使用 std::ios::sync_with_stdio(fasle) 打印时，在 Valgrind 的 LEAK SUMMARY 中获取“仍然可达”

Question

Consider the simple code below:

#include <iostream>
#include <algorithm>
#include <vector>

void print_vector( const std::vector<int> &inputVector )
{
    std::ios_base::sync_with_stdio(false);

    for ( const int &p : inputVector )
    {
        std::cout << p << "  ";
    }

    std::cout << "\n";
}

int main() {

    std::vector<int> s{ 5, 7, 4, 2, 8, 6, 1, 9, 0, 3 };

    std::sort( s.begin(), s.end(), [](const int &a, const int &b)
    {
        return a > b;
    });

    print_vector( s );

    return 0;
}

When using the above printing function I get a "still reachable" in the LEAK SUMMARY when profiling with valgrind.

I used the following for compiling: (gcc version 9)

g++ --std=c++17 -Wall sorting_stl.cc -o sorting_stl.o

and the following valgrind command

valgrind --leak-check=full --show-leak-kinds=all -v ./sorting_stl.o 

The full LEAK SUMMARY at the end:

LEAK SUMMARY:
    definitely lost: 0 bytes in 0 blocks
    indirectly lost: 0 bytes in 0 blocks
      possibly lost: 0 bytes in 0 blocks
    still reachable: 122,880 bytes in 6 blocks
         suppressed: 0 bytes in 0 blocks

Removing the std::ios::sync_with_stdio(false); and using std::endl; resolves the error. I don't understand the reason though why this happens, or if it is an error here. Is it a good idea in general to use std::sync_with_stdio(false);

Answer 1

It's fine.

This counts as a "false positive"; some aspects of the standard library deliberately don't do clean-up. This is one of them.

It's not a leak that'll get worse over time. Don't worry about it.

Ideally Valgrind would be neighbourly and would filter this out, but apparently it's been an issue for a long time .

You can read more about the various report levels in the Valgrind FAQ .