fork and existing threads?

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Threads on Linux nowadays try to stay POSIX compliant. Only the calling thread is replicated, not other threads (note that e.g. on Solaris you can choose what fork does depending on what library you link to)

From http://www.opengroup.org/onlinepubs/000095399/functions/fork.html (POSIX 2004):

A process shall be created with a
single thread. If a multi-threaded
process calls fork(), the new process
shall contain a replica of the calling
thread and its entire address space,
possibly including the states of
mutexes and other resources.
Consequently, to avoid errors, the
child process may only execute
async-signal-safe operations until
such time as one of the exec functions
is called. Fork
handlers may be established by means
of the pthread_atfork() function in
order to maintain application
invariants across fork() calls.

The POSIX 2018 specification of fork() is similar.

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