vs printf/std:cout bug

printf.cpp

#include <stdlib.h>
#include <process.h>
#include <iostream>

void work1(void* p)
{
	while (1) {
		printf("hello world");
		_sleep(1);
	}

}

void work2(void* p)
{
	while (1) {
		std::cout << "hello world";
		_sleep(1);
	}
}

int main()
{
	for (size_t i = 0; i < 100; i++)
	{
		_beginthread(work1, 0, NULL);
	}

	for (size_t i = 0; i < 100; i++)
	{
		_beginthread(work2, 0, NULL);
	}

	while (1)
	{
		_sleep(10000);
	}

	return 0;
}