man2/openat.2

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  26 .TH OPENAT 2 2006-03-06 "Linux 2.6.16" "Linux Programmer's Manual"
  27 .SH NAME
  28 openat \- open a file relative to a directory file descriptor
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <fcntl.h>
  32 .sp
  33 .BI "int openat(int " dirfd ", const char *" pathname ", int " flags );
  34 .BI "int openat(int " dirfd ", const char *" pathname ", int " flags \
  35 ", mode_t " mode );
  36 .fi
  37 .SH DESCRIPTION
  38 The
  39 .BR openat ()
  40 system call operates in exactly the same way as
  41 .BR open (2),
  42 except for the differences described in this manual page.
  43
  44 If the pathname given in
  45 .I pathname
  46 is relative, then it is interpreted relative to the directory
  47 referred to by the file descriptor
  48 .IR dirfd
  49 (rather than relative to the current working directory of
  50 the calling process, as is done by
  51 .BR open (2)
  52 for a relative pathname).
  53
  54 If the pathname given in
  55 .I pathname
  56 is relative and
  57 .I dirfd
  58 is the special value
  59 .BR AT_FDCWD ,
  60 then
  61 .I pathname
  62 is interpreted relative to the current working
  63 directory of the calling process (like
  64 .BR open (2)).
  65
  66 If the pathname given in
  67 .IR pathname
  68 is absolute, then
  69 .I dirfd
  70 is ignored.
  71 .SH "RETURN VALUE"
  72 On success,
  73 .BR openat ()
  74 returns a new file descriptor.
  75 On error, \-1 is returned and
  76 .I errno
  77 is set to indicate the error.
  78 .SH ERRORS
  79 The same errors that occur for
  80 .BR open (2)
  81 can also occur for
  82 .BR openat ().
  83 The following additional errors can occur for
  84 .BR openat ():
  85 .TP
  86 .B EBADF
  87 .I dirfd
  88 is not a valid file descriptor.
  89 .TP
  90 .B ENOTDIR
  91 .I pathname
  92 is a relative path and
  93 .I dirfd
  94 is a file descriptor referring to a file other than a directory.
  95 .SH NOTES
  96 .BR openat ()
  97 and other similar system calls suffixed "at" are supported
  98 for two reasons.
  99
 100 First,
 101 .BR openat ()
 102 allows an application to avoid race conditions that could
 103 occur when using
 104 .BR open (2)
 105 to open files in directories other than the current working directory.
 106 These race conditions result from the fact that some component
 107 of the directory prefix given to
 108 .BR open ()
 109 could be changed in parallel with the call to
 110 .BR open ().
 111 Such races can be avoided by
 112 opening a file descriptor for the target directory,
 113 and then specifying that file descriptor as the
 114 .I dirfd
 115 argument of
 116 .BR openat ().
 117
 118 Second,
 119 .BR openat ()
 120 allows the implementation of a per-thread "current working
 121 directory", via file descriptor(s) maintained by the application.
 122 (This functionality can also be obtained by tricks based
 123 on the use of
 124 .IR /proc/self/fd/ dirfd,
 125 but less efficiently.)
 126 .SH "CONFORMING TO"
 127 This system call is non-standard but is proposed
 128 for inclusion in a future revision of POSIX.1.
 129 A similar system call exists on Solaris.
 130 .\" The 'at' suffix in Solaris is actually double sensed.  It
 131 .\" primarily referred to "extended *at*tributes", which are
 132 .\" handled by Solaris' O_XATTR flag, but was also intended
 133 .\" to refer to the notion of "at a relative location".
 134 .\"
 135 .\" See the following for a discussion of the inconsistent
 136 .\" naming of the *at() functions:
 137 .\" http://www.opengroup.org/austin/mailarchives/ag/msg09103.html
 138 .\" Subject:    RE: The naming of at()s is a difficult matter
 139 .\" From:       Don Cragun
 140 .\" Date:       Tue, 14 Feb 2006 14:56:50 -0800 (PST)
 141 .\"
 142 .SH VERSIONS
 143 .BR openat ()
 144 was added to Linux in kernel 2.6.16.
 145 .SH "SEE ALSO"
 146 .BR linkat (2),
 147 .BR mkdirat (2),
 148 .BR mknodat (2),
 149 .BR open (2),
 150 .BR path_resolution (2),
 151 .BR renameat (2),
 152 .BR symlinkat (2),
 153 .BR unlinkat (2),
 154 .BR mkfifoat (3).
 155 .\" FIXME . eventually list other *at.2 pages here?