An OS/2 application that has been loaded into memory and prepared for execution is called a process. As mentioned earlier, a process consists of the code, data, and other resources (for example, open file handles) that belong to the application. Each process has at least one thread, called the main thread or thread 1.
When OS/2 runs an application, it confirms that the process code and data are in memory and that the main thread's registers and stack are set before starting the application. Each application has access to all resources of the computer, such as memory, disk drives, screen, keyboard, and the CPU itself. The system carefully manages these resources so that applications can access them without conflict.
A process can have more than one thread. OS/2 creates the first thread of execution for a process when it starts the executable file. More threads can be created with DosCreateThread. Each thread runs independently, with its own stack and register values. Unless the application changes a thread's priority, each thread gets a slice of the CPU in a round-robin strategy. All the threads in a process share the application's globally defined variables and other resources (open file handles, and so on).
A process or thread ends when it calls DosExit. OS/2 automatically closes any files or other resources left open by the process when the process ends. When a thread ends, however, any open resources remain open until another thread closes them or the process ends. A process can direct OS/2 to carry out other actions when the process ends, by using DosExitList to create a list of termination functions. OS/2 calls the termination functions, in the order given, when the process is about to end. If the thread has registered any exception handlers, the exception handlers will also be called before the thread ends.