man2/dup.2

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   3 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   4 .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
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  27 .\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
  28 .\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
  29 .\"   Fixed typoes.
  30 .\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
  31 .\" Modified 2002-09-28, aeb
  32 .\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some
  33 .\"     details for dup2().
  34 .\"
  35 .TH DUP 2 2008-01-12 "Linux" "Linux Programmer's Manual"
  36 .SH NAME
  37 dup, dup2 \- duplicate a file descriptor
  38 .SH SYNOPSIS
  39 .nf
  40 .B #include <unistd.h>
  41 .sp
  42 .BI "int dup(int " oldfd );
  43 .BI "int dup2(int " oldfd ", int " newfd );
  44 .fi
  45 .SH DESCRIPTION
  46 .BR dup ()
  47 and
  48 .BR dup2 ()
  49 create a copy of the file descriptor
  50 .IR oldfd .
  51
  52 .BR dup ()
  53 uses the lowest-numbered unused descriptor for the new descriptor.
  54
  55 .BR dup2 ()
  56 .RI "makes " newfd " be the copy of " oldfd ", closing " newfd
  57 first if necessary, but note the following:
  58 .IP * 3
  59 If
  60 .I olfd
  61 is not a valid file descriptor, then the call fails, and
  62 .I newfd
  63 is not closed.
  64 .IP *
  65 If
  66 .I oldfd
  67 is a valid file descriptor, and
  68 .I newfd
  69 has the same value as
  70 .IR oldfd ,
  71 then
  72 .BR dup2 ()
  73 does nothing, and returns
  74 .IR newfd .
  75 .PP
  76 After a successful return from
  77 .BR dup ()
  78 or
  79 .BR dup2 (),
  80 the old and new file descriptors may be used interchangeably.
  81 They refer to the same open file description (see
  82 .BR open (2))
  83 and thus share file offset and file status flags;
  84 for example, if the file offset is modified by using
  85 .BR lseek (2)
  86 on one of the descriptors, the offset is also changed for the other.
  87
  88 The two descriptors do not share file descriptor flags
  89 (the close-on-exec flag).
  90 The close-on-exec flag
  91 .RB ( FD_CLOEXEC ;
  92 see
  93 .BR fcntl (2))
  94 for the duplicate descriptor is off.
  95 .SH "RETURN VALUE"
  96 .BR dup ()
  97 and
  98 .BR dup2 ()
  99 return the new descriptor, or \-1 if an error occurred (in which case,
 100 .I errno
 101 is set appropriately).
 102 .SH ERRORS
 103 .TP
 104 .B EBADF
 105 .I oldfd
 106 isn't an open file descriptor, or
 107 .I newfd
 108 is out of the allowed range for file descriptors.
 109 .TP
 110 .B EBUSY
 111 (Linux only) This may be returned by
 112 .BR dup2 ()
 113 during a race condition with
 114 .BR open (2)
 115 and
 116 .BR dup ().
 117 .TP
 118 .B EINTR
 119 The
 120 .BR dup2 ()
 121 call was interrupted by a signal.
 122 .TP
 123 .B EMFILE
 124 The process already has the maximum number of file
 125 descriptors open and tried to open a new one.
 126 .SH "CONFORMING TO"
 127 SVr4, 4.3BSD, POSIX.1-2001.
 128 .\" SVr4 documents additional
 129 .\" EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
 130 .\" The EBUSY return is Linux-specific.
 131 .SH NOTES
 132 The error returned by
 133 .BR dup2 ()
 134 is different from that returned by
 135 .BR fcntl( "..., " F_DUPFD ", ..." )
 136 when
 137 .I newfd
 138 is out of range.
 139 On some systems
 140 .BR dup2 ()
 141 also sometimes returns
 142 .B EINVAL
 143 like
 144 .BR F_DUPFD .
 145
 146 If
 147 .I newfd
 148 was open, any errors that would have been reported at
 149 .BR close (2)
 150 time are lost.
 151 A careful programmer will not use
 152 .BR dup2 ()
 153 without closing
 154 .I newfd
 155 first.
 156 .SH "SEE ALSO"
 157 .BR close (2),
 158 .BR fcntl (2),
 159 .BR open (2)