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ACCESS(2) | System Calls Manual | ACCESS(2) |
NAME¶
access
,
eaccess
,
faccessat
—
check accessibility of a file
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include
<unistd.h>
int
access
(const
char *path, int
mode);
int
eaccess
(const
char *path, int
mode);
int
faccessat
(int
fd, const char
*path, int
mode, int
flag);
DESCRIPTION¶
Theaccess
() and
eaccess
() system calls check 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 eaccess
() system call uses the effective
user ID and the group access list to authorize the request; 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
().
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 to the file is denied if:- [
ENOTDIR
] - A component of the path prefix is not a directory.
- [
ENAMETOOLONG
] - A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
- [
ENOENT
] - The named file does not exist.
- [
ELOOP
] - Too many symbolic links were 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.
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¶
chmod(2), intro(2), stat(2)STANDARDS¶
Theaccess
() system call is expected to
conform to IEEE Std 1003.1-1990
(“POSIX.1”). The
faccessat
() system call follows The Open
Group Extended API Set 2 specification.
HISTORY¶
Theaccess
() function appeared in
Version 7 AT&T UNIX. The
faccessat
() system call appeared in
FreeBSD 8.0.
SECURITY CONSIDERATIONS¶
Theaccess
() 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
access
() to simulate access checks for the
real user or group ID. The eaccess
() system
call likewise may be subject to races if used inappropriately.
access
() 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
stat(2).April 10, 2008 | Debian |