### Working with Nonthreaded Software

Thread-safe code is code that works properly in a threaded environment. To work with nonthread-safe code, associate the global lock with all calls to such code.

You can implement the lock on the side of the routine user or the routine provider. For example, you can implement the lock on the side of the routine user if you write a new application like an RPC server that uses threads, and you link it with a library that does not. Or, if you have access to the nonthreaded code, the locks can be placed on the side of the routine provider, within the actual routine. Implement the locks as follows:

1. Associate one lock, a global lock, with execution of such code.

2. Require all of your threads to lock prior to execution of nonthreaded code.

3. Perform an unlock when execution is complete.

By using the global lock, you ensure that only one thread executes in outside libraries, which may call each other, and in unknown code. Using a single global lock is safer than using multiple local locks because it is difficult to be aware of everything a library may be doing or of the interactions that library can have with other libraries.

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