opendarwin man page for dup

DUP(2)							      BSD System Calls Manual							    DUP(2)

NAME
     dup, dup2 -- duplicate an existing file descriptor

SYNOPSIS
     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int newd);

DESCRIPTION
     Dup() duplicates an existing object descriptor and returns its value to the calling process (newd = dup(oldd)).  The argument oldd is a small
     non-negative integer index in the per-process descriptor table.  The value must be less than the size of the table, which is returned by
     getdtablesize(2).	The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.

     The object referenced by the descriptor does not distinguish between oldd and newd in any way.  Thus if newd and oldd are duplicate refer-
     ences to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and
     asynchronous I/O options are shared between the references.  If a separate pointer into the file is desired, a different object reference to
     the file must be obtained by issuing an additional open(2) call.  The close-on-exec flag on the new file descriptor is unset.

     In dup2(), the value of the new descriptor newd is specified.  If this descriptor is already in use, the descriptor is first deallocated as
     if a close(2) call had been done first.

RETURN VALUES
     The value -1 is returned if an error occurs in either call.  The external variable errno indicates the cause of the error.

ERRORS
     Dup() and dup2() fail if:

     [EBADF]		Oldd or newd is not a valid active descriptor

     [EMFILE]		Too many descriptors are active.

SEE ALSO
     accept(2), open(2), close(2), fcntl(2), pipe(2), socket(2), socketpair(2), getdtablesize(2)

STANDARDS
     Dup() and dup2() are expected to conform to IEEE Std 1003.1-1988 (``POSIX.1'').

4th Berkeley Distribution					   June 4, 1993 					 4th Berkeley Distribution