Updated: 2022/Sep/29
Please read Privacy Policy. It's for your privacy.
ACCESS(2) System Calls Manual ACCESS(2)
NAME
access, faccessat - check access permissions of a file or pathname
LIBRARY
Standard C Library (libc, -lc)
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
The access() 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
chmod(2), execve(2), stat(2), secure_path(3)
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.
NetBSD 10.99 January 12, 2013 NetBSD 10.99