Reading more bytes than a file contains yields EOF instead of data

[jdm]

The most recent Rust upgrade for Servo has uncovered a problem with the code at larsbergstrom/servo@3445fc9#diff-b8f19d42333587901f805bdc9be6df88R16 . A call to reader.read_bytes(1024) on a file that is <1024 bytes long does not yield any data, and instead returns an immediate EOF error result.

[alexcrichton]

Hm, it may be something else going on, this appears to work for me:

$ cat foo.rs
use std::io::File;

fn main() {
    let mut buf = [0, .. 2048];
    let mut f = File::open(&Path::new("foo.rs"));
    println!("{}", f.read(buf));
}
$ rustc foo.rs
$ ./foo
Ok(149)

[zkamsler]

This appears to the documented behavior of read_bytes (not be be confused with read). It basically reads until it has read the specified number of bytes or there is an error.

[jdm]

Wow. That seems like a really niche use case in which it's acceptable to potentially lose bytes.

[alexcrichton]

Ah sorry, yes, I see what's going on now. read_bytes is for reading an exact number of bytes, not for generally "read some bytes".

Perhaps is should be renamed to read_exact, it may have made more sense when conditions existed.

[jdm]

I'll note that using push_bytes doesn't seem to be much of an improvement if you're trying to read a file in chunks; I'm seeing similar behaviour with it.

[zkamsler]

push_bytes will also bubble up an EOF if there are less than the amount requested, but the bytes that were read will still have been pushed to the buffer. All read_bytes does is call push_bytes on a new ~[u8].

See source:
https://github.com/mozilla/rust/blob/b4d324334cb48198c27d782002d75eba14a6abde/src/libstd/io/mod.rs#L380

But it definitely is not that great if you don't care about reading the exact number of bytes.

[sfackler]

👍 to read_exact.