ACCESS(2) | System Calls Manual | ACCESS(2) |
access
, faccessat
— check accessibility of a file
#include
<unistd.h>
int
access
(const
char *path, int
mode);
int
faccessat
(int
fd, const char
*path, int mode,
int flag);
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
access
().AT_SYMLINK_NOFOLLOW
AT_SYMLINK_NOFOLLOW_ANY
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
.
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.
access
() or
faccessat
() will fail if:
EINVAL
]ENOTDIR
]ENAMETOOLONG
]{NAME_MAX}
characters, or an entire path name exceeded
{PATH_MAX}
characters.ENOENT
]ELOOP
]ELOOP
]EROFS
]ETXTBSY
]EACCES
]EFAULT
]EIO
]Also, the faccessat
() system call may fail
if:
EBADF
]AT_FDCWD
nor a valid file descriptor.EINVAL
]ENOTDIR
]AT_FDCWD
nor a
file descriptor associated with a directory.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 .
The access
() function appeared in
Version 7 AT&T UNIX.
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.
September 15, 2014 | macOS 14.6 |