Does scanf() Always Outperform iostream in C Input Speed?

Can scanf() Outpace iostream in C Performance?

The notion that scanf() provides faster input handling than cin in C has sparked a debate among programmers. This article delves into the performance comparison between these methods, examining the evidence and providing a detailed explanation.

Performance Testing

To assess the performance difference, a simple program was created that reads a list of numbers from standard input and performs an XOR operation on them. Both cin and scanf() were used in separate implementations:

• cin Version:

#include <iostream>

int main() {
  int parity = 0;
  int x;

  while (std::cin >> x)
    parity ^= x;
  std::cout << parity << std::endl;

  return 0;
}

• scanf Version:

#include <stdio.h>

int main() {
  int parity = 0;
  int x;

  while (1 == scanf("%d", &x))
    parity ^= x;
  printf("%d\n", parity);

  return 0;
}

Results

Using a file containing over 33 million random numbers, the execution times were measured:

• cin Version: 24.3 seconds
• scanf Version: 6.4 seconds

Optimization Considerations

Changing compiler optimization settings had minimal impact on the results.

Enhanced iostream Performance

It was discovered that the performance gap is partly due to iostream maintaining synchronization with the C I/O functions. By disabling this synchronization with the command std::ios::sync_with_stdio(false), iostream's performance improved significantly:

#include <iostream>

int main() {
  int parity = 0;
  int x;

  std::ios::sync_with_stdio(false);

  while (std::cin >> x)
    parity ^= x;
  std::cout << parity << std::endl;

  return 0;
}

New Results

After disabling iostream synchronization:

• cin Version with Synchronization: 21.9 seconds
• scanf Version: 6.8 seconds
• cin Version without Synchronization: 5.5 seconds

Conclusion

The evidence suggests that scanf() is indeed faster than cin for raw input handling in C . However, if iostream's synchronization is disabled, iostream becomes the superior choice, outperforming both scanf() and the optimized cin version.

Therefore, while scanf() may be faster for specific scenarios, iostream offers better control over synchronization and a wider range of functionality. When speed is crucial, disabling iostream synchronization is recommended.

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