为什么在使用println后，在clojure中使用'print'打印的字符串只出现在我的控制台中？

Question

I have the following code in clojure:

(do
    (println "starting....")
    (sig! a 0)
    (sig! b 0)
    (future
      (Thread/sleep 4000)
      (println "switch 1")
      (sig! a 1)
      (sig! b 0)
      (Thread/sleep 4000)
      (println "switch 2")
      (sig! a 0)
      (sig! b 1)
      (Thread/sleep 4000)
      (println "switch 3")
      (sig! a 1)
      (sig! b 1)) nil))

I have some watchers that listen to the atoms a, b, s, and c1 and print out some strings when they change. Using (print (str uid ":" @o "\\n") or the println version. The clojure docs say print and println should behave exactly the same way, with the exception of the newline in println , however I'm not seeing this. When the watcher uses println I see the strings right away. When the watcher uses print . I only see the output when the next println is called. Below I have the output with println vs with print . With print I only see the values up to the last println .

starting....
switch 1
a:1
s:1
switch 2
a:0
b:1
s:0
s:1
switch 3 

Now with println

starting....
switch 1
a:1

s:1

switch 2
a:0
b:1


s:0

s:1

switch 3
a:1

s:0

c1:1

As you can see the changes picked up by the watchers are printed in this case. I'm using clojure 1.5. It almost seems like a print requires a println to flush its contents to console.

Answer 1

After looking more closely at the Clojure source code for println and print . The difference is that println calls prn and print calls pr . In the prn the function newline is called. It then checks for flush-on-new-line and if it is true it explicitly calls flush . Therefore calling print with any number of newlines ("\\n") in the string will not flush to the output stream, you must either call newline , or call flush directly.

Answer 2

Output doesn't actually go to console until it is flushed. Generally, printing a newline character is the default signal for the console to flush the output. You can generally force this in most languages with a call to a flush function, in most IO libraries.

Answer 3

According to the Clojure documentation , output is flushed by default on every newline. So you'll probably have to explicitly flush the output to get what you want.