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Unix permissions
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access(2) System Calls Manual access(2) NAME
access() - determine accessibility of a file SYNOPSIS
DESCRIPTION
The system call checks the file pointed to by path for accessibility according to the bit pattern contained in amode. 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 to test for permissions: Read access Write access Execute (search) access 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, 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 or entry can match a user if that user has a nonnull sup- plementary groups list. If any matching entry has the appropriate permission bit set, access is permitted. Access Control Lists - JFS File Systems Only Read, write, and execute/search permissions are checked against the file's access control list (ACL). 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 ACL entries and ends when the first entry matching the user ID or any of the group IDs is encountered. If a shared text file is currently open for execution, reports that it is not writable, regardless of its access control list. However, 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. Security Restrictions If the path is valid and the real user ID is superuser or a user with and privileges, always returns except when amode includes the path is not a directory, and none of the execute bits are set in the file's mode. See privileges(5) for more information about privileged access on systems that support fine-grained privileges. RETURN VALUE
returns the following values: Successful completion. The requested access is permitted. Failure. is set to indicate the error. ERRORS
If fails, is set to one of the following values. Search permission is denied on a component of the path prefix. The access control list does not permit the requested access and the real user ID is not a user with or privileges. path points outside the allocated address space for the process. The reliable detection of this error is implementation dependent. Too many symbolic links were encountered in translating the path name. The length of the specified path name exceeds bytes, or the length of a component of the path name exceeds bytes while is in effect. Read, write, or execute (search) permission is requested for a null path name. The named file does not exist. A component of the path prefix is not a directory. Write access is requested for a file on a read-only file system. Write access is requested for a pure procedure (shared text) file that is being executed. SEE ALSO
acl(2), chmod(2), stat(2), setacl(2), acl(5), aclv(5), privileges(5), unistd(5). STANDARDS CONFORMANCE
access(2)
