FORM build_fieldcat CHANGING pt_fieldcat TYPE slis_t_fieldcat_alv.
DATA:
ls_fieldcat TYPE slis_fieldcat_alv.
...
CLEAR ls_fieldcat.
ls_fieldcat-fieldname = 'MATNR'.
ls_fieldcat-ref_tabname = 'VBAP'.
ls_fieldcat-ref_fieldname = 'MATNR'.
ls_fieldcat-outputlen = 30.
APPEND ls_fieldcat TO pt_fieldcat.
...