c++ io streams sync_with_stdio no difference

Question

For some reason I'm unable to get my output stream to run any faster with the line

std::ios_base::sync_with_stdio(false);

included at the beginning of my program. I'm testing this with these two programs:

#include <iostream>

int main() {
    for (int i = 0; i < 500000; i++)
        std::cout << "Hello World\n";
}

and

#include <iostream>

int main() {
    std::ios_base::sync_with_stdio(false);
    for (int i = 0; i < 500000; i++)
        std::cout << "Hello World\n";       
}

The run time for each program is as follows

first_test (synced)

real    0m1.095s
user    0m0.472s
sys     0m0.299s

second_test (with synced turned off)

real    0m1.091s
user    0m0.471s
sys     0m0.299s

I'm compiling with g++ -O3 main.cpp. I'm on a mac running 10.11.1.

Is there any way to get the output stream to perform faster by turning sync_with_stdio off?

Answer 1

Is there any way to get the output stream to perform faster by turning sync_with_stdio off?

No, it's not mandatory that code will run any faster. That option allows you to mix I/O to/from std::cout/std::cin (and friends) with I/O to/from stdout/stdin (and friends):

std::cout << "test";
printf("another test";

It is implementation defined whether I/O is buffered (and how) and it is implementation defined how to achieve this result. Consequence of this is that it is implementation defined whether this mechanism will slow down your code or not (and then if you will have any benefit turning it off).

You tried and in your case your compiler has a good standard library implementation without any visible price to keep this synchronization.

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That said also note that measurement of this type are hard to implement correctly. How did you get those values? Moreover same test should be repeated multiple times to calculate an average. Also note that buffering itself may bias this results. Please note that outputting to console will also flat your results. Final note: I'd also try with a higher value than 500000, timing around 1 ms may be not as accurate as you may wish.