NAME
access
, faccessat
— check access permissions of a
file or pathname
SYNOPSIS
#include
<unistd.h>
int
access
(const
char *path, int
amode);
#include <fcntl.h>
#include <unistd.h>
int
faccessat
(int
fd, const char
*path, int amode,
int flag);
DESCRIPTION
The
access
()
function checks the accessibility of the file named by
path for the access permissions indicated by
amode. The value of amode is the
bitwise inclusive OR of the access permissions to be checked
(R_OK
for read permission,
W_OK
for write permission, and
X_OK
for execute/search permission) or the existence
test, F_OK
. All components of the pathname
path are checked for access permissions (including
F_OK
).
The real user ID is used in place of the effective user ID and the real group access list (including the real group ID) is used in place of the effective ID for verifying permission.
If the invoking process has superuser privileges,
access
()
will always indicate success for R_OK
and
W_OK
, regardless of the actual file permission bits.
Likewise, for X_OK
, if the file has any of the
execute bits set and path is not a directory,
access
() will indicate success.
The
faccessat
()
function is equivalent to access
() except that where
path specifies a relative path, the file whose
accessibility is checked is determined relative to the directory associated
with file descriptor fd instead of the current working
directory.
If
faccessat
()
is passed the special value AT_FDCWD
(defined in
<fcntl.h>
) in the
fd parameter, the current working directory is used.
If flag is also zero, the behavior is identical to a
call to access
().
Values for flag are constructed by
bitwise-inclusive ORing 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 IDs.
RETURN VALUES
If path cannot be found or if any of the desired access modes would not be granted, then a -1 value is returned; otherwise a 0 value is returned.
ERRORS
Access to the file is denied if:
- [
ENOTDIR
] - A component of the path prefix is not a directory.
- [
ENAMETOOLONG
] - A component of a pathname exceeded
{NAME_MAX}
characters, or an entire path name exceeded{PATH_MAX}
characters. - [
ENOENT
] - The named file does not exist.
- [
ELOOP
] - Too many symbolic links were encountered in translating the pathname.
- [
EROFS
] - Write access is requested for a file on a read-only file system.
- [
ETXTBSY
] - Write access is requested for a pure procedure (shared text) file presently being executed.
- [
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. The owner of a file has permission checked with respect to the “owner” read, write, and execute mode bits, members of the file's group other than the owner have permission checked with respect to the “group” mode bits, and all others have permissions checked with respect to the “other” mode bits.
- [
EPERM
] - Write access has been requested and the named file has its immutable flag set (see chflags(2)).
- [
EFAULT
] - path points outside the process's allocated address space.
- [
EIO
] - An I/O error occurred while reading from or writing to the file system.
- [
EINVAL
] - An invalid value was specified for amode.
Additionally, faccessat
() will fail
if:
- [
EINVAL
] - The value of the flag argument was neither zero nor
AT_EACCESS
. - [
EBADF
] - The path argument specifies a relative path and the
fd argument is neither
AT_FDCWD
nor a valid file descriptor. - [
ENOTDIR
] - The path argument specifies a relative path and the fd argument is a valid file descriptor but it does not reference a directory.
- [
EACCES
] - The path argument specifies a relative path but search permission is denied for the directory which the fd file descriptor references.
SEE ALSO
STANDARDS
The access
() and
faccessat
() functions conform to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
access
() first appeared as an internal
kernel function in Version 1 AT&T UNIX
and was reimplemented in C before the release of
Version 4 AT&T UNIX. It was first
promoted to a system call in the Programmer's Workbench (PWB/UNIX), which
was later ported to Version 7 AT&T UNIX
and 2BSD.
The faccessat
() function appeared in
OpenBSD 5.0.
AUTHORS
Ken Thompson first implemented the
access
() kernel function in C.
CAVEATS
access
() and
faccessat
() should never be used for actual access
control. Doing so can result in a time of check vs. time of use security
hole.