man2/access.2

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   3 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   4 .\"                               1993 Michael Haardt, Ian Jackson.
   5 .\" and Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
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  27 .\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu)
  28 .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
  29 .\"   Removed note about old kernel (pre-1.1.44) using wrong id on path.
  30 .\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
  31 .\"   Stated more clearly how it behaves with symbolic links.
  32 .\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
  33 .\" Modified 1996-09-07 by Michael Haardt:
  34 .\"   Restrictions for NFS
  35 .\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
  36 .\" Modified 1998-01-13 by Michael Haardt:
  37 .\"   Using access is often insecure
  38 .\" Modified 2001-10-16 by aeb
  39 .\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
  40 .\" Modified 2004-06-23 by Michael Kerrisk
  41 .\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
  42 .\"
  43 .TH ACCESS 2 2007-07-10 "Linux" "Linux Programmer's Manual"
  44 .SH NAME
  45 access \- check real user's permissions for a file
  46 .SH SYNOPSIS
  47 .nf
  48 .B #include <unistd.h>
  49 .sp
  50 .BI "int access(const char *" pathname ", int " mode );
  51 .fi
  52 .SH DESCRIPTION
  53 .BR access ()
  54 checks whether the calling process can access the file
  55 .IR pathname .
  56 If
  57 .I pathname
  58 is a symbolic link, it is dereferenced.
  59
  60 The
  61 .I mode
  62 specifies the accessibility check(s) to be performed,
  63 and is either the value
  64 .BR F_OK ,
  65 or a mask consisting of the bitwise OR of one or more of
  66 .BR R_OK ", " W_OK ", and " X_OK .
  67 .\" FIXME . the kernel currently allows F_OK to be specified
  68 .\" in conjunction with any of the other flags.
  69 .\" Test whether other implementations permit this and if not
  70 .\" file a bug report.
  71 .B F_OK
  72 tests for the existence of the file.
  73 .BR R_OK ", " W_OK ", and " X_OK
  74 test whether the file exists and grants read, write, and
  75 execute permissions, respectively.
  76
  77 The check is done using the calling process's
  78 .I real
  79 UID and GID, rather than the effective IDs as is done when
  80 actually attempting an operation (e.g.,
  81 .BR open (2))
  82 on the file.
  83 This allows set-user-ID programs to
  84 easily determine the invoking user's authority.
  85
  86 If the calling process is privileged (i.e., its real UID is zero),
  87 then an
  88 .B X_OK
  89 check is successful for a regular file if execute permission
  90 is enabled for any of the file owner, group, or other.
  91 .SH "RETURN VALUE"
  92 On success (all requested permissions granted), zero is returned.
  93 On error (at least one bit in
  94 .I mode
  95 asked for a permission that is denied, or some other error occurred),
  96 \-1 is returned, and
  97 .I errno
  98 is set appropriately.
  99 .SH ERRORS
 100 .BR access ()
 101 shall fail if:
 102 .TP
 103 .B EACCES
 104 The requested access would be denied to the file, or search permission
 105 is denied for one of the directories in the path prefix of
 106 .IR pathname .
 107 (See also
 108 .BR path_resolution (7).)
 109 .TP
 110 .B ELOOP
 111 Too many symbolic links were encountered in resolving
 112 .IR pathname .
 113 .TP
 114 .B ENAMETOOLONG
 115 .I pathname
 116 is too long.
 117 .TP
 118 .B ENOENT
 119 A component of
 120 .I pathname
 121 does not exist or is a dangling symbolic link.
 122 .TP
 123 .B ENOTDIR
 124 A component used as a directory in
 125 .I pathname
 126 is not, in fact, a directory.
 127 .TP
 128 .B EROFS
 129 Write permission was requested for a file on a read-only file system.
 130 .PP
 131 .BR access ()
 132 may fail if:
 133 .TP
 134 .B EFAULT
 135 .I pathname
 136 points outside your accessible address space.
 137 .TP
 138 .B EINVAL
 139 .I mode
 140 was incorrectly specified.
 141 .TP
 142 .B EIO
 143 An I/O error occurred.
 144 .TP
 145 .B ENOMEM
 146 Insufficient kernel memory was available.
 147 .TP
 148 .B ETXTBSY
 149 Write access was requested to an executable which is being
 150 executed.
 151 .SH "CONFORMING TO"
 152 SVr4, 4.3BSD, POSIX.1-2001.
 153 .SH NOTES
 154 .PP
 155 .BR Warning :
 156 Using
 157 .BR access ()
 158 to check if a user is authorized to, for example,
 159 open a file before actually doing so using
 160 .BR open (2)
 161 creates a security hole, because the user might exploit the short time
 162 interval between checking and opening the file to manipulate it.
 163 .BR "For this reason, the use of this system call should be avoided" .
 164 .PP
 165 .BR access ()
 166 returns an error if any of the access types in
 167 .I mode
 168 is denied, even if some of the other access types in
 169 .I mode
 170 are permitted.
 171 .PP
 172 If the calling process has appropriate privileges (i.e., is superuser),
 173 POSIX.1-2001 permits implementation to indicate success for an
 174 .B X_OK
 175 check even if none of the execute file permission bits are set.
 176 .\" HPU-UX 11 and Tru64 5.1 do this.
 177 Linux does not do this.
 178 .PP
 179 A file is only accessible if the permissions on each of the
 180 directories in the path prefix of
 181 .I pathname
 182 grant search (i.e., execute) access.
 183 If any directory is inaccessible, then the
 184 .BR access ()
 185 call will fail, regardless of the permissions on the file itself.
 186 .PP
 187 Only access bits are checked, not the file type or contents.
 188 Therefore, if a directory is found to be writable,
 189 it probably means that files can be created in the directory,
 190 and not that the directory can be written as a file.
 191 Similarly, a DOS file may be found to be "executable," but the
 192 .BR execve (2)
 193 call will still fail.
 194 .PP
 195 .BR access ()
 196 may not work correctly on NFS file systems with UID mapping enabled,
 197 because UID mapping is done on the server and hidden from the client,
 198 which checks permissions.
 199 .SH BUGS
 200 In kernel 2.4 (and earlier) there is some strangeness in the handling of
 201 .B X_OK
 202 tests for superuser.
 203 If all categories of execute permission are disabled
 204 for a non-directory file, then the only
 205 .BR access ()
 206 test that returns \-1 is when
 207 .I mode
 208 is specified as just
 209 .BR X_OK ;
 210 if
 211 .B R_OK
 212 or
 213 .B W_OK
 214 is also specified in
 215 .IR mode ,
 216 then
 217 .BR access ()
 218 returns 0 for such files.
 219 .\" This behavior appears to have been an implementation accident.
 220 Early 2.6 kernels (up to and including 2.6.3)
 221 also behaved in the same way as kernel 2.4.
 222
 223 In kernels before 2.6.20,
 224 .BR access ()
 225 ignored the effect of the
 226 .B MS_NOEXEC
 227 flag if it was used to
 228 .BR mount (2)
 229 the underlying file system.
 230 Since kernel 2.6.20,
 231 .BR access ()
 232 honors this flag.
 233 .SH "SEE ALSO"
 234 .BR chmod (2),
 235 .BR chown (2),
 236 .BR faccessat (2),
 237 .BR open (2),
 238 .BR setgid (2),
 239 .BR setuid (2),
 240 .BR stat (2),
 241 .BR euidaccess (3),
 242 .BR credentials (7),
 243 .BR path_resolution (7)