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 "() and " dup2 ()
  44 create a copy of the file descriptor
  45 .IR oldfd .
  46
  47 After a successful return from \fBdup\fR() or \fBdup2\fR(),
  48 the old and new file descriptors may be used interchangeably.
  49 They refer to the same open file description (see
  50 .BR open (2))
  51 and thus share file offset and file status flags;
  52 for example, if the file offset is modified by using
  53 .BR lseek (2)
  54 on one of the descriptors, the position is also changed for the other.
  55
  56 The two descriptors do not share file descriptor flags
  57 (the close-on-exec flag).
  58 The close-on-exec flag
  59 .RB ( FD_CLOEXEC ;
  60 see
  61 .BR fcntl (2))
  62 for the duplicate descriptor is off.
  63
  64 .BR dup ()
  65 uses the lowest-numbered unused descriptor for the new descriptor.
  66
  67 .BR dup2 ()
  68 .RI "makes " newfd " be the copy of " oldfd ", closing " newfd
  69 first if necessary.
  70 .SH "RETURN VALUE"
  71 .BR dup "() and " dup2 ()
  72 return the new descriptor, or \-1 if an error occurred (in which case,
  73 .I errno
  74 is set appropriately).
  75 .SH ERRORS
  76 .TP
  77 .B EBADF
  78 .I oldfd
  79 isn't an open file descriptor, or
  80 .I newfd
  81 is out of the allowed range for file descriptors.
  82 .TP
  83 .B EBUSY
  84 (Linux only) This may be returned by
  85 .BR dup2 ()
  86 during a race condition with open() and dup().
  87 .TP
  88 .B EINTR
  89 The
  90 .BR dup2 ()
  91 call was interrupted by a signal.
  92 .TP
  93 .B EMFILE
  94 The process already has the maximum number of file
  95 descriptors open and tried to open a new one.
  96 .SH WARNINGS
  97 The error returned by
  98 .BR dup2 ()
  99 is different from that returned by
 100 .BR fcntl( "..., " F_DUPFD ", ..." )
 101 when
 102 .I newfd
 103 is out of range. On some systems
 104 .BR dup2 ()
 105 also sometimes returns
 106 .B EINVAL
 107 like
 108 .BR F_DUPFD .
 109
 110 If
 111 .I newfd
 112 was open, any errors that would have been reported at
 113 .IR close ()
 114 time, are lost. A careful programmer will not use
 115 .BR dup2 ()
 116 without closing
 117 .I newfd
 118 first.
 119 .SH "CONFORMING TO"
 120 SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents additional
 121 EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
 122 The EBUSY return is Linux-specific.
 123 .SH "SEE ALSO"
 124 .BR close (2),
 125 .BR fcntl (2),
 126 .BR open (2)