check accessibility of a file
Standard C Library (libc, -lc)
, const char
() system calls check the
accessibility of the file named by the path
argument for the access permissions indicated by the
argument. The value of
is either the bitwise-inclusive OR of
the access permissions to be checked (
for read permission,
permission), or the existence test (
For additional information, see the
section of intro(2)
() system call uses the effective
user ID and the group access list to authorize the request; the
() 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.
() system call is equivalent to
() except in the case where
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
instead of the current working directory.
() is passed the special value
parameter, the current working directory
is used and the behavior is identical to a call to
(). Values for
are constructed by a bitwise-inclusive
OR of flags from the following list, defined in
- 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
Even if a process's real or effective user has appropriate privileges and
indicates success for
, the file may
not actually have execute permission bits set. Likewise for
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
is set to indicate the error.
Access to the file is denied if:
- A component of the path prefix is not a directory.
- A component of a pathname exceeded 255 characters, or an entire path name
exceeded 1023 characters.
- The named file does not exist.
- Too many symbolic links were encountered in translating the pathname.
- Write access is requested for a file on a read-only file system.
- Write access is requested for a pure procedure (shared text) file
presently being executed.
- Permission bits of the file mode do not permit the requested access, or
search permission is denied on a component of the path prefix.
- The path argument points outside the
process's allocated address space.
- An I/O error occurred while reading from or writing to the file
() system call may fail
- The path argument does not specify an
absolute path and the fd argument is
AT_FDCWD nor a valid file
- The value of the flag argument is not
- The path argument is not an absolute path
and fd is neither
AT_FDCWD nor a file descriptor
associated with a directory.
() system call is expected to
conform to IEEE Std 1003.1-1990
() system call follows The Open
Group Extended API Set 2 specification.
() function appeared in
Version 7 AT&T UNIX
() system call appeared in
() system call is a potential
security hole due to race conditions and should never be used. Set-user-ID and
set-group-ID applications should restore the effective user or group ID, and
perform actions directly rather than use
() to simulate access checks for the
real user or group ID. The
call likewise may be subject to races if used inappropriately.
() remains useful for providing clues
to users as to whether operations make sense for particular filesystem objects
(e.g. 'delete' menu item only highlighted in a writable folder ... avoiding
interpretation of the st_mode bits that the application might not understand
-- e.g. in the case of AFS). It also allows a cheaper file existence test than