This function returns the handle of the window corresponding to a particular device context.

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

HDC     hdc;   /*  Device-context handle. */
HWND    hwnd;  /*  Window handle. */

hwnd = WinWindowFromDC(hdc);