Implementing a Multithreaded Fork

I am trying to checkpoint a multithreaded application. F开发者_JAVA百科or single threaded applications, forking a process as a checkpoint is an efficient technique. However, there is no such thing as a mulithreaded fork. Any idea of how to implement your own mulithreaded fork? Any reference to such work will be greatly appreciated.

There is no portable way to implement a variant of fork that preserves all threads using the interfaces provided by POSIX. On some systems such as Linux, you could implement a highly non-portable, highly fragile version of this either:

1. using ptrace to trace all threads (to stop them), then making new kernel threads in the child process to duplicate each thread in the parent and assigning them the original stack addresses, instruction pointers, register values, etc. You'd also need to patch up the thread descriptors to know their new kernelspace thread ids, and you'd need to avoid race conditions in this if the thread was in the middle of querying its thread id.

2. using vfork followed by SIGSTOP to halt the parent process and give yourself a chance to recreate its thread state without things changing under you. This seems possible but sufficiently difficult I'd get a headache trying to go into detail, I think...

3. (newly added) catch each thread in signal handlers before forking, and save the ucontext_t argument to the signal handler. Then fork and make new kernel threads (using clone), have them signal themselves, then overwrite the ucontext_t the signal handler gets to have the signal handler return back into the context of the original thread you're trying to duplicate. Of course this would all require very clever synchronization...

Alternatively, you could look for a kernel-based "process hibernation" approach to checkpointing that would not be so hackish...

What do you mean by "multithreaded fork"? A function that makes a copy of a multithreaded process, so that the forked process has just as many threads as the old one? A function that makes a new thread which copies the state of the old one?

The latter isn't possible, since the address space is shared. A copy of the current thread's state would be using the current thread's stack, and the new thread and the old thread would fight over the stack.

See also:

• Multithreaded fork
• fork and existing threads?