Does std::cout write data directly to the file descriptor?

std::cout does not write data directly to the file descriptor. It uses buffering to improve performance.

Data is first placed in a buffer, and only when the buffer is full, or when std::flush, std::endl are called, or when the program terminates, is the data sent to the file descriptor (usually standard output — stdout).

Example:

#include <iostream>

int main() {
    std::cout << "Hello, world!"; // data in buffer
    std::cout.flush(); // forcibly send data to the file descriptor
    return 0;
}

Thus, buffering reduces the number of system write calls, which improves performance.