Is C IOStream Performance Actually Slower Than printf/scanf?

How to Optimize IOStream Performance in C

C users often prefer the printf/scanf family of functions over C IOStreams, despite the latter's interface advantages. Performance concerns are often cited as the primary reason for this preference.

Buffering

Enlarging the buffer size of the underlying streambuf can significantly improve performance by reducing HDD hits and system calls. This is done with:

char Buffer[N];
std::ifstream file("file.txt");
file.rdbuf()->pubsetbuf(Buffer, N);

Locale Handling

Locales can introduce performance overhead due to character conversion, filtering, and dynamic dispatch. Setting the locale to the default C locale, which disables these operations, can improve performance:

std::locale::global(std::locale("C"));

Synchronization

Synchronization with C stdio (std::ios_base::sync_with_stdio(false)) offers no observable performance benefits.

Measurements

Benchmarking using different compilers and platforms reveals varying results:

• g 3.4.2 on SUSE 10p3: ~20% slowdown for C
• g 4.7.2 on Ubuntu 11.10: C 25% faster
• g 4.4.5 on Ubuntu Linux 10.10: C 17% faster
• g on macOS X: C 111% slower
• clang 3.8.0 on Kubuntu 16.04: C 66% faster

These results indicate that IOStream performance improvements vary depending on specific implementation implementations. Therefore, there is no universal solution to optimize IOStreams across platforms.

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