Question

I’m very new to this Parallel::ForkManager module in Perl and it has a lot of credits, so I think it supports what I need and I just haven’t figured out yet.

What I need to do is in each child process, it writes some updates into a global hash map, according to the key value computed in each child process.
However, when I proceed to claim a hash map outside the for loop and expect the hash map is updated after the loop, it turns out that the hash map stays empty.
This means although the update inside the loop succeeds (by printing out the value), outside the loop it is not.

Does anybody know how to write such a piece of code that does what I want?

Answer 1

This isn’t really a Perl-specific problem, but a matter of understanding Unix-style processes. When you fork a new process, none of the memory is shared by default between processes. There are a few ways you can achieve what you want, depending on what you need.

One easy way would be to use something like BerkeleyDB to tie a hash to a file on disk. The tied hash can be initialized before you fork and then each child process would have access to it. BerkeleyDB files are designed to be safe to access from multiple processes simultaneously.

A more involved method would be to use some form of inter-process communication. For all the gory details of achieving such, see the perlipc manpage, which has details on several IPC methods supported by Perl.

A final approach, if your Perl supports it, is to use threads and share variables between them.