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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)access.2 8.2 (Berkeley) 4/1/94 29.\" $FreeBSD: release/9.0.0/lib/libc/sys/access.2 208027 2010-05-13 12:07:55Z uqs $ 30.\" 31.Dd October 30, 2012 32.Dt ACCESS 2 33.Os 34.Sh NAME 35.Nm access , 36.Nm eaccess , 37.Nm faccessat 38.Nd check accessibility of a file 39.Sh LIBRARY 40.Lb libc 41.Sh SYNOPSIS 42.In unistd.h 43.Ft int 44.Fn access "const char *path" "int mode" 45.Ft int 46.Fn eaccess "const char *path" "int mode" 47.Ft int 48.Fn faccessat "int fd" "const char *path" "int mode" "int flag" 49.Sh DESCRIPTION 50The 51.Fn access 52and 53.Fn eaccess 54system calls check the accessibility of the 55file named by 56the 57.Fa path 58argument 59for the access permissions indicated by 60the 61.Fa mode 62argument. 63The value of 64.Fa mode 65is either the bitwise-inclusive OR of the access permissions to be 66checked 67.Dv ( R_OK 68for read permission, 69.Dv W_OK 70for write permission, and 71.Dv X_OK 72for execute/search permission), 73or the existence test 74.Pq Dv F_OK . 75.Pp 76For additional information, see the 77.Sx "File Access Permission" 78section of 79.Xr intro 2 . 80.Pp 81The 82.Fn eaccess 83system call uses 84the effective user ID and the group access list 85to authorize the request; 86the 87.Fn access 88system call uses 89the real user ID in place of the effective user ID, 90the real group ID in place of the effective group ID, 91and the rest of the group access list. 92.Pp 93The 94.Fn faccessat 95system call is equivalent to 96.Fn access 97except in the case where 98.Fa path 99specifies a relative path. 100In this case the file whose accessibility is to be determined is 101located relative to the directory associated with the file descriptor 102.Fa fd 103instead of the current working directory. 104If 105.Fn faccessat 106is passed the special value 107.Dv AT_FDCWD 108in the 109.Fa fd 110parameter, the current working directory is used and the behavior is 111identical to a call to 112.Fn access . 113Values for 114.Fa flag 115are constructed by a bitwise-inclusive OR of flags from the following 116list, defined in 117.In fcntl.h : 118.Bl -tag -width indent 119.It Dv AT_EACCESS 120The checks for accessibility are performed using the effective user and group 121IDs instead of the real user and group ID as required in a call to 122.Fn access . 123.El 124.Pp 125Even if a process's real or effective user has appropriate privileges 126and indicates success for 127.Dv X_OK , 128the file may not actually have execute permission bits set. 129Likewise for 130.Dv R_OK 131and 132.Dv W_OK . 133.Sh RETURN VALUES 134.Rv -std 135.Sh ERRORS 136Access to the file is denied if: 137.Bl -tag -width Er 138.It Bq Er ENOTDIR 139A component of the path prefix is not a directory. 140.It Bq Er ENAMETOOLONG 141A component of a pathname exceeded 255 characters, 142or an entire path name exceeded 1023 characters. 143.It Bq Er ENOENT 144The named file does not exist. 145.It Bq Er ELOOP 146Too many symbolic links were encountered in translating the pathname. 147.It Bq Er EROFS 148Write access is requested for a file on a read-only file system. 149.It Bq Er ETXTBSY 150Write access is requested for a pure procedure (shared text) 151file presently being executed. 152.It Bq Er EACCES 153Permission bits of the file mode do not permit the requested 154access, or search permission is denied on a component of the 155path prefix. 156.It Bq Er EFAULT 157The 158.Fa path 159argument 160points outside the process's allocated address space. 161.It Bq Er EIO 162An I/O error occurred while reading from or writing to the file system. 163.El 164.Pp 165Also, the 166.Fn faccessat 167system call may fail if: 168.Bl -tag -width Er 169.It Bq Er EBADF 170The 171.Fa path 172argument does not specify an absolute path and the 173.Fa fd 174argument is 175neither 176.Dv AT_FDCWD 177nor a valid file descriptor. 178.It Bq Er EINVAL 179The value of the 180.Fa flag 181argument is not valid. 182.It Bq Er ENOTDIR 183The 184.Fa path 185argument is not an absolute path and 186.Fa fd 187is neither 188.Dv AT_FDCWD 189nor a file descriptor associated with a directory. 190.El 191.Sh SEE ALSO 192.Xr chmod 2 , 193.Xr intro 2 , 194.Xr stat 2 195.Sh STANDARDS 196The 197.Fn access 198system call is expected to conform to 199.St -p1003.1-90 . 200The 201.Fn faccessat 202system call follows The Open Group Extended API Set 2 specification. 203.Sh HISTORY 204The 205.Fn access 206function appeared in 207.At v7 . 208The 209.Fn faccessat 210system call appeared in 211.Dx 2.3 . 212.Sh SECURITY CONSIDERATIONS 213The 214.Fn access 215system call 216is a potential security hole due to race conditions and 217should never be used. 218Set-user-ID and set-group-ID applications should restore the 219effective user or group ID, 220and perform actions directly rather than use 221.Fn access 222to simulate access checks for the real user or group ID. 223The 224.Fn eaccess 225system call 226likewise may be subject to races if used inappropriately. 227.Pp 228.Fn access 229remains useful for providing clues to users as to whether operations 230make sense for particular filesystem objects (e.g. 'delete' menu 231item only highlighted in a writable folder ... avoiding interpretation 232of the st_mode bits that the application might not understand -- 233e.g. in the case of AFS). 234It also allows a cheaper file existence test than 235.Xr stat 2 . 236