ACCESS(2) | System Calls Manual | ACCESS(2) |
access
, faccessat
—
#include <unistd.h>
int
access
(const
char *path, int
mode);
#include
<fcntl.h>
int
faccessat
(int
fd, const char
*path, int mode,
int flags);
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
W_OK
X_OK
F_OK
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:
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).)
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.
EACCES
]EBADF
]AT_FDCWD
.EFAULT
]EINVAL
]EIO
]ELOOP
]ENAMETOOLONG
]NAME_MAX
}
characters, or an entire path name exceeded
{PATH_MAX
} characters.ENOENT
]ENOTDIR
]EROFS
]ETXTBSY
]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.
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.
January 12, 2013 | NetBSD 9.0 |