fildes is a file descriptor referring to an open file, and
fildes2 is a non-negative integer less than the maximum number
of open files available.
dup2 causes fildes2 to refer to the same file as fildes.
If fildes2 already referred to an open file, not fildes,
it is closed first.
If fildes2 refers to fildes, or if fildes is not a valid
open file descriptor, fildes2 will not be closed first.
Return values
Upon successful completion a non-negative integer, namely, the
file descriptor, is returned.
Otherwise, a value of -1 is returned and errno
is set to indicate the error.
Errors
dup2 will fail if one or more of the following are true:
EBADF
fildes is not a valid open file descriptor.
EBADF
fildes2 is negative or greater than or equal to the maximum number of
open files available.
EINTR
a signal was caught during the dup2 call.
EMFILE
The maximum number of file descriptors are currently open.