ACCESS(2)                   BSD System Calls Manual                  ACCESS(2)

NAME
     access, faccessat -- check accessibility of a file

SYNOPSIS
     #include <unistd.h>

     int
     access(const char *path, int mode);

     int
     faccessat(int fd, const char *path, int mode, int flag);

DESCRIPTION
     The access() system call checks the accessibility of the file named by
     the path argument for the access permissions indicated by the mode argu-
     ment.  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 per-
     mission, 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 acces-
     sibility is to be determined is located relative to the directory associ-
     ated with the file descriptor fd instead of the current working direc-
     tory.  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().

     Even if a process has appropriate privileges and indicates success for
     X_OK, the file may not actually have execute permission bits set.  Like-
     wise 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() or faccessat() will fail if:

     [EINVAL]           The value of the mode argument is invalid.

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ENAMETOOLONG]     A component of a pathname exceeded {NAME_MAX} charac-
                        ters, or an entire path name exceeded {PATH_MAX} char-
                        acters.

     [ENOENT]           The named file does not exist.

     [ELOOP]            Too many symbolic links were encountered in translat-
                        ing 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 allo-
                        cated address space.

     [EIO]              An I/O error occurred while reading from or writing to
                        the file system.

     Also, the 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
     The access() system call is expected to conform to ISO/IEC 9945-1:1990
     (``POSIX.1'').  The faccessat() system call is expected to conform to
     POSIX.1-2008 .

HISTORY
     The access() function appeared in Version 7 AT&T UNIX.

SECURITY CONSIDERATIONS
     The result of access() should not be used to make an actual access con-
     trol 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 inter-
     face 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.

BSD                           September 15, 2014                           BSD