tpcc of sqlite does not work with multithreaded mode

[lijianan98]

Hi, I ported your implementation of tpcc benchmark for sqlite.
Firstly I tested the benchmark by loading with warehouse number = 4 as below:
./build-release/tpcc_load -w 4 -d tpcc.db
The code compiles and runs with no problems when I set database connection to be 1 like below:
./tpcc_start -w 4 -c 1 -t 2000 -d tpcc.db.

However, when I set the database connection to be 2 like below:
./tpcc_start -w 4 -c 2 -t 2000 -d tpcc.db.
There will occur constraint error:

CHECKING IF SQLITE IS THREADSAFE: RETURN VALUE = 1
***************************************
*** ###easy### TPC-C Load Generator ***
***************************************
option w with value '4'
option c with value '2'
option t (number of transactions) with value '2000'
option d with value 'tpcc.db'
<Parameters>
  [warehouse]: 4
 [connection]: 2
     [rampup]: 10 (sec.)
    [measure]: 20 (sec.)
RAMP-UP TIME.(10 sec.)
thread_main: opening db, thread id = 140629658339072
thread_main: opening db, thread id = 140629649946368
thread_main: opened db, thread id = 140629658339072
thread_main: opened db, thread id = 140629649946368
neword 0:4
neword: error: UNIQUE constraint failed: orders.o_id, orders.o_d_id, orders.o_w_id
neword 0:4
neword: error: UNIQUE constraint failed: orders.o_id, orders.o_d_id, orders.o_w_id
neword 0:4
neword: error: UNIQUE constraint failed: orders.o_id, orders.o_d_id, orders.o_w_id
....
error at thread_main
thread_main: error: cannot commit - no transaction is active

Thank you so much and I really appreciate any idea about possible reasons of what is happening