bsd/man/man2/dup.2

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  34 .\"     @(#)dup.2       8.1 (Berkeley) 6/4/93
  35 .\"
  36 .Dd June 4, 1993
  37 .Dt DUP 2
  38 .Os BSD 4
  39 .Sh NAME
  40 .Nm dup ,
  41 .Nm dup2
  42 .Nd duplicate an existing file descriptor
  43 .Sh SYNOPSIS
  44 .Fd #include <unistd.h>
  45 .Ft int
  46 .Fn dup "int oldd"
  47 .Ft int
  48 .Fn dup2 "int oldd" "int newd"
  49 .Sh DESCRIPTION
  50 .Fn Dup
  51 duplicates an existing object descriptor and returns its value to
  52 the calling process
  53 .Fa ( newd
  54 =
  55 .Fn dup oldd ) .
  56 The argument
  57 .Fa oldd
  58 is a small non-negative integer index in
  59 the per-process descriptor table.  The value must be less
  60 than the size of the table, which is returned by
  61 .Xr getdtablesize 2 .
  62 The new descriptor returned by the call
  63 is the lowest numbered descriptor
  64 currently not in use by the process.
  65 .Pp
  66 The object referenced by the descriptor does not distinguish
  67 between
  68 .Fa oldd
  69 and
  70 .Fa newd
  71 in any way.
  72 Thus if
  73 .Fa newd
  74 and
  75 .Fa oldd
  76 are duplicate references to an open
  77 file,
  78 .Xr read 2 ,
  79 .Xr write 2
  80 and
  81 .Xr lseek 2
  82 calls all move a single pointer into the file,
  83 and append mode, non-blocking I/O and asynchronous I/O options
  84 are shared between the references.
  85 If a separate pointer into the file is desired, a different
  86 object reference to the file must be obtained by issuing an
  87 additional
  88 .Xr open 2
  89 call.
  90 The close-on-exec flag on the new file descriptor is unset.
  91 .Pp
  92 In
  93 .Fn dup2 ,
  94 the value of the new descriptor
  95 .Fa newd
  96 is specified.  If this descriptor is already
  97 in use, the descriptor is first deallocated as if a
  98 .Xr close 2
  99 call had been done first.
 100 .Sh RETURN VALUES
 101 The value -1 is returned if an error occurs in either call.
 102 The external variable
 103 .Va errno
 104 indicates the cause of the error.
 105 .Sh ERRORS
 106 .Fn Dup
 107 and
 108 .Fn dup2
 109 fail if:
 110 .Bl -tag -width Er
 111 .It Bq Er EBADF
 112 .Fa Oldd
 113 or
 114 .Fa newd
 115 is not a valid active descriptor
 116 .It Bq Er EMFILE
 117 Too many descriptors are active.
 118 .El
 119 .Sh SEE ALSO
 120 .Xr accept 2 ,
 121 .Xr open 2 ,
 122 .Xr close 2 ,
 123 .Xr fcntl 2 ,
 124 .Xr pipe 2 ,
 125 .Xr socket 2 ,
 126 .Xr socketpair 2 ,
 127 .Xr getdtablesize 2
 128 .Sh STANDARDS
 129 .Fn Dup
 130 and
 131 .Fn dup2
 132 are expected to conform to
 133 .St -p1003.1-88 .