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DUP(2) NetBSD System Calls Manual DUP(2)
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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
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, 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),
The dup() and dup2() functions conform to ISO/IEC 9945-1:1990
The dup3() function is inspired from Linux and appeared in NetBSD 6.0.
NetBSD 6.0 January 23, 2012 NetBSD 6.0