man2/access.2

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   3 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   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 by Michael Chastain (mec@shell.portal.com):
  28 .\"   Removed note about old kernel (pre-1.1.44) using wrong id on path.
  29 .\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
  30 .\"   Stated more clearly how it behaves with symbolic links.
  31 .\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
  32 .\" Modified 1996-09-07 by Michael Haardt:
  33 .\"   Restrictions for NFS
  34 .\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
  35 .\" Modified 1998-01-13 by Michael Haardt:
  36 .\"   Using access is often insecure
  37 .\" Modified 2001-10-16 by aeb
  38 .\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
  39 .\" Modified 2004-06-23 by Michael Kerrisk
  40 .\"
  41 .TH ACCESS 2 2004-06-23 "Linux" "Linux Programmer's Manual"
  42 .SH NAME
  43 access \- check user's permissions for a file
  44 .SH SYNOPSIS
  45 .nf
  46 .B #include <unistd.h>
  47 .sp
  48 .BI "int access(const char *" pathname ", int " mode );
  49 .fi
  50 .SH DESCRIPTION
  51 .B access
  52 checks whether the process would be allowed to read,
  53 write or test for existence of the file (or other file system
  54 object) whose name is
  55 .IR pathname .
  56 If
  57 .I pathname
  58 is a symbolic link permissions of the file referred to by this
  59 symbolic link are tested.
  60
  61 .I mode
  62 is a mask consisting of one or more of
  63 .BR R_OK ", " W_OK ", " X_OK " and " F_OK .
  64
  65 .BR R_OK ", " W_OK " and " X_OK
  66 request checking whether the file exists and has read, write and
  67 execute permissions, respectively.
  68 .B F_OK
  69 just requests checking for the existence of the file.
  70
  71 The tests depend on the permissions of the directories
  72 occurring in the path to the file, as given in
  73 .IR pathname ,
  74 and on the permissions of directories and files referred to by symbolic
  75 links encountered on the way.
  76
  77 The check is done with the process's
  78 .I real
  79 UID and GID, rather than with the effective IDs as is done when
  80 actually attempting an operation.
  81 This is to allow set-user-ID programs to
  82 easily determine the invoking user's authority.
  83
  84 Only access bits are checked, not the file type or contents.  Therefore, if
  85 a directory is found to be "writable," it probably means that files can be
  86 created in the directory, and not that the directory can be written as a
  87 file.  Similarly, a DOS file may be found to be "executable," but the
  88 .BR execve (2)
  89 call will still fail.
  90
  91 If the process has appropriate privileges, an implementation may
  92 indicate success for
  93 .B X_OK
  94 even if none of the execute file permission bits are set.
  95 .SH "RETURN VALUE"
  96 On success (all requested permissions granted), zero is returned.
  97 On error (at least one bit in
  98 .I mode
  99 asked for a permission that is denied, or some other error occurred),
 100 \-1 is returned, and
 101 .I errno
 102 is set appropriately.
 103 .SH ERRORS
 104 .B access
 105 shall fail if:
 106 .TP
 107 .B EACCES
 108 The requested access would be denied to the file or search permission
 109 is denied for one of the directories in the path prefix of
 110 .IR pathname .
 111 (See also
 112 .BR path_resolution (2).)
 113 .TP
 114 .B ELOOP
 115 Too many symbolic links were encountered in resolving
 116 .IR pathname .
 117 .TP
 118 .B ENAMETOOLONG
 119 .I pathname
 120 is too long.
 121 .TP
 122 .B ENOENT
 123 A directory component in
 124 .I pathname
 125 would have been accessible but does not exist or was a dangling
 126 symbolic link.
 127 .TP
 128 .B ENOTDIR
 129 A component used as a directory in
 130 .I pathname
 131 is not, in fact, a directory.
 132 .TP
 133 .B EROFS
 134 Write permission was requested for a file on a read-only filesystem.
 135 .PP
 136 .B access
 137 may fail if:
 138 .TP
 139 .B EFAULT
 140 .I pathname
 141 points outside your accessible address space.
 142 .TP
 143 .B EINVAL
 144 .I mode
 145 was incorrectly specified.
 146 .TP
 147 .B EIO
 148 An I/O error occurred.
 149 .TP
 150 .B ENOMEM
 151 Insufficient kernel memory was available.
 152 .TP
 153 .B ETXTBSY
 154 Write access was requested to an executable which is being
 155 executed.
 156 .SH RESTRICTIONS
 157 .B access
 158 returns an error if any of the access types in the requested call
 159 fails, even if other types might be successful.
 160 .PP
 161 .B access
 162 may not work correctly on NFS file systems with UID mapping enabled,
 163 because UID mapping is done on the server and hidden from the client,
 164 which checks permissions.
 165 .PP
 166 Using
 167 .B access
 168 to check if a user is authorized to e.g. open a file before actually
 169 doing so using
 170 .BR open (2)
 171 creates a security hole, because the user might exploit the short time
 172 interval between checking and opening the file to manipulate it.
 173 .SH "CONFORMING TO"
 174 SVID, AT&T, POSIX, X/OPEN, 4.3BSD
 175 .SH "SEE ALSO"
 176 .BR chmod (2),
 177 .BR chown (2),
 178 .BR open (2),
 179 .BR path_resolution (2),
 180 .BR setgid (2),
 181 .BR setuid (2),
 182 .BR stat (2)