Описание тега dup
NoneDup() is a c system call that duplicates a file descriptor
Usage: int dup(int oldfd); (#include <unistd.h>)
The C system call dup duplicates a file descriptor. It returns a second file descriptor that points to the same file table entry as precious file descriptor does, so that we can treat the two file descriptors as identical.
The dup system call uses the lowest-numbered unused file descriptor as its new file descriptor. Compare dup2 which has two arguments, and that you specify the new destination file descriptor number in the second argument.
See http://man7.org/linux/man-pages/man2/dup.2.html for more details.