This example uses WinQueryLboxCount to find the number of list box items and selects them all.

#define INCL_WINLISTBOXES
#define INCL_WINWINDOWMGR
#include <OS2.H>

LONG cWindows;
HWND hwndWindowLB;

cWindows = WinQueryLboxCount(hwndWindowLB);

/* Loop through all windows, selecting them all */
while (cWindows)
{
   WinSendMsg(hwndWindowLB,
              LM_SELECTITEM,
              (MPARAM)--cWindows,
              (MPARAM)TRUE);
}