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28 .\" @(#)dup.2 8.1 (Berkeley) 6/4/93
37 .Nd duplicate an existing file descriptor
45 .Fn dup2 "int oldd" "int newd"
50 duplicates an existing object descriptor and returns its value to
57 is a small non-negative integer index in
58 the per-process descriptor table.
59 The new descriptor returned by the call
60 is the lowest numbered descriptor
61 currently not in use by the process.
63 The object referenced by the descriptor does not distinguish
73 are duplicate references to an open
79 calls all move a single pointer into the file,
80 and append mode, non-blocking I/O and asynchronous I/O options
81 are shared between the references.
82 If a separate pointer into the file is desired, a different
83 object reference to the file must be obtained by issuing an
87 The close-on-exec flag on the new file descriptor is unset.
91 the value of the new descriptor
94 If this descriptor is already in use and
98 the descriptor is first deallocated as if the
100 system call had been used.
103 is not a valid descriptor, then
112 is a valid descriptor, then
114 is successful, and does nothing.
118 system call allows file descriptors to be duplicated with restrictions on
121 The value -1 is returned if an error occurs in either call.
122 The external variable
124 indicates the cause of the error.
128 system call fails if:
134 is not a valid active descriptor
136 Too many descriptors are active.
140 system call fails if:
145 argument is not a valid active descriptor or the
147 argument is negative or exceeds the maximum allowable descriptor number
154 .Xr getdtablesize 2 ,
164 system calls are expected to conform to
171 functions appeared in