性能差异C和C ++风格文件IO之间 [英] Performance Difference Between C and C++ Style File IO
问题描述
我总是听说C ++文件I / O操作是非常非常慢，则C风格的I / O。但我没有找到比较他们实际上有多慢是任何实际的参考，所以我决定测试它在我的机器（Ubuntu的12.04，GCC 4.6.3，EXT4分区格式）。
I've always heard that C++ file I/O operations are much much slower then 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).
首先，我在磁盘写了〜900MB的文件。
First I wrote a ~900MB file in the disk.
C ++（的ofstream ）：器163s
C++ (ofstream): 163s
ofstream file("test.txt");
for(register int i = 0; i < 100000000; i++)
file << i << endl;
C（ fprintf中）：12S
FILE *fp = fopen("test.txt", "w");
for(register int i = 0; i < 100000000; i++)
fprintf(fp, "%d\n", i);
我期待这样的输出，它表明写入文件位于C慢得多++的C.然后我读使用C和C ++的I / O相同的文件。是什么让我惊呼在性能几乎没有区别，而从文件中读取。
I was expecting such output, it shows that writing to a file is much slower in C++ the 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的）：12S
C++ (ifstream): 12s
int n;
ifstream file("test.txt");
for(register int i = 0; i < 100000000; i++)
file >> n;
C（的fscanf ）：12S
FILE *fp = fopen("test.txt", "r");
for(register int i = 0; i < 100000000; i++)
fscanf(fp, "%d", &n);
那么，为什么要花这么长时间使用流来执行的写作？或者说，为什么使用流读取是如此之快比写作？
So, why is taking so long to execute writing using stream? Or, why reading using stream is so fast compared to writing?
结论：的罪魁祸首是的std :: ENDL ，因为答案和评论人士指出。转产文件＆LT;＆LT; I＆LT;＆LT; ENDL;
至文件＆LT;＆LT; I＆LT;＆LT;的'\\ n'; 已从器163s缩短运行时间为16S
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 << '\n'; has reduced running time to 16s from 163s.
推荐答案
您正在使用 ENDL 来打印一个换行符。这是这里的问题，因为它的更多的不仅仅是打印一个换行符＆MDASH; ENDL 也的刷新的这是一个昂贵的操作缓冲区（如果你这样做，在每次迭代）。
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).
使用 \\ n 如果你的意思是这样：
Use \n if you mean so:
file << i << '\n';
而且，必须编译在释放模式的code（即打开优化）。
And also, must compile your code in release mode (i.e turn on the optimizations).
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