io_cancel.2: Improve description

[thirdparty/man-pages.git] / man2 / access.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"             and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu)
.\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
.\"   Removed note about old kernel (pre-1.1.44) using wrong id on path.
.\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
.\"   Stated more clearly how it behaves with symbolic links.
.\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
.\" Modified 1996-09-07 by Michael Haardt:
.\"   Restrictions for NFS
.\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
.\" Modified 1998-01-13 by Michael Haardt:
.\"   Using access is often insecure
.\" Modified 2001-10-16 by aeb
.\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
.\" Modified 2004-06-23 by Michael Kerrisk
.\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
.\"
.TH ACCESS 2 2013-02-28 "Linux" "Linux Programmer's Manual"
.SH NAME
access \- check real user's permissions for a file
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "int access(const char *" pathname ", int " mode );
.fi
.SH DESCRIPTION
.BR access ()
checks whether the calling process can access the file
.IR pathname .
If
.I pathname
is a symbolic link, it is dereferenced.

The
.I mode
specifies the accessibility check(s) to be performed,
and is either the value
.BR F_OK ,
.\" F_OK is defined as 0 on every system that I know of.
or a mask consisting of the bitwise OR of one or more of
.BR R_OK ", " W_OK ", and " X_OK .
.B F_OK
tests for the existence of the file.
.BR R_OK ", " W_OK ", and " X_OK
test whether the file exists and grants read, write, and
execute permissions, respectively.

The check is done using the calling process's
.I real
UID and GID, rather than the effective IDs as is done when
actually attempting an operation (e.g.,
.BR open (2))
on the file.
This allows set-user-ID programs to
easily determine the invoking user's authority.

If the calling process is privileged (i.e., its real UID is zero),
then an
.B X_OK
check is successful for a regular file if execute permission
is enabled for any of the file owner, group, or other.
.SH "RETURN VALUE"
On success (all requested permissions granted), zero is returned.
On error (at least one bit in
.I mode
asked for a permission that is denied, or some other error occurred),
\-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.BR access ()
shall fail if:
.TP
.B EACCES
The requested access would be denied to the file, or search permission
is denied for one of the directories in the path prefix of
.IR pathname .
(See also
.BR path_resolution (7).)
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR pathname .
.TP
.B ENAMETOOLONG
.I pathname
is too long.
.TP
.B ENOENT
A component of
.I pathname
does not exist or is a dangling symbolic link.
.TP
.B ENOTDIR
A component used as a directory in
.I pathname
is not, in fact, a directory.
.TP
.B EROFS
Write permission was requested for a file on a read-only file system.
.PP
.BR access ()
may fail if:
.TP
.B EFAULT
.I pathname
points outside your accessible address space.
.TP
.B EINVAL
.I mode
was incorrectly specified.
.TP
.B EIO
An I/O error occurred.
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ETXTBSY
Write access was requested to an executable which is being
executed.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
.PP
.BR Warning :
Using
.BR access ()
to check if a user is authorized to, for example,
open a file before actually doing so using
.BR open (2)
creates a security hole, because the user might exploit the short time
interval between checking and opening the file to manipulate it.
.BR "For this reason, the use of this system call should be avoided" .
(In the example just described,
a safer alternative would be to temporarily switch the process's
effective user ID to the real ID and then call
.BR open (2).)
.PP
.BR access ()
always dereferences symbolic links.
If you need to check the permissions on a symbolic link, use
.BR faccessat (2)
with the flag
.BR AT_SYMLINK_NOFOLLOW .
.PP
.BR access ()
returns an error if any of the access types in
.I mode
is denied, even if some of the other access types in
.I mode
are permitted.
.PP
If the calling process has appropriate privileges (i.e., is superuser),
POSIX.1-2001 permits an implementation to indicate success for an
.B X_OK
check even if none of the execute file permission bits are set.
.\" HPU-UX 11 and Tru64 5.1 do this.
Linux does not do this.
.PP
A file is accessible only if the permissions on each of the
directories in the path prefix of
.I pathname
grant search (i.e., execute) access.
If any directory is inaccessible, then the
.BR access ()
call will fail, regardless of the permissions on the file itself.
.PP
Only access bits are checked, not the file type or contents.
Therefore, if a directory is found to be writable,
it probably means that files can be created in the directory,
and not that the directory can be written as a file.
Similarly, a DOS file may be found to be "executable," but the
.BR execve (2)
call will still fail.
.PP
.BR access ()
may not work correctly on NFS file systems with UID mapping enabled,
because UID mapping is done on the server and hidden from the client,
which checks permissions.
Similar problems can occur to FUSE mounts.
.SH BUGS
In kernel 2.4 (and earlier) there is some strangeness in the handling of
.B X_OK
tests for superuser.
If all categories of execute permission are disabled
for a nondirectory file, then the only
.BR access ()
test that returns \-1 is when
.I mode
is specified as just
.BR X_OK ;
if
.B R_OK
or
.B W_OK
is also specified in
.IR mode ,
then
.BR access ()
returns 0 for such files.
.\" This behavior appears to have been an implementation accident.
Early 2.6 kernels (up to and including 2.6.3)
also behaved in the same way as kernel 2.4.

In kernels before 2.6.20,
.BR access ()
ignored the effect of the
.B MS_NOEXEC
flag if it was used to
.BR mount (2)
the underlying file system.
Since kernel 2.6.20,
.BR access ()
honors this flag.
.SH "SEE ALSO"
.BR chmod (2),
.BR chown (2),
.BR faccessat (2),
.BR open (2),
.BR setgid (2),
.BR setuid (2),
.BR stat (2),
.BR euidaccess (3),
.BR credentials (7),
.BR path_resolution (7)