Calling fork( ) in a Multithreaded Environment

The fork( ) system call creates an exact duplicate of the address space from which it is called, resulting in two address spaces executing the same code. Problems can occur if the forking address space has multiple threads executing at the time of the fork( ). When multithreading is a result of library invocation, threads are not necessarily aware of each other's presence, purpose, actions, and so on. Suppose that one of the other threads (any thread other than the one doing the fork( )) has the job of deducting money from your checking account. Clearly, you do not want this to happen twice as a result of some other thread's decision to call fork( ).

Because of these types of problems, which in general are problems of threads modifying persistent state, POSIX defined the behavior of fork( ) in the presence of threads to propagate only the forking thread. This solves the problem of improper changes being made to persistent state. However, it causes other problems, as discussed in the next paragraph.

In the POSIX model, only the forking thread is propagated. All the other threads are eliminated without any form of notice; no cancels are sent and no handlers are run. However, all the other portions of the address space are cloned, including all the mutex state. If the other thread has a mutex locked, the mutex will be locked in the child process, but the lock owner will not exist to unlock it. Therefore, the resource protected by the lock will be permanently unavailable.

The fact that there may be mutexes outstanding only becomes a problem if your code attempts to lock a mutex that could be locked by another thread at the time of the fork( ). This means that you cannot call outside of your own code between the call to fork( ) and the call to exec( ). Note that a call to malloc( ), for example, is a call outside of the currently executing application program and may have a mutex outstanding. The following code obeys these guidelines and is therefore safe:

fork ();

a = 1+2; /* some inline processing */


Similarly, if your code calls some of your own code that does not make any calls outside of your code and does not lock any mutexes that could possibly be locked in another thread, then your code is safe.

One solution to the problem of calling fork( ) in a multithreaded environment exists. (Note that this method will not work for server application code or any other application code that is invoked by a callback from a library.) Before an application performs a fork( ) followed by something other than exec( ), it must cancel all of the other threads. After it joins the canceled threads, it can safely fork( ) because it is the only thread in existence. This means that libraries that create threads must establish cancel handlers that propagate the cancel to the created threads and join them. The application should save enough state so that the threads can be recreated and restarted after the fork( ) processing completes.