Query in alv

Hi all,

I have got a query in ALV. I want to build the field catalog . I am using the following statement:

CALL FUNCTION 'REUSE_ALV_FIELDCATALOG_MERGE'

EXPORTING

I_PROGRAM_NAME = SY-REPID

I_INTERNAL_TABNAME = 'ITAB_DATA1'

i_bypassing_buffer = 'X'

i_buffer_active = ' '

CHANGING

CT_FIELDCAT = I_FIELDCAT

EXCEPTIONS

INCONSISTENT_INTERFACE = 1

PROGRAM_ERROR = 2

OTHERS = 3.

The data declarations are as follows :

TYPE-POOLS : SLIS.

DATA :

ITAB_DATA! type standard table of ZDETAILS1.

DATA : I_FIELDCAT TYPE SLIS_T_FIELDCAT_ALV.

But the Changing parameter I_FIELDCAT is never getting filled up.

Please clarify me on how to populate the Changing parameter I_FIELDCAT.

Kindly reply as fast as possible.

Regards,

Vijay