man-pages/man2/access.2

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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                               1993 Michael Haardt, Ian Jackson.
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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
.\"
.TH ACCESS 2 2004-06-23 "Linux" "Linux Programmer's Manual"
.SH NAME
access \- check 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 process would be allowed to read,
write or test for existence of the file (or other file system
object) whose name is
.IR pathname .
If
.I pathname
is a symbolic link permissions of the file referred to by this
symbolic link are tested.

.I mode
is a mask consisting of one or more of
.BR R_OK ", " W_OK ", " X_OK " and " F_OK .

.BR R_OK ", " W_OK " and " X_OK
request checking whether the file exists and has read, write and
execute permissions, respectively.
.B F_OK
just requests checking for the existence of the file.

The tests depend on the permissions of the directories
occurring in the path to the file, as given in
.IR pathname ,
and on the permissions of directories and files referred to by symbolic
links encountered on the way.

The check is done with the process's
.I real
UID and GID, rather than with the effective IDs as is done when
actually attempting an operation.
This is to allow set-user-ID programs to
easily determine the invoking user's authority.

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.

If the process has appropriate privileges, an implementation may
indicate success for
.B X_OK
even if none of the execute file permission bits are set.
.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 (2).)
.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 filesystem.
.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 "LINUX NOTES"
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 ()
honours this flag.
.SH RESTRICTIONS
.BR access ()
returns an error if any of the access types in the requested call
fails, even if other types might be successful.
.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.
.PP
Using
.BR access ()
to check if a user is authorized to e.g. 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.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
.SH "SEE ALSO"
.BR chmod (2),
.BR chown (2),
.BR faccessat (2),
.BR open (2),
.BR path_resolution (2),
.BR setgid (2),
.BR setuid (2),
.BR stat (2)