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関数とIO#eof?メソッド ——過去にEOFに出会ったのか、それとも今現在EOFなのか * C言語とPascalにおけるファイルの終端 * ユーザにとってわかりやすいファイルの終端 * まとめ
1.04 EOFフラグの除去 ——モードで挙動が変化するのは良くない * stdioのEOFフラグ * RubyにおけるEOFフラグ * EOFフラグの再実装の試み * まとめ
---------------------------------------- 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/