Performance Difference Between C and C++ Style File IO

Question

I've always heard that C++ file I/O operations are much much slower than C style I/O. But I didn't find any practical references on comparatively how slow they actually are, so I decided to test it in my machine (Ubuntu 12.04, GCC 4.6.3, ext4 partition format).

First I wrote a ~900MB file in the disk.

C++ (ofstream): 163s

ofstream file("test.txt");
    
for(register int i = 0; i < 100000000; i++) 
    file << i << endl;

C (fprintf): 12s

FILE *fp = fopen("test.txt", "w");
    
for(register int i = 0; i < 100000000; i++) 
    fprintf(fp, "%d
", i);

I was expecting such output, it shows that writing to a file is much slower in C++ than in C. Then I read the same file using C and C++ I/O. What made me exclaimed that there is almost no difference in performance while reading from file.

C++ (ifstream): 12s

int n;
ifstream file("test.txt");

for(register int i = 0; i < 100000000; i++) 
    file >> n;

C (fscanf): 12s

FILE *fp = fopen("test.txt", "r");
    
for(register int i = 0; i < 100000000; i++) 
    fscanf(fp, "%d", &n);

So, why is taking so long to execute writing using stream? Or, why reading using stream is so fast compared to writing?

Conclusion: The culprit is the std::endl, as the answers and the comments have pointed out. Changing the line file << i << endl; to file << i << ' '; has reduced running time to 16s from 163s.

Answer 1

You're using endl to print a newline. That is the problem here, as it does more than just printing a newline — endl also flushes the buffer which is an expensive operation (if you do that in each iteration).

Use if you mean so:

file << i << '
';

And also, must compile your code in release mode (i.e turn on the optimizations).