S_IRUSR/S_IWUSR/S_IRGRP/S_IWGRP

S_IRUSR
Permits the file's owner to read it.

S_IWUSR
Permits the file's owner to write to it.

S_IRGRP
Permits the file's group to read it.

S_IWGRP
Permits the file's group to write to it.