man2/dup.2

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   4 .\"                               1993 Michael Haardt, Ian Jackson.
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  26 .\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
  27 .\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
  28 .\"   Fixed typoes.
  29 .\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
  30 .\" Modified 2002-09-28, aeb
  31 .\"
  32 .TH DUP 2 1994-08-21 "Linux 1.1.46" "Linux Programmer's Manual"
  33 .SH NAME
  34 dup, dup2 \- duplicate a file descriptor
  35 .SH SYNOPSIS
  36 .nf
  37 .B #include <unistd.h>
  38 .sp
  39 .BI "int dup(int " oldfd );
  40 .BI "int dup2(int " oldfd ", int " newfd );
  41 .fi
  42 .SH DESCRIPTION
  43 .BR dup ()
  44 and
  45 .BR dup2 ()
  46 create a copy of the file descriptor
  47 .IR oldfd .
  48
  49 After a successful return from \fBdup\fR() or \fBdup2\fR(),
  50 the old and new file descriptors may be used interchangeably.
  51 They refer to the same open file description (see
  52 .BR open (2))
  53 and thus share file offset and file status flags;
  54 for example, if the file offset is modified by using
  55 .BR lseek (2)
  56 on one of the descriptors, the offset is also changed for the other.
  57
  58 The two descriptors do not share file descriptor flags
  59 (the close-on-exec flag).
  60 The close-on-exec flag
  61 .RB ( FD_CLOEXEC ;
  62 see
  63 .BR fcntl (2))
  64 for the duplicate descriptor is off.
  65
  66 .BR dup ()
  67 uses the lowest-numbered unused descriptor for the new descriptor.
  68
  69 .BR dup2 ()
  70 .RI "makes " newfd " be the copy of " oldfd ", closing " newfd
  71 first if necessary.
  72 .SH "RETURN VALUE"
  73 .BR dup ()
  74 and
  75 .BR dup2 ()
  76 return the new descriptor, or \-1 if an error occurred (in which case,
  77 .I errno
  78 is set appropriately).
  79 .SH ERRORS
  80 .TP
  81 .B EBADF
  82 .I oldfd
  83 isn't an open file descriptor, or
  84 .I newfd
  85 is out of the allowed range for file descriptors.
  86 .TP
  87 .B EBUSY
  88 (Linux only) This may be returned by
  89 .BR dup2 ()
  90 during a race condition with
  91 .BR open ()
  92 and
  93 .BR dup ().
  94 .TP
  95 .B EINTR
  96 The
  97 .BR dup2 ()
  98 call was interrupted by a signal.
  99 .TP
 100 .B EMFILE
 101 The process already has the maximum number of file
 102 descriptors open and tried to open a new one.
 103 .SH WARNINGS
 104 The error returned by
 105 .BR dup2 ()
 106 is different from that returned by
 107 .BR fcntl( "..., " F_DUPFD ", ..." )
 108 when
 109 .I newfd
 110 is out of range. On some systems
 111 .BR dup2 ()
 112 also sometimes returns
 113 .B EINVAL
 114 like
 115 .BR F_DUPFD .
 116
 117 If
 118 .I newfd
 119 was open, any errors that would have been reported at
 120 .IR close ()
 121 time, are lost. A careful programmer will not use
 122 .BR dup2 ()
 123 without closing
 124 .I newfd
 125 first.
 126 .SH "CONFORMING TO"
 127 SVr4, SVID, POSIX, X/OPEN, 4.3BSD. SVr4 documents additional
 128 EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
 129 The EBUSY return is Linux-specific.
 130 .SH "SEE ALSO"
 131 .BR close (2),
 132 .BR fcntl (2),
 133 .BR open (2)