File descriptor copying tests with fork() and dup2()

fd1.cpp

#include <cstdio>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/wait.h>

using namespace std;

int main() {
	int fd = open("fd.txt", O_CREAT | O_WRONLY | O_TRUNC, 644);
	char buf[14] = "Hello_world!\n";
	write(fd, buf, 13);
	close(fd);

	fd = open("fd.txt", O_RDONLY);
	char rbuf[20] = {0};
	int pid = fork();
	if (pid) {
		// parent
		lseek(fd, 1, SEEK_SET);
		read(fd, rbuf, 4);
		printf("Read from parent: %s\n", rbuf);
		waitpid(pid, 0, 0);
	}
	else {
		// child
		sleep(1);
		read(fd, rbuf, 4);
		printf("Read from child: %s\n", rbuf);
	}

	return 0;
}

fd2.cpp

#include <cstdio>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/wait.h>

using namespace std;

int main() {
	int fd = open("fd.txt", O_CREAT | O_WRONLY | O_TRUNC, 644);
	char buf[14] = "Hello_world!\n";
	write(fd, buf, 13);
	close(fd);

	fd = open("fd.txt", O_RDONLY);
	char rbuf[20] = {0};
	dup2(fd, 42);

	// original
	lseek(fd, 1, SEEK_SET);
	read(fd, rbuf, 4);
	printf("Read from fd: %s\n", rbuf);

	// dup2
	sleep(1);
	read(42, rbuf, 4);
	printf("Read from dup2: %s\n", rbuf);

	return 0;
}