Why is C 's `cin` Significantly Slower Than Python for Reading Lines from stdin?

Why is Reading Lines from stdin Much Slower in C than Python?

Background:

Reading lines of string input from stdin using C can be significantly slower than its Python equivalent. This discrepancy arises due to default settings in C that require more system calls.

Default Behavior of Input Streams:

By default, the C input stream cin is synchronized with the standard C I/O library (stdio). This synchronization ensures that input is not buffered and is read one character at a time. While this behavior prevents conflicts between the cin stream and stdio functions, it also incurs a performance penalty.

Use of System Calls:

When cin is synchronized with stdio, each character read triggers a system call, a costly operation. This overhead significantly slows down the input process, especially when reading large amounts of data.

Solution:

To improve the performance of line reading in C , one can disable the synchronization between cin and stdio. This can be achieved by invoking the sync_with_stdio(false) method.

Alternatively, using the fgets function instead of getline also bypasses the synchronization overhead.

Performance Comparison:

Here's a performance comparison between the original (synchronized) C implementation and various approaches:

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

Conclusion:

Disabling the synchronization between cin and stdio or using fgets can significantly improve the performance of line reading in C , making it comparable to Python's efficiency.

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