Shared memory in bash

[Peng Yu]

I see that mmap can be used to share data between parent and child.

https://youtu.be/WgVSq-sgHOc?t=123

In bash, I only know a subshell can access data in the parent shell,
but not the reverse. One could use files to share data from the
subshell to the parent shell. But is there a more efficient way to do
so, such as by mmap?

-- 
Regards,
Peng