access - determine accessibility of file

     #include <sys/types.h>
     #include <unistd.h>

     #define R_OK   4   /* test for read permission */
     #define W_OK   2   /* test for write permission */
     #define X_OK   1   /* test for execute (search) permission */
     #define F_OK   0   /* test for presence of file */

     int access(const char *path, mode_t mode)

     Access checks the given file path for accessibility  according  to  mode,
     which  is  an  inclusive  or of the bits R_OK, W_OK and X_OK.  Specifying
     mode as F_OK (i.e., 0) tests whether the directories leading to the  file
     can be searched and the file exists.

     The real user ID and the group access list (including the real group  ID)
     are  used  in  verifying  permission,  so  this call is useful to set-UID

     Notice that only access bits are checked.  A directory may  be  indicated
     as  writable  by  access, but an attempt to open it for writing will fail
     (although files may be created there); a file may  look  executable,  but
     execve will fail unless it is in proper format.

     If path cannot be found or if any of the desired access modes  would  not
     be granted, then a -1 value is returned; otherwise a 0 value is returned.

     Access to the file is denied if one or more of the following are true:

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

     [ENAMETOOLONG] The path name exceeds PATH_MAX characters.

     [ENOENT]       The named file does not exist.

     [EACCES]       Search permission is denied for a component  of  the  path

     [ELOOP]        Too many symbolic links were  encountered  in  translating
                    the pathname.  (Minix-vmd)

     [EROFS]        Write access is requested for a file on a  read-only  file

     [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.  The owner  of  a  file  has
                    permission  checked  with  respect  to the ``owner'' read,
                    write, and execute mode bits, members of the file's  group
                    other  than the owner have permission checked with respect
                    to  the  ``group''  mode  bits,  and   all   others   have
                    permissions  checked  with  respect  to the ``other'' mode

     [EFAULT]       Path points outside the process's allocated address space.

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

     chmod(2), stat(2).