In Postgres C extension development, the argument of function is a tuple, how to iterate all its attributes

This is my .c file

Datum show_line(PG_FUNCTION_ARGS)
{
    /* Get Input*/
    HeapTupleHeader tup_hdr;
    TupleDesc tup_desc;
    Oid tupType;
    int32 tupTypmod;
    bool isNull;
    // int tuplen;
    // bool *nulls;
    int i;
    tup_hdr = PG_GETARG_HEAPTUPLEHEADER(0);
    elog(NOTICE, "Get tup_hdr");
    tupType = HeapTupleHeaderGetTypeId(tup_hdr);
    tupTypmod = HeapTupleHeaderGetTypMod(tup_hdr);
    tup_desc = lookup_rowtype_tupdesc(tupType, tupTypmod);
    // tup_desc = lookup_rowtype_tupdesc(
    //     HeapTupleHeaderGetTypeId(tup_hdr),
    //     HeapTupleHeaderGetTypMod(tup_hdr));

    Datum value = GetAttributeByNum(tup_hdr, 5, &isNull);
    if (isNull)
    {
        elog(NOTICE, "Get value is NULL");
    }
    else
    {
        elog(NOTICE, "Get value %f", DatumGetFloat8(value));
    }

    elog(NOTICE, "Get tup_desc");
    elog(NOTICE, "get tupType %d", tupType);
    elog(NOTICE, "get tupTypmod %d", tupTypmod);
    if (tup_desc == NULL)
    {
        elog(NOTICE, "tup_desc is NULL");
        PG_RETURN_INT32(0);
    }

    int natts = tup_desc->natts;
    elog(NOTICE, "Get natts %d", natts);

    for (i = 0; i natts; i++)
    {
        Form_pg_attribute attr;
        char *name;
        elog(NOTICE, "Get attr %d, Step 1", i);
        attr = TupleDescAttr(tup_desc, i);
        elog(NOTICE, "Get attr %d, Step 2", i);
        name = NameStr(attr->attname);
        elog(NOTICE, "Attribute %d: %s", i, name);
    }
    PG_RETURN_INT32(0);
}

this is SQL file

CREATE
OR REPLACE FUNCTION show_line(iris) RETURNS int AS 'MODULE_PATHNAME',
'show_line' LANGUAGE C STRICT;

I can successfully compile and install it in Postgres. But when execute it, the psql will crash

test_db=# SELECT
    show_line(iris) as target
from
    iris
limit
    4;
NOTICE:  Get tup_hdr
NOTICE:  Get value 0.000000
NOTICE:  Get tup_desc
NOTICE:  get tupType 24617
NOTICE:  get tupTypmod -1
SSL SYSCALL error: EOF detected
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.
!?>

It seems it crashed when run in

int natts = tup_desc->natts;

I follow the function in example composite_to_json in src/backend/utils/adt/json.c