Getting NOT NULL constraint from pg_attribute

[Wu Ivy]

Hi developers,

I’m currently building a Postgres C extension that fetch data from a Postgres 
table.
Since the table can be large, in order to prevent memory overrun, I use 
SPI_cursor_fetch to fetch chunks of data. The result rows are saved in 
SPITupleTable* SPI_tuptable and attributes are saved in SPI_tuptable->tupdesc. 
In order to process my data, I need to get information of column nullability 
(whether column has NOT NULL constrain). I can get this information by calling:

        TupleDesc tupdesc = SPI_tuptable->tupdesc;
        bool is_nullable = TupleDescAttr(tupdesc, column_num - 1) -> attnotnull;
However, the result (is_nullable) is always 0, meaning the column does not have 
NOT NULLl constraint, even for columns that do have the NOT NULL constraint.

Any idea of why is it happening? 
Thanks in advance!

Best,
Ivy