Automatic cout flushing

Question

Good day,

I wrote a Java program that starts multiple C++ written programs using the Process object and Runtime.exec() function calls. The C++ programs use cout and cin for their input and output. The Java program sends information and reads information from the C++ programs input stream and outputstream.

I then have a string buffer that builds what a typical interaction of the program would look like by appending to the string buffer the input and output of the C++ program. The problem is that all the input calls get appended and then all the output calls get posted. For example, and instance of the StringBuffer might be something like this...

2
3
Please enter two numbers to add. Your result is 5

when the program would look like this on a standard console

Please enter two numbers to add. 2
3
Your result is 5

The problem is that I am getting the order of the input and output all out of wack because unless the C++ program calls the cout.flush() function, the output does not get written before the input is given.

Is there a way to automatically flush the buffer so the C++ program does not have to worry about calling cout.flush()? Similiar to as if the C++ program was a standalone program interacting with the command console, the programmer doesn't always need the cout.flush(), the command console still outputs the data before the input.

Thank you,

If this is an interactive program then it's good to flush manually anyway. `std::cout << "Please enter two numbers to add. " << std::flush` — cdhowie, Jun 13 '12 at 17:21
If the Java is sending the data, I don't think this is theoretically solvable in the general case, Note that you can do the same thing from a batch file: `echo 2 & echo 3 & myprogram.exe <<<"2\n3` and it would look just the same. The problem here has nothing to do with flushing or buffering. — Mooing Duck, Mar 05 '15 at 22:31

score 6 · Answer 1 · edited Sep 22 '16 at 17:46

6

In case someone comes looking for a way to set cout to always flush. Which can be totally fair when doing some coredump investigation or the like.

Have a look to std::unitbuf.

std::cout << std::unitbuf;

At the beginning of the program.

It will flush at every insertion by default.

edited Sep 22 '16 at 17:46

Jarod42

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answered Mar 05 '15 at 22:20

Mario Corchero

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score 3 · Answer 2 · edited May 23 '17 at 12:32

I can't guarantee that it will fix all of your problems, but to automatically flush the stream when you're couting you can use endl

e.g.:

cout << "Please enter two numbers to add: " << endl;

using "\n" doesn't flush the stream, like if you were doing:

cout << "Please enter two numbers to add:\n";

Keep in mind that using endl can be (relatively) slow if you're doing a lot of outputting

See this question for more info

Automatic cout flushing

2 Answers2