Bug #13965

Surprising behavior when using

Added by davemyron (Dave Myron) about 3 years ago. Updated about 3 years ago.

Target version:
ruby -v:


I was trying to write the contents of an array to a Tempfile using the #open() + block syntax: { |f| my_array.each { |item| f.puts item }}

However, I was finding that only most of the items were being written to the file which, in my case, was Very Bad.

Witness (with Ruby 2.4.1p111; same behavior in 2.4.3):

>> { |f| (1..10000).each { |i| f.puts i }; `tail -n1 #{f.path}` }
=> "8416"
>> { |f| (1..100000).each { |i| f.puts i }; `tail -n1 #{f.path}` }
=> "98835"
>> { |f| f.puts "Test\n"; `tail -n1 #{f.path}` }
=> ""

All very surprising results! When using block syntax, I expect to be able to interact with the Tempfile inside the block and then forget about it.

Storing the path of the Tempfile and accessing it after the block is closed produces the expected results:

>> path = ""; { |f| path = f.path; f.puts "Test\n" }; `tail -n1 #{path}`
=> "Test\n"
>> path = ""; { |f| path = f.path; (1..10000).each { |i| f.puts i }}; `tail -n1 #{path}`
=> "10000\n"

I suspect that it's due to some buffering. Nonetheless, it's unexpected to have to store the path to the Tempfile when using the block syntax and to interact with after closing the block.

Updated by davemyron (Dave Myron) about 3 years ago

This appears to be the behavior of + block, as well:

>>"/tmp/test.txt","w+"){|f| (1..10000).each { |i| f.puts i }; `tail -n1 #{f.path}` }
=> "8416"

Updated by davemyron (Dave Myron) about 3 years ago

To workaround the unexpected behavior, either use IO#sync = true or IO#flush after writing to the file.

Updated by nobu (Nobuyoshi Nakada) about 3 years ago

  • Status changed from Open to Rejected

Yes, it's a nature of buffering.

Updated by davemyron (Dave Myron) about 3 years ago

Could a small documentation blurb alleviate future headaches for other users?

If the docs had said something like "Due to buffering, the contents of a file may be incomplete until flushed" I would not have been (so) surprised.

Also available in: Atom PDF