DUP(2) BSD System Calls Manual DUP(2)
dup, dup2, dup3 -- duplicate an existing file descriptor
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
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, it is first deallocated as if a close(2)
call had been done first. When newd equals oldd, dup2() just returns
without affecting the close-on-exec flag.
In dup3(), both the value of the new descriptor and the close-on-exec
flag on the new file descriptor are specified: newd specifies the value
and the O_CLOEXEC bit in flags specifies the close-on-exec flag. Unlike
dup2(), if oldd and newd are equal then dup3() fails. Otherwise, if
flags is zero then dup3() is identical to a call to dup2().
Upon successful completion, the value of the new descriptor is returned.
The value -1 is returned if an error occurs in either call. The external
variable errno indicates the cause of the error.
dup() will fail if:
[EBADF] oldd is not a valid active descriptor.
[EMFILE] Too many descriptors are active.
dup2() and dup3() will fail if:
[EBADF] oldd is not a valid active descriptor or newd is nega-
tive or greater than or equal to the process's
[EINTR] An interrupt was received.
[EIO] An I/O error occurred while writing to the file sys-
In addition, dup3() will return the following error:
[EINVAL] oldd is equal to newd or flags is invalid.
accept(2), close(2), fcntl(2), getrlimit(2), open(2), pipe(2), socket(2),
dup() and dup2() conform to IEEE Std 1003.1-2008 (``POSIX.1''). The
dup3() function is expected to conform to a future revision of that stan-
The dup() system call first appeared in Version 3 AT&T UNIX, dup2() in
Version 7 AT&T UNIX, and dup3() in OpenBSD 5.7.
BSD March 27, 2017 BSD