Swift cannot output when using NSTimer

问题

I tried output to STDOUT every second by using NSTimer.

I wrote the following code and saved it as sample.swift.

#!/usr/bin/swift

import AppKit

class Foo : NSObject {
  override init() {
    super.init()
    NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "bar", userInfo: nil, repeats: true)
  }

  func bar() {
    print("buz")
  }
}

let h = Foo()

// Output successful
// while true {
//   print("Foo!")
// }

NSRunLoop.mainRunLoop().run()

Then, I executed the following command and can see buz.

okada@iyokan$ ./sample.swift
buz
buz
buz
buz

So, I executed the following command but I cannot see any buz.

okada@iyokan$ ./sample.swift > sample.log # wait 5 seconds
okada@iyokan$ cat sample.log
okada@iyokan$

By the way, I tried while loop version (activated comment out the above code) then I could get buz by two procedures.

Why I could not get any buz? Please teach me.

回答1:

That is a "normal" behaviour of the stdio library. A file descriptor can be "buffered", which means that the result of all printf() and other output operations goes to an internal buffer first. When that buffer reaches a certain size (e.g. 8KB), its contents is written to the actual file.

There are three modes: "unbuffered" (all output is written to the file immediately), "line buffered" (output is collected until a newline characters is printed), and "fully buffered".

According to Is stdout line buffered, unbuffered or indeterminate by default?:

Unix convention is that stdin and stdout are line-buffered when associated with a terminal, and fully-buffered (aka block-buffered) otherwise.

This explains why you see the output of each print operation immediately when the output goes to the terminal, but not when output is redirected to a file.

With