FreeBSD man pages : dup (2)
DUP(2) FreeBSD System Calls Manual DUP(2)
dup, dup2 - duplicate an existing file descriptor
Standard C Library (libc, -lc)
dup2(int oldd, int newd);
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 references 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 and oldd != newd, the descriptor is first
deallocated as if a close(2) call had been used. If oldd is not a valid
descriptor, then newd is not closed. If oldd == newd and oldd is a valid
descriptor, then dup2() is successful, and does nothing.
The value -1 is returned if an error occurs in either call. The external
variable errno indicates the cause of the error.
Dup() and dup2() fail if:
[EBADF] Oldd or newd is not a valid active descriptor
[EMFILE] Too many descriptors are active.
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
The dup() and dup2() function calls are expected to conform to ISO/IEC
A dup() and a dup2() function call appeared in Version 7 AT&T UNIX.
FreeBSD 4.8 June 4, 1993 FreeBSD 4.8