[Discuss-gnuradio] segmentation fault when using a new added block

[intermilan]

hi all:
    I want to use the gr-howto-write-a-block-3.3.0 (my version of the Gnuradio is 3.3.0)to add a new package
or a new block to the gnuradio and grc. At first I did not change anything in the gr-howto-write-a-block-3.3.0,
so the name of the new package should be howto,and the name of the block should be howto.square_ff() and
howto.square2_ff().I run the follow command as follow:
    sudo ./bootstrap
    sudo ./configure --prefix=/usr/local
    sudo make
    sudo make check
    sudo make install
 After all commands done,everything seems fine (no error appear after make check)and the package looks like  been installed.
Then I write a small python program to test the new block.Here are the codes:
#! /usr/bin/env python
from gnuradio import gr
import howto
class example_block(gr.top_block):
    def __init__(self):
        gr.top_block.__init__(self)
        src = "">                                 True)
        thr = gr.throttle(gr.sizeof_float,
                          32000)
        sqr =howto.square_ff()
        dst = gr.file_sink(gr.sizeof_float,
                           "/home/tianxia515/result.dat")

        self.connect(src,thr)
        self.connect(thr,sqr)
        self.connect(sqr,dst)

if __name__ == '__main__':
    tb = example_block()
    tb.run()

But when I run the program in the shell, there is error "segmentation fault".I do not know why does this error happen.
I did not change anything in the gr-howto-write-a-block-3.3.0. I hope someone can help me to fix this problem and
tell me why this error happen.
Thanks in advance.

inter.