Weird behaviour of File.open

Questions / Help
1

Can’t figure out what’s happening here

 Erlang/OTP 27 [erts-15.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] 

Elixir 1.18.0 (compiled with Erlang/OTP 27)

iex(1)> File.open("/tmp/x.png", [:read, :binary], fn f -> IO.read(f, 1000) end)
{:ok,
 <<194, 137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82, 0, 0, 7,
   39, 0, 0, 2, 195, 164, 8, 6, 0, 0, 0, 194, 165, 123, 57, 194, 133, 0, 0, 1,
   90, 105, 67, 67, 80, 73, 67, 67, ...>>}
iex(2)> File.read("/tmp/x.png")
{:ok,
 <<137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82, 0, 0, 7, 39, 0,
   0, 2, 228, 8, 6, 0, 0, 0, 165, 123, 57, 133, 0, 0, 1, 90, 105, 67, 67, 80,
   73, 67, 67, 32, 80, 114, 111, ...>>}
iex(3)>

PNG file starts with bytes 137, 80 and so on, but IO.read inside File.open inserts extra byte at the beginning

2

Looks like any file containing bytes with MSB set gets screwed up

3

Use IO.binread - IO.read is specified to return a valid UTF-8 string

2 Likes
4

Thanks, that helps!