Why C++ output is too much slower than C?

Question

I am actually a fan of C++, but today I figured out very slow file output of my program. So, I designed an experiment to compare speed of C++ file output with C. Suppose we have this piece of code :

int Num = 20000000;
vector <int> v;
for ( int i = 0; i < Num; i++ )
{
    v.push_back(i);
}

Now I run two separate code, one in C++ :

int now = time(0);
cout << "start" << endl;
ofstream fout("c++.txt");
for(size_t i = 0; i < v.size(); ++i)
{
    fout<< v[i] << endl;
}
fout.close();
cout << time(0) - now << endl;

and one in C :

int now = time(0);
printf("start\n");
FILE *fp = fopen("c.txt", "w");
for(size_t i = 0; i < v.size(); ++i)
{
    fprintf(fp, "%d\n", v[i]);
}
fclose(fp);
printf("%ld\n", time(0) - now);

C++ program works surprisingly slower! On my system, C program runs in 3 seconds while C++ program takes about 50 seconds to run! Is there any reasonable explanation for this?

both are C++ if you are using vectors, you are just using the older C style libraries in C++ — Keith Nicholas, May 03 '13 at 03:57
If you really want to know use the -S option in the compiler to see the assembly file. — aaronman, May 03 '13 at 03:59
Related: http://stackoverflow.com/questions/4340396/does-the-c-standard-mandate-poor-performance-for-iostreams-or-am-i-just-deali, There are better ones, but I can't find them atm... — Mysticial, May 03 '13 at 03:59
Might make a good example question for teaching beginners that `endl` is evil (if there isn't one already). favourited. — Christian Rau, May 03 '13 at 08:19

score 27 · Accepted Answer · answered May 03 '13 at 04:00

27

It's likely because of how often you are flushing the stream to disk in the C++ code. Inserting endl into a stream inserts a new line and flushes the buffer, while fprintf doesn't cause a buffer flush.

So your C++ example performs 20,000,000 buffer flushes while your C example will only flush to disk when the file handles buffer is full.

answered May 03 '13 at 04:00

shf301

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I agree - this is exactly what the problem is. – rohitsan May 03 '13 at 04:03
Thank you for your response. So, How can I change the C++ code such that it does not perform that much buffer flushes? Of course, I need the endl. So what should I do? – Ehsan Haghshenas May 03 '13 at 04:08
4

@user2345493 You don't need `endl`, you can use `"\n"` instead. See http://stackoverflow.com/questions/213907/c-stdendl-vs-n – shf301 May 03 '13 at 04:09
1

Yes, you are completely right. I changed it and it is also fast. Thank you for your help ;) – Ehsan Haghshenas May 03 '13 at 04:11

Why C++ output is too much slower than C?

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