[Discuss-gnuradio] what does noutput mean in a FFT block whic fft_len=2048?

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hi,

i want deal the data after FFT in my own block,i  copy a  fft_len*sizeof(gr_complex) length of data from in to a buffer,then  deal the data ,and  copy the dealed data from buffer to the out,

in a FFT blocks,if there is 

 size_t block_size=output_signature()->sizeof_stream_item(0);

when i use memcpy(out,in,block_size*noutput ); it works normally ,

but when i do it like this :memcpy(out,in, fft_len*sizeof(gr_complex) );,it does not work 

does the result of block_size*noutput  have any relations with fft_len*sizeof(gr_complex)

or some other suggestions

thanks,

BZS