7e6cabd06e
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
244 lines
6.3 KiB
Groff
244 lines
6.3 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)access.2 8.2 (Berkeley) 4/1/94
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.\" $FreeBSD$
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.\"
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.Dd September 15, 2014
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.Dt ACCESS 2
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.Os
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.Sh NAME
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.Nm access ,
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.Nm eaccess ,
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.Nm faccessat
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.Nd check accessibility of a file
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft int
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.Fn access "const char *path" "int mode"
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.Ft int
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.Fn eaccess "const char *path" "int mode"
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.Ft int
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.Fn faccessat "int fd" "const char *path" "int mode" "int flag"
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.Sh DESCRIPTION
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The
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.Fn access
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and
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.Fn eaccess
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system calls check the accessibility of the
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file named by
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the
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.Fa path
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argument
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for the access permissions indicated by
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the
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.Fa mode
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argument.
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The value of
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.Fa mode
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is either the bitwise-inclusive OR of the access permissions to be
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checked
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.Dv ( R_OK
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for read permission,
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.Dv W_OK
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for write permission, and
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.Dv X_OK
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for execute/search permission),
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or the existence test
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.Pq Dv F_OK .
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.Pp
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For additional information, see the
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.Sx "File Access Permission"
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section of
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.Xr intro 2 .
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.Pp
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The
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.Fn eaccess
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system call uses
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the effective user ID and the group access list
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to authorize the request;
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the
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.Fn access
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system call uses
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the real user ID in place of the effective user ID,
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the real group ID in place of the effective group ID,
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and the rest of the group access list.
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.Pp
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The
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.Fn faccessat
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system call is equivalent to
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.Fn access
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except in the case where
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.Fa path
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specifies a relative path.
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In this case the file whose accessibility is to be determined is
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located relative to the directory associated with the file descriptor
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.Fa fd
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instead of the current working directory.
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If
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.Fn faccessat
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is passed the special value
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.Dv AT_FDCWD
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in the
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.Fa fd
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parameter, the current working directory is used and the behavior is
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identical to a call to
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.Fn access .
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Values for
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.Fa flag
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are constructed by a bitwise-inclusive OR of flags from the following
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list, defined in
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.In fcntl.h :
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.Bl -tag -width indent
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.It Dv AT_EACCESS
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The checks for accessibility are performed using the effective user and group
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IDs instead of the real user and group ID as required in a call to
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.Fn access .
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.El
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.Pp
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Even if a process's real or effective user has appropriate privileges
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and indicates success for
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.Dv X_OK ,
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the file may not actually have execute permission bits set.
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Likewise for
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.Dv R_OK
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and
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.Dv W_OK .
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.Sh RETURN VALUES
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.Rv -std
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.Sh ERRORS
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.Fn access ,
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.Fn eaccess ,
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or
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.Fn faccessat
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The value of the
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.Fa mode
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argument is invalid.
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.It Bq Er ENOTDIR
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A component of the path prefix is not a directory.
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.It Bq Er ENAMETOOLONG
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A component of a pathname exceeded 255 characters,
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or an entire path name exceeded 1023 characters.
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.It Bq Er ENOENT
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The named file does not exist.
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.It Bq Er ELOOP
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Too many symbolic links were encountered in translating the pathname.
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.It Bq Er EROFS
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Write access is requested for a file on a read-only file system.
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.It Bq Er ETXTBSY
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Write access is requested for a pure procedure (shared text)
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file presently being executed.
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.It Bq Er EACCES
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Permission bits of the file mode do not permit the requested
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access, or search permission is denied on a component of the
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path prefix.
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.It Bq Er EFAULT
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The
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.Fa path
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argument
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points outside the process's allocated address space.
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.It Bq Er EIO
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An I/O error occurred while reading from or writing to the file system.
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.El
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.Pp
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Also, the
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.Fn faccessat
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system call may fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa path
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argument does not specify an absolute path and the
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.Fa fd
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argument is
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neither
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.Dv AT_FDCWD
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nor a valid file descriptor.
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.It Bq Er EINVAL
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The value of the
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.Fa flag
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argument is not valid.
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.It Bq Er ENOTDIR
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The
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.Fa path
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argument is not an absolute path and
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.Fa fd
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is neither
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.Dv AT_FDCWD
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nor a file descriptor associated with a directory.
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.El
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.Sh SEE ALSO
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.Xr chmod 2 ,
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.Xr intro 2 ,
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.Xr stat 2
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.Sh STANDARDS
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The
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.Fn access
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system call is expected to conform to
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.St -p1003.1-90 .
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The
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.Fn faccessat
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system call follows The Open Group Extended API Set 2 specification.
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.Sh HISTORY
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The
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.Fn access
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function appeared in
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.At v7 .
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The
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.Fn faccessat
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system call appeared in
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.Fx 8.0 .
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.Sh SECURITY CONSIDERATIONS
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The
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.Fn access
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system call
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is a potential security hole due to race conditions and
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should never be used.
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Set-user-ID and set-group-ID applications should restore the
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effective user or group ID,
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and perform actions directly rather than use
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.Fn access
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to simulate access checks for the real user or group ID.
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The
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.Fn eaccess
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system call
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likewise may be subject to races if used inappropriately.
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.Pp
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.Fn access
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remains useful for providing clues to users as to whether operations
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make sense for particular filesystem objects (e.g. 'delete' menu
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item only highlighted in a writable folder ... avoiding interpretation
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of the st_mode bits that the application might not understand --
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e.g. in the case of AFS).
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It also allows a cheaper file existence test than
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.Xr stat 2 .
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