Strange behaviour of std::cout in Linux

Question

I am trying to print results in 2 nested for cycles using std::cout. However, the results are not printed to the console immediately, but with delay (a

Answer 1

std::cout is an stream, and it is buffered. You can flush it by several ways:

std::cout.flush();

std::cout << std::flush;

std::cout << std::endl;  // same as:  std::cout << "\n" << std::flush`

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johny:

I am flushing the buffer before the cycle using std::endl. The problem arises when printing dot representing % of processed data inside the cycle.

If you flush the buffer before the cycle, that does not affect the output in the cycle. You have to flush in or after the cycle, to see the output.

Answer 2

If you don't flush your output, your output is not guaranteed to be visible outside your program. The fact that it is not printing in your terminal is just a consequence of the default behavior in linux to do line buffering when the output is a tty. If you run your program on linux with its output piped to another thing, like

 ./your_program | cat

Then the default buffer will be MUCH larger, most likely it will be at least 4096 bytes. So nothing will get displayed until the big buffer gets full. but really, the behaviour is OS specific unless you flush std::cout yourself.

To flush std::cout, use :

std::cout << std::flush;

also, using

std::cout << std::endl;

is a shortcut to

std::cout << '\n' << std::flush;