This option causes the address space in question to be shared between parent and child. It has no effect on how the original underlying backing store was mapped.
This option prevents the address space in question from being inherited at all. The address space will be unmapped in the child.
This option causes the child to inherit the address space as copy-on-write. This option also has an unfortunate side effect of causing the parent address space to become copy-on-write when the parent forks. If the original mapping was MAP_SHARED, it will no longer be shared in the parent after the parent forks and there is no way to get the previous shared-backing-store mapping without unmapping and remapping the address space in the parent.