This call queries the pointer-conversion procedure associated with a class.

#define INCL_WINTHUNKAPI /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

PSZ    pszClassName;  /*  Window-class name. */
PFN    thunkpr;       /*  Pointer-conversion procedure identifier. */

thunkpr = WinQueryClassThunkProc(pszClassName);