How to do an lseek on an open file, and read N bytes?

Question

I'd like to use Elixir to read a binary file that has a fixed-length header, and data structures within that are of determinable length. These files can be huge, and I really don't want to be forced to read the entire thing to memory. Maybe my search-fu is lacking, but I didn't find anything like this in the docs or elsewhere.

Answer 1

From the File docs:

to read from a specific position in a file, use :file.pread/3:

File.write!("example.txt", "Eats, Shoots & Leaves")
file = File.open!("example.txt")
:file.pread(file, 15, 6)
#=> {:ok, "Leaves"}

Alternatively, if you need to keep track of the current position, use :file.position/2 and :file.read/2:

:file.position(file, 6)
#=> {:ok, 6}
:file.read(file, 6)
#=> {:ok, "Shoots"}
:file.position(file, {:cur, -12})
#=> {:ok, 0}
:file.read(file, 4)
#=> {:ok, "Eats"}