I use sleep function in ubuntu ,but printf function run very slowly in while loop. why?

Question

I use ubuntu 14.04 version system for below code. I use below code : (below code is infinite Loop)

#include <stdio.h>
#include <unistd.h>

int flag=0;

int main(void){

    printf("program start.\n");   
    printf("PID=%d\n",getpid());    
    printf("flag=%d\n",flag);
    //-----------I feel weired below do...while... sentences----------//
    do{
        printf("loop_");
        sleep(1);
       }while(flag==0);

    printf("program exit.\n");    
    return 0;
}

in the beginning below print result :

root@ubuntu:~/Desktop/my_test_code# ./issue 
program start.
PID=3113
flag=0
...........//start waiting here,and don't print "loop_"

then after I waited for long time this program prints a lot of "loop_". I find it very strange, and should print a string "loop_" and then, wait a second, then print a "loop_" again, and so on, why me waiting for a long time, do it start to print a lot of "loop_" ?

anybody has any idea for my issue . thank you in advance.

`printf` is line buffered. It will not flush to stdout until a newline is encountered. So either put a `"\n"` at the end of your print string or explicitly `fflush`. — kaylum, Dec 09 '15 at 09:44
Possible duplicate of [When does printf fail to print?](http://stackoverflow.com/questions/9317529/when-does-printf-fail-to-print) — n. m. could be an AI, Dec 09 '15 at 09:56

score 5 · Answer 1 · answered Dec 09 '15 at 09:43

5

printf bufferizes its output (until a given size or \n), so you don't see the output until it flushes it.

changing your loop to

printf("loop_");
fflush(stdout);
sleep(1);

should solve your issue.

answered Dec 09 '15 at 09:43

Jarod42

203,559
14
181
302

score 0 · Accepted Answer · answered Dec 09 '15 at 12:51

Root cause:

Usually stdout's buffer are getting flushed by \n so here loop goes on and stdout did not get chance to buffered its data properly.

Solution:

There are three way here

1) Use \n at the end of printf

printf("loop_\n");

2) call fflush(stdout): after printf in loop.

3) Disabled the buffering for stdout stream using below call in your program

setvbuf(stdout, NULL, _IONBF, 0);

I use sleep function in ubuntu ,but printf function run very slowly in while loop. why?

2 Answers2