Why Is C \'s `stdin` Line Reading Significantly Slower Than Python\'s, and How Can It Be Improved?

Why Is Reading Lines from stdin Significantly Slower in C Than Python?

In this article, we aim to investigate why reading lines of string data from standard input (stdin) using C is generally much slower than its Python counterpart. We begin by examining the code provided, identifying potential areas of concern.

The C Code

string input_line;
long line_count = 0;
time_t start = time(NULL);
int sec;
int lps;

while (cin) {
    getline(cin, input_line);
    if (!cin.eof())
        line_count++;
}

This code employs getline() to read lines of text from stdin and counts the number of lines in a file. However, it reads each character of the file one at a time within an inner loop. This approach is inefficient and leads to excessive system calls, resulting in slow execution.

Default Settings of C Streams

Unlike Python, C streams have default settings that prioritize synchronization with the C-style standard input and output functions. This synchronization ensures that both C and C streams access input and output resources consistently. However, this synchronization also prevents C streams from using more efficient buffering mechanisms.

cin is synchronized with stdio, which causes it to avoid any input buffering. As a result, cin reads only one character at a time, making the process more time-consuming.

Performance Improvements

To improve the performance of the C code, we can disable the synchronization between cin and stdio. By adding the statement cin.sync_with_stdio(false) at the beginning of the program, we can allow C streams to buffer their I/O independently, leading to significant speed enhancements.

Alternative: Using fgets

Another effective approach to enhance performance is to use fgets() instead of getline(). fgets is a C function that reads a specified number of characters from stdin into a character array, providing greater control over the buffering process.

Benchmark Results

To demonstrate the performance difference, a file containing 100 million lines was used for benchmarking. The results using the original (synchronized) C code, C with synchronization disabled, and Python were as follows:

Implementation	Lines per Second
Python (default)	3,571,428
cin (default/naive)	819,672
cin (no sync)	12,500,000
fgets	14,285,714
wc (not a fair comparison)	54,644,808

Disabling synchronization in C resulted in a significant improvement, while using fgets offered even better performance. It's important to note that using wc is not a fair comparison as it is designed specifically for counting lines.

Conclusion

By understanding the default settings of C streams and implementing appropriate optimizations, such as disabling stream synchronization or using fgets, the performance of C code for reading lines from stdin can be dramatically improved, making it on par with or even faster than its Python counterpart.

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