Why is C Standard Input Reading Slower Than Python's, and How Can It Be Improved?

Why C Standard Input Reading Falls Short Against Python's Efficiency

While comparing Python and C 's performance in reading lines of string input from standard input (stdin), a striking disparity emerged: C code ran significantly slower. This prompted an investigation into the underlying reasons for this performance gap.

The issue stems from the default settings of C . By default, istreams, such as cin, synchronize with stdio, resulting in the avoidance of input buffering. To optimize performance, this synchronization can be disabled using the std::ios_base::sync_with_stdio(false) statement. This adjustment alleviates a major performance constraint.

Standard input streams operate differently by design between Python and C . Python's is typically buffered, while C 's is unbuffered by default. Under normal circumstances, buffering reduces system calls, improving efficiency. However, the separate implementations and buffers of FILE* based stdio and iostreams in C pose potential issues when used together.

To avoid buffer inconsistencies and unexpected results, C synchronizes streams with stdio by default. While this safeguards against potential problems, it introduces overhead, especially when processing large amounts of input.

To achieve maximum performance, programmers can disable synchronization using the sync_with_stdio method. This optimization allows C standard streams to buffer independently, significantly improving speed in certain scenarios.

Performance Benchmark

For a more comprehensive comparison, here's a performance benchmark with 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	54,644,808	(Not a fair comparison)

As evident, disabling synchronization and using fgets result in notable performance enhancements in C .

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