path: root/lib/libc/sys/access.2

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.\"     @(#)access.2	8.2 (Berkeley) 4/1/94
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.Dd $Mdocdate: March 30 2013 $
.Dt ACCESS 2
.Os
.Sh NAME
.Nm access ,
.Nm faccessat
.Nd check access permissions of a file or pathname
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn access "const char *path" "int amode"
.Fd #include <fcntl.h>
.Ft int
.Fn faccessat "int fd" "const char *path" "int amode" "int flag"
.Sh DESCRIPTION
The
.Fn access
function checks the accessibility of the file named by
.Fa path
for the access permissions indicated by
.Fa amode .
The value of
.Fa amode
is the bitwise inclusive
.Tn OR
of the access permissions to be checked
.Pf ( Dv R_OK
for read permission,
.Dv W_OK
for write permission, and
.Dv X_OK
for execute/search permission) or the existence test,
.Dv F_OK .
All components of the pathname
.Fa path
are checked for access permissions (including
.Dv F_OK ) .
.Pp
The real user ID is used in place of the effective user ID
and the real group access list
(including the real group ID) is
used in place of the effective ID for verifying permission.
.Pp
If the invoking process has superuser privileges,
.Fn access
will always indicate success for
.Dv R_OK
and
.Dv W_OK ,
regardless of the actual file permission bits.
Likewise, for
.Dv X_OK ,
if the file has any of the execute bits set and
.Fa path
is not a directory,
.Fn access
will indicate success.
.Pp
The
.Fn faccessat
function is equivalent to
.Fn access
except that where
.Fa path
specifies a relative path,
the file whose accessibility is checked is determined relative to
the directory associated with file descriptor
.Fa fd
instead of the current working directory.
.Pp
If
.Fn faccessat
is passed the special value
.Dv AT_FDCWD
(defined in
.In fcntl.h )
in the
.Fa fd
parameter, the current working directory is used.
If
.Fa flag
is also zero, the behavior is identical to a call to
.Fn access .
.Pp
Values for
.Fa flag
are constructed by bitwise-inclusive
.Tn OR Ns ing
flags from the following list defined in
.In fcntl.h :
.Pp
.Bl -tag -width AT_EACCESS -offset indent -compact
.It Dv AT_EACCESS
The checks for accessibility are performed using the effective user
and group IDs instead of the real user and group IDs.
.El
.Sh RETURN VALUES
If
.Fa path
cannot be found or if any of the desired access modes would not be granted,
then a \-1 value is returned; otherwise a 0 value is returned.
.Sh ERRORS
Access to the file is denied if:
.Bl -tag -width Er
.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.
The owner of a file has permission checked with respect to the
.Dq owner
read, write, and execute mode bits, members of the file's group other
than the owner have permission checked with respect to the
.Dq group
mode bits, and all others have permissions checked with respect to the
.Dq other
mode bits.
.It Bq Er EPERM
Write access has been requested and the named file has its immutable
flag set (see
.Xr chflags 2 ) .
.It Bq Er EFAULT
.Fa path
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.
.It Bq Er EINVAL
An invalid value was specified for
.Ar amode .
.El
.Pp
Additionally,
.Fn faccessat
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa path
argument specifies a relative path and the
.Fa fd
argument is neither
.Dv AT_FDCWD
nor a valid file descriptor.
.It Bq Er ENOTDIR
The
.Fa path
argument specifies a relative path and the
.Fa fd
argument is a valid file descriptor but it does not reference a directory.
.It Bq Er EACCES
The
.Fa path
argument specifies a relative path but search permission is denied
for the directory which the
.Fa fd
file descriptor references.
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr stat 2
.Sh STANDARDS
The
.Fn access
and
.Fn faccessat
functions conform to
.St -p1003.1-2008 .
.Sh HISTORY
.Fn access
first appeared as an internal kernel function in
.At v1
and was reimplemented in C before the release of
.At v4 .
It was first promoted to a system call in the Programmer's Workbench
(PWB/UNIX), which was later ported to
.At v7
and
.Bx 2 .
.Pp
The
.Fn faccessat
function appeared in
.Ox 5.0 .
.Sh AUTHORS
.An Ken Thompson
first implemented the
.Fn access
kernel function in C.
.Sh CAVEATS
.Fn access
and
.Fn faccessat
should never be used for actual access control.
Doing so can result in a time of check vs. time of use security hole.