I have this code:
a = File.open("/dev/urandom")
b = a.read(1024)
a.close
puts b
I expected to get the first 1024 bytes from the /dev/urandom device\file, instead I got an Error which says read accepts only slice and not Integer.
So I tried to do it like that:
b = a.read(("a" * 1000).to_slice)
But then I got back "1000" in the output.
What is the right way to read x bytes from a file in Crystal ?
What you did is not very ideal, but it actually worked. IO#read(Slice(UInt8)) returns the number of bytes actually read, in case the file is smaller than what you requested or the data isn't available for some other reason. In other words, it's a partial read. So you get 1000 in b because the slice you passed was filled with 1000 bytes. There's IO#read_fully(Slice(UInt8)) which blocks until it fulfilled as much of the request as possible, but too can't guarantee it in any case.
A better approach looks like this:
File.open("/dev/urandom") do |io|
buffer = Bytes.new(1000) # Bytes is an alias for Slice(UInt8)
bytes_read = io.read(buffer)
# We truncate the slice to what we actually got back,
# /dev/urandom never blocks, so this isn't needed in this specific
# case, but good practice in general
buffer = buffer[0, bytes_read]
pp buffer
end
IO also provides various convenience functions for reading strings until specific tokens or up to a limit, in various encodings. Many types also implement the from_io interface, which allows you to easily read structured data.
