libc/sys/access.2

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.\"     @(#)access.2	8.2 (Berkeley) 4/1/94
.\" $FreeBSD: release/9.0.0/lib/libc/sys/access.2 208027 2010-05-13 12:07:55Z uqs $
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.Dd October 30, 2012
.Dt ACCESS 2
.Os
.Sh NAME
.Nm access ,
.Nm eaccess ,
.Nm faccessat
.Nd check accessibility of a file
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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
.Pq Dv F_OK .
.Pp
For additional information, see the
.Sx "File Access Permission"
section of
.Xr intro 2 .
.Pp
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.
.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'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:
.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 255 characters,
or an entire path name exceeded 1023 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 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
.Dx 2.3 .
.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.
.Pp
.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 .