libc/sys/access.2

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.\"     @(#)access.2	8.2 (Berkeley) 4/1/94
.\" $FreeBSD: src/lib/libc/sys/access.2,v 1.7.2.5 2002/10/12 22:02:53 rwatson Exp $
.\" $DragonFly: src/lib/libc/sys/access.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
.\"
.Dd August 23, 2009
.Dt ACCESS 2
.Os
.Sh NAME
.Nm access ,
.Nm faccessat
.Nd check access permissions of a file or pathname
.Sh LIBRARY
.Lb libc
.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 flags"
.Sh DESCRIPTION
The
.Fn access
function checks the accessibility of the
file named by
.Fa path
for the access permissions indicated by
.Fa mode .
The value of
.Fa mode
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
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 .
.Pp
The
.Fn faccessat
function
operates as the
.Fn access
function but uses either the effective and group IDs or real ones depending on
the value of
.Fa flags .
.Pp
The values for the
.Fa flags
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
Use effective user and group IDs.
.El
.Pp
If
.Fa path
specifies a relative path the file to access is determined relative to the
directory associated with the file descriptor
.Fa fd
instead of the current working directory.
If
.Fa fd
is the special value
.Dv AT_FDCWD
the current working directory is used and the behavior is identical to a call to
.Fn access .
.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
The
.Fn access
and
.Fn faccessat
functions can fail with:
.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.  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 EIO
An I/O error occurred while reading from or writing to the file system.
.El
.Pp
In addition the
.Fn faccessat
function can fail with:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa fd
is not a valid file descriptor.
.It Bq Er ENOTDIR
.Fa path
is relative and
.Fa fd
does not point to a directory.
.It Bq Er EINVAL
.Fa flags
contains unsupported flags.
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr stat 2
.Sh STANDARDS
The
.Fn access
function call is expected to conform to
.St -p1003.1-90 .
.Pp
The
.Fn faccessat
function call is expected to conform to
.St -p1003.1-2008 .
.Sh CAVEAT
.Fn Access
is a potential security hole due to race conditions and
should never be used.
Setuid and setgid applications should either use the
.Fn faccessat
function or restore the effective uid or gid
and perform actions directly rather than use
.Fn access
to simulate access checks for the real user of group id.
.Sh HISTORY
An
.Fn access
function call appeared in
.At v7 .
.Pp
The
.Fn faccessat
system call appeared in
.Dx 2.3 .