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.\"     @(#)access.2	8.2 (Berkeley) 4/1/94
.\" $FreeBSD$
.\"
.Dd September 15, 2014
.Dt ACCESS 2
.Os
.Sh NAME
.Nm access ,
.Nm faccessat
.Nd check accessibility of a file
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn access "const char *path" "int mode"
.Ft int
.Fn faccessat "int fd" "const char *path" "int mode" "int flag"
.Sh DESCRIPTION
The
.Fn access
system call checks 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
.Pq Dv F_OK .
.Pp
For additional information, see the
.Sx "File Access Permission"
section of
.Xr intro 2 .
.Pp
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.
.Pp
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
.In fcntl.h :
.Bl -tag -width indent
.It 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
.Bl -tag -width indent
.It Dv AT_SYMLINK_NOFOLLOW
If
.Fa path
names a symbolic link, the status of the symbolic link is returned.
.El
.Pp
Even if a process 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
.Fn access
or
.Fn faccessat
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value of the
.Fa mode
argument is invalid.
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 
.Dv {NAME_MAX}
characters, or an entire path name exceeded 
.Dv {PATH_MAX}
characters.
.It Bq Er ENOENT
The named file does not exist.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EROFS
Write access is requested for a file on a read-only file system.
.It Bq Er ETXTBSY
Write access is requested for a pure procedure (shared text)
file presently being executed.
.It 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.
.It Bq Er EFAULT
The
.Fa path
argument
points outside the process's allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.El
.Pp
Also, the
.Fn faccessat
system call may fail if:
.Bl -tag -width Er
.It 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.
.It Bq Er EINVAL
The value of the
.Fa flag
argument is not valid.
.It 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 is expected to conform to POSIX.1-2008 .
.Sh HISTORY
The
.Fn access
function appeared in
.At v7 .
.Sh SECURITY CONSIDERATIONS
The result of
.Fn access
should not be used to make an actual access control decision, since its
response, even if correct at the moment it is formed, may be outdated at the
time you act on it.
.Fn access
results should only be used to pre-flight, such as when configuring user
interface elements or for optimization purposes.  The actual access control
decision should be made by attempting to execute the relevant system call while
holding the applicable credentials, and properly handling any resulting errors;
and this must be done even though
.Fn access
may have predicted success.
.Pp
Additionally, 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.