Applications always have at least one thread of execution-thread 1. Using multiple threads of execution, an application can do several things at the same time.
For example, a simple Presentation Manager application consists of a single process with two threads:
OS/2 creates the first thread of execution for a process when it starts the executable file. To create another thread of execution, a thread calls DosCreateThread, specifying the address within the program module where the thread begins asynchronous execution. Although a thread can execute any part of the application, including a part being executed by another thread, threads typically are used to execute separate sections of the application. By using several threads, the system can distribute the available CPU time and enable an application to carry out several tasks simultaneously. For example, an application can load a file and prompt the user for input at the same time.
Each thread in a process has a unique stack and register context. Threads shares the resources of the process with the other threads in the process. For example, threads in the same process have access to the memory spaces of other threads within the process. However, threads of one process do not have access to the data spaces of other processes.
Each thread has a priority, that determines the amount of CPU time the thread is allocated. Threads inherit the priority of the thread that creates them. The priority of a thread can be changed by the application; see Changing the Priority of a Thread for details.
An application can use DosSuspendThread and DosResumeThread to suspend and resume the execution of a given thread. When an application suspends a thread, the thread remains suspended until the application calls DosResumeThread.
When an application has more than one thread, it might be necessary to ensure that one thread is finished executing before another thread uses a shared resource, such as a disk file. DosWaitThread causes the application to wait until a specific thread has finished. DosWaitThread can also be used to determine the state of a thread; the function can return immediately with an error value if the identified thread is still running.
A thread ends when it calls DosExit.