Why we cannot accept() a socket on some process and recv() data from its child?

You can't send file descriptors via shared memory, AFAIK. So what your're doing is essentially sending a small integer to a worker process.

I'm quite sure your descriptors won't be valid outside the process creating/owning them. It should be a bit like with local pointers. They're valid for the process that created them only (due to virtual memory addressing and stuff).

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