What Happens When I Call fork() in Unix?

c unix fork process
Rob P. picture Rob P. · Sep 17, 2011 · Viewed 9.9k times · Source

I've tried to look this up, but I'm struggling a bit to understand the relation between the Parent Process and the Child Process immediately after I call fork().

Are they completely separate processes, only associated by the id/parent id? Or do they share memory? For example the 'code' section of each process - is that duplicated so that each process has it's own identical copy, or is that 'shared' in some way so that only one exists?

I hope that makes sense.

In the name of full disclosure this is 'homework related'; while not a direct question from the book, I have a feeling it's mostly academic and, in practice, I probably don't need to know.

Answer

jpa picture jpa · Sep 17, 2011

As it appears to the process, the entire memory is duplicated.

In reality, it uses "copy on write" system. The first time either process changes its memory after fork(), a separate copy is made of the modified page (usually 4kB).

Usually the code segment of a process is not modified, in which case it remains shared.

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