I have a c++ application, which contains large amount of std::cout. It runs on linux 2.6.x. I need to test the performance of the application, so i am thinking of redirecting the std::cout to /dev/null. In C, i could simply use dup2. Is there an equivalent in c++ to redirect std::ostream to a file or /dev/null?
3 Answers
The dup2 trick will still work in C++, since just like <stdio.h>, <iostream> is just a buffering layer atop the UNIX system calls.
You can also do this at the C++ level by disconnecting the buffer from std::cout:
std::cout.rdbuf( NULL ); Besides severing the relationship between std::cout and any actual output device, this will set the std::ios::badbit flag which will prevent any output conversions (e.g. numbers to text) from occuring. Performance should be much better than with the filesystem-level hack.
7 Comments
Tony Delroy
+1 for an informative post, but re "I need to test the performance of the application" / "Performance should be much better than with the filesystem-level hack." - it could be argued that avoiding conversions is no longer testing the application performance.
Potatoswatter
@TonyD Asking how to test performance is a loaded question. Performance is tested by carefully replicating real-world conditions, and nobody here can answer that. I'm assuming he wants to test how it would perform if it weren't for all the output. If that's not a practical use, then the question is pointless.
Jimm
This is really good answer. A quick Question, can you suggest some resource to learn more about iostreams?
Potatoswatter
@Jimm I don't personally use any resources besides the ISO standard document. But there's always cppreference.com and I have this in my bookmarks: horstmann.com/cpp/streams.txt
Tony Delroy
@Potatoswatter: in my experience, it's common to want to know the performance including the generation of output, but without the time taken by the I/O device to write. For example, to be able to say things like "on that spec CPU/ram etc. it'll take 10 second + however long your disk needs to write the output", or "generation of output took 12 seconds, but when writing that to disk it took 40 seconds overall - how about we get a SSD?".
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You can do the exact same thing in C++. Both C and C++ both rely on the underlying operating system for IO, and redirecting fd 1 will affect std::cout just like it affects stdout.
(of course for testing you can just run the command with > /dev/null on the command line...)
Comments
An alternative way would be to symlink your file to /dev/null.
% ln -s /dev/null core % ls -l core lrwx-xr-x 1 john users 9 Nov 18 12:26 core -> /dev/null To truly test your program speed however I would suggest to comment out the writes to your file and calculate the execution time difference, because writing to /dev/null might have different overhead than writing to a normal file.
dup2method from Potatoswatter works like a charm.