#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main()
{

  int fd1,fd2;
  char str[11] = "ciao mamma\n";

  fd1 = open("prova.txt", O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR | S_IXUSR);

  fd2 = dup2(fd1,fd2);

  write(fd1, str, 11);
  write(fd2, str, 11);

  close(fd1);
  close(fd2);
}