DUP(2) BSD System Calls Manual DUP(2)
dup, dup2, dup3 -- duplicate an existing file descriptor
Standard C Library (libc, -lc)
dup2(int oldd, int newd);
dup3(int oldd, int newd, int flags);
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(3). The new descriptor returned by the call is the lowest num-
bered 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, the descriptor is first deallocated as if a close(2) call had been done first. If
newd and oldd are the same, the call has no effect.
dup3() behaves exactly like dup2() only it allows extra flags to be set on the returned file
descriptor. The following flags are valid:
O_CLOEXEC Set the ``close-on-exec'' property.
O_NONBLOCK Sets non-blocking I/O.
Return EPIPE instead of raising SIGPIPE.
The value -1 is returned if an error occurs in either call. The external variable errno
indicates the cause of the error.
All three functions may fail if:
[EBADF] oldd is not a valid active descriptor or newd is not in the range of
valid file descriptors.
The dup() function may also fail if:
[EMFILE] Too many descriptors are active.
The dup3() function will also fail if:
[EINVAL] flags is other than O_NONBLOCK or O_CLOEXEC.
accept(2), close(2), fcntl(2), open(2), pipe(2), socket(2), socketpair(2), getdtablesize(3)
The dup() and dup2() functions conform to ISO/IEC 9945-1:1990 (``POSIX.1'').
The dup3() function is inspired from Linux and appeared in NetBSD 6.0.
BSD January 23, 2012 BSD