man2/dup.2

   1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   2 .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
   3 .\" and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpages@gmail.com>
   4 .\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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  27 .\"
  28 .\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
  29 .\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
  30 .\"   Fixed typos.
  31 .\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
  32 .\" Modified 2002-09-28, aeb
  33 .\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some
  34 .\"     details for dup2().
  35 .\" 2008-10-09, mtk: add description of dup3()
  36 .\"
  37 .TH DUP 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  38 .SH NAME
  39 dup, dup2, dup3 \- duplicate a file descriptor
  40 .SH SYNOPSIS
  41 .nf
  42 .B #include <unistd.h>
  43 .PP
  44 .BI "int dup(int " oldfd );
  45 .BI "int dup2(int " oldfd ", int " newfd );
  46
  47 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  48 .BR "#include <fcntl.h>" "              /* Obtain O_* constant definitions */
  49 .B #include <unistd.h>
  50 .PP
  51 .BI "int dup3(int " oldfd ", int " newfd ", int " flags );
  52 .fi
  53 .SH DESCRIPTION
  54 The
  55 .BR dup ()
  56 system call creates a copy of the file descriptor
  57 .IR oldfd ,
  58 using the lowest-numbered unused file descriptor for the new descriptor.
  59 .PP
  60 After a successful return,
  61 the old and new file descriptors may be used interchangeably.
  62 They refer to the same open file description (see
  63 .BR open (2))
  64 and thus share file offset and file status flags;
  65 for example, if the file offset is modified by using
  66 .BR lseek (2)
  67 on one of the file descriptors, the offset is also changed for the other.
  68 .PP
  69 The two file descriptors do not share file descriptor flags
  70 (the close-on-exec flag).
  71 The close-on-exec flag
  72 .RB ( FD_CLOEXEC ;
  73 see
  74 .BR fcntl (2))
  75 for the duplicate descriptor is off.
  76 .\"
  77 .SS dup2()
  78 The
  79 .BR dup2 ()
  80 system call performs the same task as
  81 .BR dup (),
  82 but instead of using the lowest-numbered unused file descriptor,
  83 it uses the file descriptor number specified in
  84 .IR newfd .
  85 If the file descriptor
  86 .IR newfd
  87 was previously open, it is silently closed before being reused.
  88 .PP
  89 The steps of closing and reusing the file descriptor
  90 .IR newfd
  91 are performed
  92 .IR atomically .
  93 This is important, because trying to implement equivalent functionality using
  94 .BR close (2)
  95 and
  96 .BR dup ()
  97 would be
  98 subject to race conditions, whereby
  99 .I newfd
 100 might be reused between the two steps.
 101 Such reuse could happen because the main program is interrupted
 102 by a signal handler that allocates a file descriptor,
 103 or because a parallel thread allocates a file descriptor.
 104 .PP
 105 Note the following points:
 106 .IP * 3
 107 If
 108 .I oldfd
 109 is not a valid file descriptor, then the call fails, and
 110 .I newfd
 111 is not closed.
 112 .IP *
 113 If
 114 .I oldfd
 115 is a valid file descriptor, and
 116 .I newfd
 117 has the same value as
 118 .IR oldfd ,
 119 then
 120 .BR dup2 ()
 121 does nothing, and returns
 122 .IR newfd .
 123 .\"
 124 .SS dup3()
 125 .BR dup3 ()
 126 is the same as
 127 .BR dup2 (),
 128 except that:
 129 .IP * 3
 130 The caller can force the close-on-exec flag to be set
 131 for the new file descriptor by specifying
 132 .BR O_CLOEXEC
 133 in
 134 .IR flags .
 135 See the description of the same flag in
 136 .BR open (2)
 137 for reasons why this may be useful.
 138 .IP *
 139 .\" Ulrich Drepper, LKML, 2008-10-09:
 140 .\"     We deliberately decided on this change.  Otherwise, what is the
 141 .\"     result of dup3(fd, fd, O_CLOEXEC)?
