See the CAVEATS here:
https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man2/fork.2.html
Apparently fork() without exec() isn't all that well supported.
Noticed while perusing
http://lwn.net/Articles/518306/
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Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services