NAME
access
, faccessat
— check access permissions of a
file or pathname
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
int
access
(const
char *path, int
mode);
#include
<fcntl.h>
int
faccessat
(int
fd, const char
*path, int mode,
int flags);
DESCRIPTION
Theaccess
()
function checks the accessibility of the file named by
path. The faccessat
() function
checks the accessibility of the file named by path using
fd as the starting point for relative pathnames. If
fd is AT_FDCWD
the current
directory is used. Calling access
() is equivalent to
calling faccessat
() with fd set
to AT_FDCWD
and flags set to 0.
The form of access to check is specified by the bitwise or of the following values for mode:
R_OK
- Check for read permission.
W_OK
- Check for write permission.
X_OK
- Check for execute/search permission.
F_OK
- Check only for existence.
All components of the pathname path are checked for access permissions as well.
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.
The file descriptor fd must name a directory. Search permission is required on this directory.
The flags argument to
faccessat
()
can specify the following optional behavior:
- AT_EACCESS
- Use the effective user and group IDs instead of the real user and group IDs for checking permission. See discussion below.
- AT_SYMLINK_NOFOLLOW
- Do not follow a symbolic link encountered as the last component in path.
For
access
(),
and faccessat
() when the
AT_EACCESS
flag is not passed, the real user ID and
the real group ID are used for checking permission in place of the effective
user ID and effective group ID. This affects only set-user-ID and
set-group-ID programs, which should not use these functions. (For other
programs, the real and effective IDs are the same.)
For processes running with super-user privileges, these functions
may return success for read and write checks regardless of whether read and
write permission bits are actually set. This reflects the fact that the
super-user may read and write all files regardless of permission settings.
However, even for the super-user, an execute check using
X_OK
will succeed only if the target object has at
least one of its execute permission bits set. (This does not guarantee that
the target object can necessarily be successfully executed. See
execve(2).)
RETURN VALUES
The access
() and
faccessat
() functions succeed and return 0 if, at
some point in the recent past, the target object named by
path existed and its permission settings allowed the
requested access as described above. If the requested access would not have
been granted, the object did not exist, or the path lookup failed, the value
-1 is returned and the value of errno is set to
reflect what went wrong.
ERRORS
These functions fail if:
- [
EACCES
] - Search permission is denied for fd, or for the current directory, or for a directory in the prefix of path; or the permission bits on the target file system object do not permit the requested access.
- [
EBADF
] - The file descriptor fd is not open and is not
AT_FDCWD
. - [
EFAULT
] - path points outside the process's allocated address space.
- [
EINVAL
] - The mode or flags argument contained an invalid value.
- [
EIO
] - An I/O error occurred while reading from or writing to the file system.
- [
ELOOP
] - Too many symbolic links were encountered in translating the pathname.
- [
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.
- [
ENOTDIR
] - The file descriptor fd does not name a directory, or a component of the path prefix is not a directory.
- [
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.
SEE ALSO
STANDARDS
The access
() function conforms to
IEEE Std 1003.1-1990 (“POSIX.1”).
faccessat
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
Note that faccessat
() violates the
historic convention that system calls whose names begin with `f' operate on
file handles rather than paths. There is no equivalent to
access
() for checking access properties of an
already-opened file.
SECURITY CONSIDERATIONS
Because the results of these calls reflect the state of the file system at the time they ran, and the file system can potentially be modified between that time and the time the caller attempts to act on the results, they should never be used for security enforcement.
Privileged programs that need to restrict their actions
to files or directories properly accessible to unprivileged users
must do this by
assuming or restoring an unprivileged state (see
seteuid(2)) when performing the pertinent actions. Checking in
advance (with access
() or any other method) and
performing such actions while privileged introduces a race condition that in
most cases is easily exploitable by even a naive adversary.
Even for non-privileged programs, the opportunity for the world to
change after the call runs makes access
() and
faccessat
() not very useful. In general only
F_OK
should be used, and that too, sparingly. The
other checks may occasionally be useful for user interface or diagnostic
purposes.