NAME
access
, faccessat
— check accessibility of a
file
SYNOPSIS
#include
<unistd.h>
int
access
(const
char *path, int
mode);
int
faccessat
(int
fd, const char
*path, int mode,
int flag);
DESCRIPTION
The
access
()
system call checks the accessibility of the file named by the
path argument for the access permissions indicated by
the mode argument. The value of
mode is either 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
).
For additional information, see the File Access Permission section of intro(2).
The
access
()
system call uses the real user ID in place of the effective user ID, the
real group ID in place of the effective group ID, and the rest of the group
access list.
The
faccessat
()
system call is equivalent to access
() except in the
case where path specifies a relative path. In this
case the file whose accessibility is to be determined is located relative to
the directory associated with the file descriptor fd
instead of the current working directory. If
faccessat
() is passed the special value
AT_FDCWD
in the fd parameter,
the current working directory is used and the behavior is 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
().
AT_SYMLINK_NOFOLLOW
- If path names a symbolic link, the status of the symbolic link is returned.
AT_SYMLINK_NOFOLLOW_ANY
- If path names a symbolic link, the status of the symbolic link is returned and if the path has any other symbolic links, an error is returned.
Even if a process has appropriate privileges and indicates success
for X_OK
, the file may not actually have execute
permission bits set. Likewise for R_OK
and
W_OK
.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS
access
() or
faccessat
() will fail if:
- [
EINVAL
] - The value of the mode argument is invalid.
- [
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.
- [
ELOOP
] - AT_SYMLINK_NOFOLLOW_ANY was passed and a symbolic link was 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.
- [
EFAULT
] - The path argument points outside the process's allocated address space.
- [
EIO
] - An I/O error occurred while reading from or writing to the file system.
Also, the faccessat
() system call may fail
if:
- [
EBADF
] - The path argument does not specify an absolute path
and the fd argument is neither
AT_FDCWD
nor a valid file descriptor. - [
EINVAL
] - The value of the flag argument is not valid.
- [
ENOTDIR
] - The path argument is not an absolute path and
fd is neither
AT_FDCWD
nor a file descriptor associated with a directory.
SEE ALSO
STANDARDS
The access
() system call is expected to
conform to IEEE Std 1003.1-1990
(“POSIX.1”). The faccessat
()
system call is expected to conform to POSIX.1-2008 .
HISTORY
The access
() function appeared in
Version 7 AT&T UNIX.
SECURITY CONSIDERATIONS
The result of access
() should not be used
to make an actual access control decision, since its response, even if
correct at the moment it is formed, may be outdated at the time you act on
it. access
() results should only be used to
pre-flight, such as when configuring user interface elements or for
optimization purposes. The actual access control decision should be made by
attempting to execute the relevant system call while holding the applicable
credentials, and properly handling any resulting errors; and this must be
done even though access
() may have predicted
success.
Additionally, set-user-ID and set-group-ID applications should
restore the effective user or group ID, and perform actions directly rather
than use access
() to simulate access checks for the
real user or group ID.