From: "mame (Yusuke Endoh) via ruby-core" <ruby-core@...>
Date: 2025-08-05T05:17:24+00:00
Subject: [ruby-core:122914] [Ruby Bug#21530] Is IO#eof? supposed to always block and read?

Issue #21530 has been updated by mame (Yusuke Endoh).


The short answer is: Ruby handles EOF in the Pascal style, not the C style.

In C, the `FILE` structure has an EOF flag. When a `read(2)` syscall returns 0, the EOF flag in the FILE structure is set. In the example provided, if you forcefully interrupt the input for fgets by pressing ^D twice, the EOF flag is set, and a subsequent call to `feof` returns true.

On the other hand, in Pascal and Ruby, the IO object itself does not have an EOF flag. Therefore, even if `IO#gets` is forcefully interrupted with a double ^D, the IO object does not remember this state, and a subsequent call to IO#eof? will attempt to read again, thus blocking.

This is a trade-off, and neither approach is definitively "correct,", but Ruby's stateless approach has some advantages:

* Simple and robust: There is no hidden state in an IO, which is good itself. It avoids common C bugs related to incorrect `feof()` checks.
* Flexible: It works consistently for streams that can grow over time, like sockets or files being appended to (similar to tail -f).

What @nobu said is the second one. For example, you can continuously read from standard input or a growing file:

```
$ ruby -e 'p [1, $stdin.read]; p [2, $stdin.read]'
foo^D^D[1, "foo"]
bar^D^D[2, "bar"]
```

FYI, a more detailed answer is written in the Japanese book "API design case study" by @akr who designed Ruby's IO. You may want to read it :-)

https://gihyo.jp/book/2016/978-4-7741-7802-8

> 1.02 feof���������IO#eof?������������ ���������������EOF���������������������������������������������EOF���������
> * C���������Pascal���������������������������������
> * ������������������������������������������������������������
> * ���������

> 1.04 EOF������������������ ���������������������������������������������������������
> * stdio���EOF���������
> * Ruby������������EOF���������
> * EOF������������������������������
> * ���������


----------------------------------------
Bug #21530: Is IO#eof? supposed to always block and read?
https://bugs.ruby-lang.org/issues/21530#change-114218

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
I'm not sure whether or not this is expected behavior, but it seems like eof? blocks when called on $stdin.

For example:

```ruby
if (str = $stdin.gets)
  $stderr.puts "read #{str}"
end

if $stdin.eof? # this call waits for input
  $stderr.puts "stdin is eof"
end
```

I think this is kind of odd behavior because if you input a string but _do not_ input a newline, then hit ^D twice, `$stdin` should be at EOF, but `eof?` will block and wait for input.  If you hit ^D a third time, $stdin will be EOF, but if you input a different character it will not be EOF.

Compare this C program:

```c
#include <stdio.h>
#include <stdlib.h>

#define BUF_SIZE 4096

int main(int argc, char *argv[]) {
    char buf[BUF_SIZE];
    if (fgets(buf, BUF_SIZE, stdin)) {
        fprintf(stderr, "read %s\n", buf);
    }

    if (feof(stdin)) { // Does not block
        fprintf(stderr, "stdin is EOF\n");
    }
}
```

If you hit ^D twice with this C program, `feof` will return true for `stdin`.  I would have expected the Ruby program and the C program to behave similarly, but they don't.  Is this expected?  The documentation indeed says that `eof?` will read, but shouldn't the IO be at EOF after the second ^D?

Thank you.



-- 
https://bugs.ruby-lang.org/
______________________________________________
 ruby-core mailing list -- ruby-core@ml.ruby-lang.org
 To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
 ruby-core info -- https://ml.ruby-lang.org/mailman3/lists/ruby-core.ml.ruby-lang.org/