Random print program, prints multiple outputs at once despite sleep() function? [duplicate]

Question

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std::cout won't print (4 answers)

Closed 8 years ago.

So, I have the program below. What it does is, it prints random numbers while formating them with a width of 10 on the console.

Now, when I added the sleep function, I expected it to print one number every 10 milliseconds (or more), but what it does is, it prints 100 outputs every 100*10 milliseconds. I was wondering, why does this happen? Does the output get buffered or what?

#include <unistd.h>
#include <iostream>
#include <cstdlib>
#include <time.h>
#include <iomanip>

int main()
{
    srand(time(0));
    for (int i = 1; i < 1000000; i++)
    {
        usleep(10*1000);    // No matter the input, the same thing happens
        cout << setw(10) << rand()%(90*i) + 10*i;
    }
}

I tried this both on windows and unix and it's the exact same thing.

Answer 1

Yes, the output does get buffered.

Use std::cout.flush(); to flush manually.

As a note, std::endl (appends a new line and) does the flush.