Bug #10039: "a+" mode for File.open doesn’t work

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-linux]
• Backport: 2.0.0: DONE, 2.1: DONE

[Quintus (Marvin Gülker)]

Hi there,

According to the documentation the "a+" file open mode should open a file:

Read-write, starts at end of file if file exists

However, it appears that the "a+" mode behaves exactly as "r+", i.e. the file pointer is placed at the beginning of the file instead of the end.

This example demonstrates the problem:

(288) [20:37:02 quintus@hades] /tmp/foobar
% cat test.txt 
test
test2
(289) [20:37:07 quintus@hades] /tmp/foobar
% irb
irb(main):001:0> f = File.open("test.txt", "a+")
=> #<File:test.txt>
irb(main):002:0> f.pos
=> 0
irb(main):003:0> f.eof?
=> false
irb(main):004:0> f.read
=> "test\ntest2\n"
irb(main):005:0> f.pos
=> 10
irb(main):006:0> f.close
=> nil
irb(main):007:0> 

The file position pointer should have been placed at the end of the file (i.e. position 10), not at the beginning (which is what "r+" is for).

Vale,
Quintus

[jaredbeck (Jared Beck)]

Writing with a+ mode appears to work fine, but the pos cursor is not updated until the first IO operation.

echo -en "test1\ntest2\n" > test

irb
irb(main):001:0> f = File.open('test', 'a+')
=> #<File:test>
irb(main):002:0> f.pos
=> 0
irb(main):003:0> f.write "test3\n"
=> 6
irb(main):004:0> f.pos
=> 18
irb(main):005:0> f.close
=> nil

cat test
test1
test2
test3

Notice how the pos cursor is updated after the first write.

EDIT: If you trace the system calls (I used dtruss on my mac), you'll see a call to lseek (man 2 lseek) after the first pos, and before the write.

[nobu (Nobuyoshi Nakada)]

• Category set to doc

Append mode doesn't move the pointer at open.
It moves it to the end at each write(2).

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed
• % Done changed from 0 to 100

Applied in changeset r46876.

---

io.c: fix rdoc of append mode

• io.c (rb_io_initialize): [DOC] fix rdoc of append mode. it does
  not move the pointer at open. [ruby-core:63747] [Bug #10039]

[nobu (Nobuyoshi Nakada)]

• Backport changed from 2.0.0: UNKNOWN, 2.1: UNKNOWN to 2.0.0: REQUIRED, 2.1: REQUIRED

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.0.0: REQUIRED, 2.1: REQUIRED to 2.0.0: REQUIRED, 2.1: DONE

Backported into ruby_2_1 branch at r47377.

[usa (Usaku NAKAMURA)]

• Backport changed from 2.0.0: REQUIRED, 2.1: DONE to 2.0.0: DONE, 2.1: DONE

backported into ruby_2_0_0 at r47498.