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@(#)access.2 8.2 (Berkeley) 4/1/94
$FreeBSD: stable/10/lib/libc/sys/access.2 208027 2010-05-13 12:07:55Z uqs $
.Dd April 10, 2008 .Dt ACCESS 2 .Os .Sh NAME .Nm access , .Nm eaccess , .Nm faccessat .Nd check accessibility of a file .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn access "const char *path" "int mode" .Ft int .Fn eaccess "const char *path" "int mode" .Ft int .Fn faccessat "int fd" "const char *path" "int mode" "int flag" .Sh DESCRIPTION The .Fn access and .Fn eaccess system calls check the accessibility of the file named by the .Fa path argument for the access permissions indicated by the .Fa mode argument. The value of .Fa mode is either the bitwise-inclusive OR of the access permissions to be checked .Dv ( R_OK for read permission, .Dv W_OK for write permission, and .Dv X_OK for execute/search permission), or the existence test
q Dv F_OK .
p For additional information, see the .Sx "File Access Permission" section of .Xr intro 2 .
p The .Fn eaccess system call uses the effective user ID and the group access list to authorize the request; the .Fn access system call uses the real user ID in place of the effective user ID, the real group ID in place of the effective group ID, and the rest of the group access list.
p The .Fn faccessat system call is equivalent to .Fn access except in the case where .Fa path specifies a relative path. In this case the file whose accessibility is to be determined is located relative to the directory associated with the file descriptor .Fa fd instead of the current working directory. If .Fn faccessat is passed the special value .Dv AT_FDCWD in the .Fa fd parameter, the current working directory is used and the behavior is identical to a call to .Fn access . Values for .Fa flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in n fcntl.h : l -tag -width indent t Dv AT_EACCESS The checks for accessibility are performed using the effective user and group IDs instead of the real user and group ID as required in a call to .Fn access . .El
p Even if a process's real or effective user has appropriate privileges and indicates success for .Dv X_OK , the file may not actually have execute permission bits set. Likewise for .Dv R_OK and .Dv W_OK . .Sh RETURN VALUES .Rv -std .Sh ERRORS Access to the file is denied if: l -tag -width Er t Bq Er ENOTDIR A component of the path prefix is not a directory. t Bq Er ENAMETOOLONG A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters. t Bq Er ENOENT The named file does not exist. t Bq Er ELOOP Too many symbolic links were encountered in translating the pathname. t Bq Er EROFS Write access is requested for a file on a read-only file system. t Bq Er ETXTBSY Write access is requested for a pure procedure (shared text) file presently being executed. t Bq Er EACCES Permission bits of the file mode do not permit the requested access, or search permission is denied on a component of the path prefix. t Bq Er EFAULT The .Fa path argument points outside the process's allocated address space. t Bq Er EIO An I/O error occurred while reading from or writing to the file system. .El
p Also, the .Fn faccessat system call may fail if: l -tag -width Er t Bq Er EBADF The .Fa path argument does not specify an absolute path and the .Fa fd argument is neither .Dv AT_FDCWD nor a valid file descriptor. t Bq Er EINVAL The value of the .Fa flag argument is not valid. t Bq Er ENOTDIR The .Fa path argument is not an absolute path and .Fa fd is neither .Dv AT_FDCWD nor a file descriptor associated with a directory. .El .Sh SEE ALSO .Xr chmod 2 , .Xr intro 2 , .Xr stat 2 .Sh STANDARDS The .Fn access system call is expected to conform to .St -p1003.1-90 . The .Fn faccessat system call follows The Open Group Extended API Set 2 specification. .Sh HISTORY The .Fn access function appeared in .At v7 . The .Fn faccessat system call appeared in .Fx 8.0 . .Sh SECURITY CONSIDERATIONS The .Fn access system call is a potential security hole due to race conditions and should never be used. Set-user-ID and set-group-ID applications should restore the effective user or group ID, and perform actions directly rather than use .Fn access to simulate access checks for the real user or group ID. The .Fn eaccess system call likewise may be subject to races if used inappropriately.
p .Fn access remains useful for providing clues to users as to whether operations make sense for particular filesystem objects (e.g. 'delete' menu item only highlighted in a writable folder ... avoiding interpretation of the st_mode bits that the application might not understand -- e.g. in the case of AFS). It also allows a cheaper file existence test than .Xr stat 2 .