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.\"	$NetBSD: access.2,v 1.7 1995/02/27 12:31:44 cgd Exp $
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.\"     @(#)access.2	8.2 (Berkeley) 4/1/94
.\"
.Dd April 1, 1994
.Dt ACCESS 2
.Os BSD 4
.Sh NAME
.Nm access ,
.Nm faccessat
.Nd check access permissions of a file or pathname
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fo access
.Fa "const char *path"
.Fa "int amode"
.Fc
.Ft int
.Fn faccessat "int fd" "const char *path" "int mode" "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 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) are
used in place of the effective ID for verifying permission.
.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
.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
If
.Fa path
cannot be found
or if any of the desired access modes would not be granted,
then a -1 value is returned and the global integer variable
.Va errno
is set to indicate the error.
Otherwise, a 0 value is returned.
.Sh ERRORS
Access to the file is denied if:
.Bl -tag -width Er
.\" ==========
.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.
.Pp
The owner of a file has permission checked
with respect to the ``owner'' read, write, and execute mode bits,
members of the file's group other than the owner have permission checked
with respect to the ``group'' mode bits,
and all others have permissions checked
with respect to the ``other'' mode bits.
.\" 
.\" ==========
.It Bq Er EFAULT
.Fa Path
points outside the process's allocated address space.
.It Bq Er EINVAL
An invalid value was specified for
.Ar amode .
.\" ==========
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.\" ==========
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.\" ==========
.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 ENOTDIR
A component of the path prefix is not a directory.
.\" ==========
.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 that is presently being executed.
.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 stat 2
.Sh STANDARDS
The
.Fn access
function conforms to 
.St -p1003.1-90 .
The
.Fn faccessat
system call is expected to conform to POSIX.1-2008 .
.Sh CAVEAT
.Fn access
is a potential security hole and
should never be used.