Thanks for getting back to me. I guess that makes sense. The only thing I was planning to use was the scipy.fromfile() function, in hopes that it would read out the binary data into a 1024-point vector, but I guess that was the wrong assumption.
Maybe the Head block is a good option - it looks like I should put that in between the complex_to_mag and the file_sink blocks, right? And there are two values, num_items and vec_length. By the documentation, it seems like num_items should be 1024 to match the complex_to_mag's vector length of 1024; however there is a mismatch there, as I get the error "Source IO size 4096 does not match sink IO size 4." I'm not sure what the problem is though.
And based on your description, vec_length should be 1 and the file sink's length should also be 1; those do not give any problem. Thanks for the help again.