimal example

TlsCallback.c

//
// Merge .CRT section to .rdata (read only)
//

#pragma comment(linker, "/merge:.CRT=.rdata")

//
// Define variables marking the begin and the end
// of the TLS callback array.
//

#pragma section(".CRT$XLA", read)
__declspec(allocate(".CRT$XLA"))
const PIMAGE_TLS_CALLBACK __xl_a = NULL;

#pragma section(".CRT$XLZ", read)
__declspec(allocate(".CRT$XLZ"))
const PIMAGE_TLS_CALLBACK __xl_z = NULL;

//
// Custom-defined "extern" variable, which can be
// later assigned to the real array of TLS callbacks.
//

#pragma section(".CRT$XLM", read)
__declspec(allocate(".CRT$XLM"))
extern const PIMAGE_TLS_CALLBACK TlsCallbackArray;

//
// Strictly speaking, _tls_start, _tls_end and _tls_index
// should be located in the '.tls' section (read-write),
// while _tls_used should be located in the '.rdata' section
// (read-only) - as defined in the tlssup.cpp of the MSVC
// runtime.
//

char _tls_start = 0;
char _tls_end = 0;
unsigned int _tls_index = 0;

const IMAGE_TLS_DIRECTORY _tls_used = {
  (ULONG_PTR)&_tls_start,        // Start of tls data
  (ULONG_PTR)&_tls_end,          // End of tls data
  (ULONG_PTR)&_tls_index,        // Address of tls_index
  (ULONG_PTR)(&__xl_a + 1),      // Pointer to call back array
  (ULONG)0,                      // Size of tls zero fill
  (ULONG)0                       // Characteristics
};

//////////////////////////////////////////////////////////////////////////

VOID
NTAPI
TlsCallback(
  PVOID DllHandle,
  DWORD Reason,
  PVOID Reserved
  )
{
  __debugbreak();
}

const PIMAGE_TLS_CALLBACK TlsCallbackArray = { &TlsCallback };