Windows: Buffer too small

#include <stdio.h>

#include <windows.h>

HINSTANCE hInst = 0;

char szAppName[] = "Basis";

char szTitle[100] = "Willys Titelleiste";

BOOL InitApplication ();

BOOL InitInstance (int);

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM);

void onPaint (HWND);

int WINAPI WinMain (  HINSTANCE hInstance,

            HINSTANCE hPrevInstance,

            LPSTR    lpCmdLine,

            int       nCmdShow)

{

  MSG msg;

  hInst = hInstance;

  if (!InitApplication () ) return FALSE;

  if (!InitInstance (nCmdShow) ) return FALSE;

  while (GetMessage (&msg, NULL, 0, 0))

  {

    TranslateMessage (&msg);

    DispatchMessage (&msg);

  }

  return (msg.wParam);

}

BOOL InitApplication ()

{

  WNDCLASSEX wc;

  wc.cbSize = sizeof(WNDCLASSEX);

  wc.style = CS_HREDRAW | CS_VREDRAW;

  wc.lpfnWndProc = (WNDPROC)WndProc;

  wc.cbClsExtra = 0;

  wc.cbWndExtra = 0;

  wc.hInstance = hInst;

  wc.hIcon = LoadIcon ( NULL, IDI_QUESTION);

  wc.hCursor = LoadCursor ( NULL, IDC_WAIT);

  wc.hbrBackground = (HBRUSH)GetStockObject(GRAY_BRUSH);

  wc.lpszMenuName = NULL;

  wc.lpszClassName = szAppName;

  wc.hIconSm = LoadIcon (NULL, IDI_WINLOGO);

  return RegisterClassEx(&wc);

}

BOOL InitInstance (int nCmdShow)

{

  HWND hWnd;

  hWnd = CreateWindow(

      szAppName,

      szTitle,

      WS_OVERLAPPEDWINDOW,

      50,//CW_USEDEFAULT,

      100,

      500,

      250,

      NULL,

      NULL,

      hInst,

      NULL);

  if (!hWnd) return (FALSE);

  ShowWindow (hWnd, nCmdShow);

  UpdateWindow (hWnd);

  return (TRUE);

}

LRESULT CALLBACK WndProc (HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)

{

  switch (message)

  {

  case WM_PAINT:

    onPaint(hWnd);

    break;

  case WM_DESTROY:

    PostQuitMessage(0);

    break;

  default:

    return DefWindowProc (hWnd, message, wParam, lParam);

  }

  return 0;

}

void onPaint (HWND hWnd)

{

  PAINTSTRUCT ps;

  HDC hdc;

  TEXTMETRIC tm;

  char szZahl[10];

  int i = 0;

  int nXStart1, nXStart2, nYStart;

  int nAveCharWidth, nLineHeight;

  char* strMetrik[] = {  "Zeichenhoehe: ",

              "Hoehe oberhalb der Basislinie:",

              "Hoehe unterhalb der Basislinie:",

              "Hoehe des Bereichs fuer Umlaute:",

              "Abstand zwischen den Zeilen:",

              "Mittlere Zeichenbreite:",

              "Maximale Zeichenbreite:"

            };

  hdc = BeginPaint (hWnd, &ps);

  GetTextMetrics(hdc, &tm);

  nLineHeight = (int)(tm.tmHeight + tm.tmExternalLeading)+2;

  nAveCharWidth = (int)tm.tmAveCharWidth;

  nXStart1 = 5*nAveCharWidth;

  nXStart2 = nXStart1 + 40*nAveCharWidth;

  nYStart = 1*nLineHeight;

  for (i=0; i < 7; i++)

  {

    TextOut (hdc, nXStart1, nYStart, strMetrik[i], strlen(strMetrik[i]) );

    switch(i)

    {

    case 0: sprintf_s(szZahl, "%31d", tm.tmHeight);

        break;

    case 1: sprintf_s(szZahl, "%31d", tm.tmAscent);

        break;

    case 2: sprintf_s(szZahl, "%31d", tm.tmDescent);

        break;

    case 3: sprintf_s(szZahl, "%31d", tm.tmInternalLeading);

        break;

    case 4: sprintf_s(szZahl, "%31d", tm.tmExternalLeading);

        break;

    case 5: sprintf_s(szZahl, "%31d", tm.tmAveCharWidth);

        break;

    case 6: sprintf_s(szZahl, "%31d", tm.tmMaxCharWidth);

        break;

    };

    TextOut(hdc, nXStart2, nYStart, szZahl, strlen(szZahl) );

    nYStart += nLineHeight;

  }

  EndPaint (hWnd, &ps);

}