ACCESS
Section: POSIX Programmer's Manual (3P)
Updated: 2013
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PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
access, faccessat
--- determine accessibility of a file relative to directory file
descriptor
SYNOPSIS
#include <unistd.h>
int access(const char *path, int amode);
int faccessat(int fd, const char *path, int amode, int flag);
DESCRIPTION
The
access()
function shall check the file named by the pathname pointed to by the
path
argument for accessibility according to the bit pattern contained in
amode,
using the real user ID in place of the effective user ID and the real
group ID in place of the effective group ID.
The value of
amode
is either the bitwise-inclusive OR of the access permissions to be
checked (R_OK, W_OK, X_OK) or the existence test (F_OK).
If any access permissions are checked, each shall be checked
individually, as described in the Base Definitions volume of POSIX.1-2008,
Section 4.4,
File Access Permissions,
except that where that description refers to execute permission for
a process with appropriate privileges, an implementation may indicate
success for X_OK even if execute permission is not granted to any user.
The
faccessat()
function shall be equivalent to the
access()
function, except in the case where
path
specifies a relative path. In this case the file whose accessibility is
to be determined shall be located relative to the directory associated
with the file descriptor
fd
instead of the current working directory. If the file descriptor was
opened without O_SEARCH, the function shall check whether directory
searches are permitted using the current permissions of the directory
underlying the file descriptor. If the file descriptor was opened with
O_SEARCH, the function shall not perform the check.
If
faccessat()
is passed the special value AT_FDCWD in the
fd
parameter, the current working directory shall be used and the behavior
shall be identical to a call to
access().
Values for
flag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
<fcntl.h>:
- AT_EACCESS
-
The checks for accessibility are performed using the effective user and
group IDs instead of the real user and group ID as required in a call
to
access().
RETURN VALUE
Upon successful completion, these functions shall return 0. Otherwise,
these functions shall return -1 and set
errno
to indicate the error.
ERRORS
These functions shall fail if:
- 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.
- ELOOP
-
A loop exists in symbolic links encountered during resolution of the
path
argument.
- ENAMETOOLONG
-
The length of a component of a pathname is longer than
{NAME_MAX}.
- ENOENT
-
A component of
path
does not name an existing file or
path
is an empty string.
- ENOTDIR
-
A component of the path prefix names an existing file that is neither
a directory nor a symbolic link to a directory, or the
path
argument contains at least one non-<slash>
character and ends with one or more trailing
<slash>
characters and the last pathname component names an existing file
that is neither a directory nor a symbolic link to a directory.
- EROFS
-
Write access is requested for a file on a read-only file system.
The
faccessat()
function shall fail if:
- EACCES
-
fd
was not opened with O_SEARCH and the permissions of the directory
underlying
fd
do not permit directory searches.
- EBADF
-
The
path
argument does not specify an absolute path and the
fd
argument is neither AT_FDCWD nor a valid file descriptor open
for reading or searching.
- ENOTDIR
-
The
path
argument is not an absolute path and
fd
is a file descriptor associated with a non-directory file.
These functions may fail if:
- EINVAL
-
The value of the amode argument is invalid.
- ELOOP
-
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the
path
argument.
- ENAMETOOLONG
-
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
- ETXTBSY
-
Write access is requested for a pure procedure (shared text) file that
is being executed.
The
faccessat()
function may fail if:
- EINVAL
-
The value of the
flag
argument is not valid.
The following sections are informative.
EXAMPLES
Testing for the Existence of a File
The following example tests whether a file named
myfile
exists in the
/tmp
directory.
-
#include <unistd.h>
...
int result;
const char *pathname = "/tmp/myfile";
result = access (pathname, F_OK);
APPLICATION USAGE
Additional values of
amode
other than the set defined in the description may be valid; for
example, if a system has extended access controls.
The use of the AT_EACCESS value for
flag
enables functionality not available in
access().
RATIONALE
In early proposals, some inadequacies in the
access()
function led to the creation of an
eaccess()
function because:
- 1.
-
Historical implementations of
access()
do not test file access correctly when the process'
real user ID is
superuser. In particular, they always return zero when testing execute
permissions without regard to whether the file is executable.
- 2.
-
The superuser has complete access to all files on a system. As a
consequence, programs started by the superuser and switched to the
effective user ID
with lesser privileges cannot use
access()
to test their file access permissions.
However, the historical model of
eaccess()
does not resolve problem (1), so this volume of POSIX.1-2008 now allows
access()
to behave in the desired way because several implementations have
corrected the problem. It was also argued that problem (2) is more
easily solved by using
open(),
chdir(),
or one of the
exec
functions as appropriate and responding to the error, rather than
creating a new function that would not be as reliable. Therefore,
eaccess()
is not included in this volume of POSIX.1-2008.
The sentence concerning appropriate privileges and execute permission
bits
reflects the two possibilities implemented by historical
implementations when checking superuser access for X_OK.
New implementations are discouraged from returning X_OK unless at least
one execution permission bit is set.
The purpose of the
faccessat()
function is to enable the checking of the accessibility of files in
directories other than the current working directory without exposure
to race conditions. Any part of the path of a file could be changed in
parallel to a call to
access(),
resulting in unspecified behavior. By opening a file descriptor for
the target directory and using the
faccessat()
function it can be guaranteed that the file tested for accessibility is
located relative to the desired directory.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(),
fstatat()
The Base Definitions volume of POSIX.1-2008,
Section 4.4,
File Access Permissions,
<fcntl.h>,
<unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.unix.org/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Testing for the Existence of a File
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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