man2/openat.2

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  26 .TH OPENAT 2 2006-03-06 "Linux" "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 #define _ATFILE_SOURCE
  32 .B #include <fcntl.h>
  33 .sp
  34 .BI "int openat(int " dirfd ", const char *" pathname ", int " flags );
  35 .BI "int openat(int " dirfd ", const char *" pathname ", int " flags \
  36 ", mode_t " mode );
  37 .fi
  38 .SH DESCRIPTION
  39 The
  40 .BR openat ()
  41 system call operates in exactly the same way as
  42 .BR open (2),
  43 except for the differences described in this manual page.
  44
  45 If the pathname given in
  46 .I pathname
  47 is relative, then it is interpreted relative to the directory
  48 referred to by the file descriptor
  49 .I dirfd
  50 (rather than relative to the current working directory of
  51 the calling process, as is done by
  52 .BR open (2)
  53 for a relative pathname).
  54
  55 If
  56 .I pathname
  57 is relative and
  58 .I dirfd
  59 is the special value
  60 .BR AT_FDCWD ,
  61 then
  62 .I pathname
  63 is interpreted relative to the current working
  64 directory of the calling process (like
  65 .BR open (2)).
  66
  67 If
  68 .I pathname
  69 is absolute, then
  70 .I dirfd
  71 is ignored.
  72 .SH "RETURN VALUE"
  73 On success,
  74 .BR openat ()
  75 returns a new file descriptor.
  76 On error, \-1 is returned and
  77 .I errno
  78 is set to indicate the error.
  79 .SH ERRORS
  80 The same errors that occur for
  81 .BR open (2)
  82 can also occur for
  83 .BR openat ().
  84 The following additional errors can occur for
  85 .BR openat ():
  86 .TP
  87 .B EBADF
  88 .I dirfd
  89 is not a valid file descriptor.
  90 .TP
  91 .B ENOTDIR
  92 .I pathname
  93 is relative and
  94 .I dirfd
  95 is a file descriptor referring to a file other than a directory.
  96 .SH VERSIONS
  97 .BR openat ()
  98 was added to Linux in kernel 2.6.16.
  99 .SH "CONFORMING TO"
 100 This system call is non-standard but is proposed
 101 for inclusion in a future revision of POSIX.1.
 102 A similar system call exists on Solaris.
 103 .\" The 'at' suffix in Solaris is actually double sensed.  It
 104 .\" primarily referred to "extended *at*tributes", which are
 105 .\" handled by Solaris' O_XATTR flag, but was also intended
 106 .\" to refer to the notion of "at a relative location".
 107 .\"
 108 .\" See the following for a discussion of the inconsistent
 109 .\" naming of the *at() functions:
 110 .\" http://www.opengroup.org/austin/mailarchives/ag/msg09103.html
 111 .\" Subject:    RE: The naming of at()s is a difficult matter
 112 .\" From:       Don Cragun
 113 .\" Date:       Tue, 14 Feb 2006 14:56:50 -0800 (PST)
 114 .\"
 115 .SH NOTES
 116 .BR openat ()
 117 and other similar system calls suffixed "at" are supported
 118 for two reasons.
 119
 120 First,
 121 .BR openat ()
 122 allows an application to avoid race conditions that could
 123 occur when using
 124 .BR open (2)
 125 to open files in directories other than the current working directory.
 126 These race conditions result from the fact that some component
 127 of the directory prefix given to
 128 .BR open (2)
 129 could be changed in parallel with the call to
 130 .BR open (2).
 131 Such races can be avoided by
 132 opening a file descriptor for the target directory,
 133 and then specifying that file descriptor as the
 134 .I dirfd
 135 argument of
 136 .BR openat ().
 137
 138 Second,
 139 .BR openat ()
 140 allows the implementation of a per-thread "current working
 141 directory", via file descriptor(s) maintained by the application.
 142 (This functionality can also be obtained by tricks based
 143 on the use of
 144 .IR /proc/self/fd/ dirfd,
 145 but less efficiently.)
 146 .SH "SEE ALSO"
 147 .BR faccessat (2),
 148 .BR fchmodat (2),
 149 .BR fchownat (2),
 150 .BR fstatat (2),
 151 .BR futimesat (2),
 152 .BR linkat (2),
 153 .BR mkdirat (2),
 154 .BR mknodat (2),
 155 .BR open (2),
 156 .BR readlinkat (2),
 157 .BR renameat (2),
 158 .BR symlinkat (2),
 159 .BR unlinkat (2),
 160 .BR mkfifoat (3)
 161 .BR path_resolution (7)