 142 If
 143 .IR oldfd
 144 equals
 145 .IR newfd ,
 146 then
 147 .BR dup3 ()
 148 fails with the error
 149 .BR EINVAL .
 150 .SH RETURN VALUE
 151 On success, these system calls
 152 return the new file descriptor.
 153 On error, \-1 is returned, and
 154 .I errno
 155 is set appropriately.
 156 .SH ERRORS
 157 .TP
 158 .B EBADF
 159 .I oldfd
 160 isn't an open file descriptor.
 161 .TP
 162 .B EBADF
 163 .I newfd
 164 is out of the allowed range for file descriptors (see the discussion of
 165 .BR RLIMIT_NOFILE
 166 in
 167 .BR getrlimit (2)).
 168 .TP
 169 .B EBUSY
 170 (Linux only) This may be returned by
 171 .BR dup2 ()
 172 or
 173 .BR dup3 ()
 174 during a race condition with
 175 .BR open (2)
 176 and
 177 .BR dup ().
 178 .TP
 179 .B EINTR
 180 The
 181 .BR dup2 ()
 182 or
 183 .BR dup3 ()
 184 call was interrupted by a signal; see
 185 .BR signal (7).
 186 .TP
 187 .B EINVAL
 188 .RB ( dup3 ())
 189 .I flags
 190 contain an invalid value.
 191 .TP
 192 .B EINVAL
 193 .RB ( dup3 ())
 194 .I oldfd
 195 was equal to
 196 .IR newfd .
 197 .TP
 198 .B EMFILE
 199 The per-process limit on the number of open file descriptors has been reached
 200 (see the discussion of
 201 .BR RLIMIT_NOFILE
 202 in
 203 .BR getrlimit (2)).
 204 .SH VERSIONS
 205 .BR dup3 ()
 206 was added to Linux in version 2.6.27;
 207 glibc support is available starting with
 208 version 2.9.
 209 .SH CONFORMING TO
 210 .BR dup (),
 211 .BR dup2 ():
 212 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 213 .PP
 214 .BR dup3 ()
 215 is Linux-specific.
 216 .\" SVr4 documents additional
 217 .\" EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
 218 .\" The EBUSY return is Linux-specific.
 219 .SH NOTES
 220 The error returned by
 221 .BR dup2 ()
 222 is different from that returned by
 223 .BR fcntl( "..., " F_DUPFD ", ..." )
 224 when
 225 .I newfd
 226 is out of range.
 227 On some systems,
 228 .BR dup2 ()
 229 also sometimes returns
 230 .B EINVAL
 231 like
 232 .BR F_DUPFD .
 233 .PP
 234 If
 235 .I newfd
 236 was open, any errors that would have been reported at
 237 .BR close (2)
 238 time are lost.
 239 If this is of concern,
 240 then\(emunless the program is single-threaded and does not allocate
 241 file descriptors in signal handlers\(emthe correct approach is
 242 .I not
 243 to close
 244 .I newfd
 245 before calling
 246 .BR dup2 (),
 247 because of the race condition described above.
 248 Instead, code something like the following could be used:
 249 .PP
 250 .EX
 251     /* Obtain a duplicate of 'newfd' that can subsequently
 252        be used to check for close() errors; an EBADF error
 253        means that 'newfd' was not open. */
 254
 255     tmpfd = dup(newfd);
 256     if (tmpfd == \-1 && errno != EBADF) {
 257         /* Handle unexpected dup() error */
 258     }
 259
 260     /* Atomically duplicate 'oldfd' on 'newfd' */
 261
 262     if (dup2(oldfd, newfd) == \-1) {
 263         /* Handle dup2() error */
 264     }
 265
 266     /* Now check for close() errors on the file originally
 267        referred to by 'newfd' */
 268
 269     if (tmpfd != \-1) {
 270         if (close(tmpfd) == \-1) {
 271             /* Handle errors from close */
 272         }
 273     }
 274 .EE
 275 .SH SEE ALSO
 276 .BR close (2),
 277 .BR fcntl (2),
 278 .BR open (2),
 279 .BR pidfd_getfd (2)