XmTextFieldSetSelection man page
XmTextFieldSetSelection — A TextField function that sets the primary selection of the text
#include <Xm/TextF.h> void XmTextFieldSetSelection( Widget widget, XmTextPosition first, XmTextPosition last, Time time);
XmTextFieldSetSelection sets the primary selection of the text in the widget. It also sets the insertion cursor position to the last position of the selection and calls the widget's XmNmotionVerifyCallback callbacks. XmTextFieldSetSelection always generates an XmNgainPrimaryCallback unless it fails to take ownership of the primary text selection.
Specifies the TextField widget ID
Marks the first character position of the text to be selected
Marks the last position of the text to be selected
Specifies the time at which the selection value is desired. This should be the same as the time of the event that triggered this request. One source of a valid time stamp is the function XtLastTimestampProcessed.
For a complete definition of TextField and its associated resources, see XmTextField(3).