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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.\" 4. 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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