As easy as my title and the rules of the C++ core guidelines sound, getting more performance out of the iostreams is no no-brainer.
Okay, let’s step back. Although I did a lot of tests, my numbers in this post are more controversial than I thought. Please let me know if you have any ideas, improvements, or clarifications, and I will add them to this post.
Here are the two performance-related rules from the guidelines to Iostreams.
- SL.io.10: Unless you use
printf-family functions call
- SL.io.50: Avoid
I assume you don’t know std::ios_base::sync_with_stdio?
Per default, operations on the C++ streams are synchronized with the C streams. This synchronization happens after each in- or output operation.
- C++ Streams: std::cin, std::cout, std::cerr, std::clog, std::wcin, std::wcout, std::wcerr, and std::wclog.
- C Streams: stdin, stdout, and stderr.
This allows it to mix C++ and C in- or output operations because operations on the C++ streams go unbuffered to the C streams. What is also important to note from the concurrency perspective: synchronized C++ streams are thread-safe. All threads can write to the C++ streams without any need for synchronization. The effect may be an interleaving of characters but not a data race.
When you set the std::ios_base::sync_with_stdio(false), the synchronization between C++ streams and C streams will not happen because the C++ stream may put its output into a buffer. Because of the buffering, the in- and output operation may become faster. You must invoke std::ios_base::sync_with_stdio(false) before any in- or output operation. If not, the behavior is implementation-defined.
I assume you noticed that I wrote quite often, maybe. That is for a reason.
Interleaving of C++ Streams and C Streams
First, I want to know what would happen when I execute the following program with various compilers.
I add a few pieces of information to get a better picture of my various compiler.
It seems that only the output on GCC is not synchronized. This observation does not hold for clang or cl.exe on Windows. A small performance test confirmed my first impression.
Performance with and without Synchronisation
Let me write a small program with and without synchronization to the console. Doing it without synchronization should be faster.
The program is relatively easy to explain. I open a file (line 1), read its entire content (line 2) into a string, and write its iterations-times to the console (line 3). This is done in the function writeToConsole(fileContent).
iterations are in my concrete case 10. In the end, I display the average time of the output operations (line 4).
The non-synchronized version of the program is quite similar to the synchronized version. Only the main function changed a bit.
I just added line (1) to the main program. Now, I hope for performance improvement.
I did my performance test with a small program and a bigger text file (600.000 characters). The bigger file gave me no new insight; therefore, I skipped it.
The results puzzled me because of Windows.
- With GCC, I had a performance improvement of about 70% in the non-synchronized variant.
- Neither Clang nor cl.exe showed any performance improvement. It seems that the non-synchronized in- and output operations are synchronized. My numbers proved my observation from the program syncWithStdio.cpp.
- Only for the record. Did you notice how slow the console on windows is?
Of course, I’m guilty. I almost always break the following rule.
Why should you avoid std::endl? Or, to say it differently: What is the difference between the manipulator std::endl and ‘\n’.
- std::endl: writes a new line and flushes the output buffer.
- ‘\n’: writes a new line.
Flushing the buffer is expensive and should, therefore, be avoided. If necessary, the buffer is automatically flushed. Honestly, I was curious to see the numbers. To make it extremely worse, here is my program, which puts a linebreak (line 3) after each character.
In the first case, I did it with std::endl (line 2); in the second case, I did it with ‘\n‘ (line 3). The program is quite similar to the previous one. The big difference is that I made 500 iterations (line 3). Why? I was astonished by the variations in the numbers. With a few iterations, I could not notice any difference. Sometimes, std::endl was two times faster than ‘\n’; sometimes, std::endl was four times slower. I got similar behavior with cl.exe or with GCC. I also did it with another GCC or cl.exe compiler. Honestly, this was not what I expected. When I did it with 500 iterations, I got the expected winner. ‘\n‘ seems to be 10% – 20% faster than std::endl. Once more, only 10% – 20% faster.
My Small Conclusion
I want to draw a small conclusion from my performance test.
- std::ios_base::sync_with_stdio(false) can make a big difference on your platform, but you lose your thread-safety guarantee.
- std::endl is not as bad as its reputation. I will not change my habit.
Only one rule exists for the sections regex, chrono, and the C standard library. You see, I have to improvise in my next post.
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