NAME
access — determine accessibility of a file
SYNOPSIS
#include <unistd.h>
int access(char *path, int amode);
DESCRIPTION
The
access()
system call checks the file pointed to by
path
for accessibility according to the bit pattern contained in
amode.
access()
uses the real user ID, not the effective user ID,
and the real group ID, not the effective group ID.
The value of
amode
is either the bit-wise inclusive OR
of the access permissions to be checked, or the existence test.
You can use the following symbolic constants, defined in
<unistd.h>,
to test for permissions:
- R_OK
Read access
- W_OK
Write access
- X_OK
Execute (search) access
- F_OK
Check existence of file
The owner of a file has permission checked
with respect to the "user" read, write, and execute mode bits.
Members of the file's group other than the owner
have permissions checked with respect to the "group" mode bits.
All others have permissions checked with respect to the "other" mode bits.
If a file is currently open for execution,
access()
reports that it is not writable, regardless of the setting of its mode.
Access Control Lists - HFS File Systems Only
Read, write, and execute/search permissions
are checked against the file's access control list (ACL).
Each mode is checked separately since different ACL entries
can grant different permissions.
The real user ID is combined with the process's real group ID
and each group in its supplementary groups list,
and the access control list is searched for a match.
Search proceeds in order of specificity and ends
when one or more matching entries are found at a specific level.
More than one
user.group
or
%.group
entry can match a user if that user has a nonnull supplementary groups list.
If any matching entry has the appropriate permission bit set,
access is permitted.
If a shared text file is currently open for execution,
access()
reports that it is not writable, regardless of its access control list.
However,
access()
does not report that a shared text file open for writing is not executable,
since the check is not easily done.
It also reports that a file on a read-only file system is not writable.
RETURN VALUE
access()
returns the following values:
- 0
Successful completion.
The requested access is permitted.
If the path is valid and the real user ID
is superuser,
access()
always returns 0, except when
amode
includes
X_OK,
the path is not a directory,
and none of the execute bits are set in the file's mode.
- -1
Failure.
errno
is set to indicate the error.
ERRORS
If
access()
fails,
errno
is set to one of the following values.
- [EACCES]
Search permission is denied on a component of the path prefix.
- [EACCES]
The access control list does not permit the requested access
and the real user ID is not a user with
appropriate privileges.
- [EFAULT]
path
points outside the allocated address space for the process.
The reliable detection of this error is implementation dependent.
- [ELOOP]
Too many symbolic links were encountered in translating the path name.
- [ENAMETOOLONG]
The length of the specified path name exceeds
PATH_MAX
bytes, or the length of a component of the path name exceeds
NAME_MAX
bytes while
_POSIX_NO_TRUNC
is in effect.
- [ENOENT]
Read, write, or execute (search) permission is requested for a null path name.
- [ENOENT]
The named file does not exist.
- [ENOTDIR]
A component of the path prefix is not a directory.
- [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 that is being executed.
STANDARDS CONFORMANCE
access(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
