getline significantly slower in C++ vs. C

While reading text files line by line, I noticed significant performance drops when using C++ std::getline compared to the C function getline, using GCC 4.8.5.

I limited my tests to a very simple program counting the number of lines.

In C++:

#include <iostream> #include <string> using namespace std; int main() { size_t n = 0; string line; while (getline(cin, line)) ++n; cout << n << endl; return 0; } 

In C:

#include <stdio.h> #include <stdlib.h> int main() { size_t n = 0; char* line = NULL; size_t len = 0; while (getline(&line, &len, stdin) != -1) ++n; printf("%zu\n", n); free(line); return 0; } 

In both cases I use the C++ compiler to make sure that the difference comes exclusively from getline and not from C vs. C++ compiler optimizations.

I start to notice performance drops after "only" a few thousand lines.

Examples with ~600, 2.5k, 20k, 157k, 10M, 40M, 50M, 60M and 80M lines:

Nb lines | Time (C) | Time (C++) | slower ----------+------------+------------+-------- 613 | 0m0.003s | 0m0.005s | 1.5x 2452 | 0m0.002s | 0m0.011s | 5.5x 19616 | 0m0.004s | 0m0.062s | 15x 156928 | 0m0.014s | 0m0.511s | 37x 10043392 | 0m0.776s | 0m31.560s | 41x 40173568 | 0m3.335s | 2m7.752s | 38x 50216960 | 0m5.543s | 2m42.116s | 18x 60260352 | 0m22.571s | 3m13.148s | 9x 80347136 | 0m27.713s | 4m18.272s | 9x 

These numbers should be taken with a pinch of salt but I think they reflect how values increase with the file size, and that there is some sort of maximum around 40-ish slower before things start to even out slightly, probably due to other limitations (hardware maybe?) instead of software issues.

Considering I read almost exclusively files with 100k+ lines I should expect a performance drop of 9 times (at best) if I stick to the C++ code.

Is there a reason for such a big difference? I understand that the STL overhead can be significant but I thought that file access would outweigh such differences.

Also, is there a way to optimize calls to std::getline (apart from using its C counterpart, obviously)?

Additional notes

• I tried with GCC 9.3.0 and got similar results
• I tried some compiler optimizations but observed no significant improvement, the build/test call is: gcc -c count.cpp && gcc -o count count.o && (time ./count < infile)
• Following @Eljay’s advice, I added ios_base::sync_with_stdio(false); and cin.tie(NULL); at the beginning of the C++ code and results are much better, although not quite on par with the C code ; it could still be acceptable for balancing performance vs. readability (note: this isolated code is readable in both C and C++ but the full code is much more readable in C++)