Thread Fundamentals

Threads are created with either the PERFORM statement or the CALL statement. You simply add the word "THREAD" after the verb. The thread runs the target of the PERFORM or CALL statement. When the target completes, the thread is destroyed.

For example, to run the paragraph OPEN-FILES in a thread, you would code:

PERFORM THREAD OPEN-FILES

The main thread continues to execute at the statement following the PERFORM, while the OPEN-FILES code begins execution in a separate thread. When OPEN-FILES finishes, the thread is destroyed.

Threads are identified by thread handles. When you create a thread, you can optionally store its handle. Thread handles are used to communicate among threads. The next example demonstrates how this works.

There are times when you will want to ensure that a thread is complete before continuing in the program. For example, if one thread is used to open an application's files while another thread gets the user's password, you will not want to continue in the program until both tasks are complete, otherwise you might try to look up the password in a file that is closed. You can use the WAIT verb to make sure that both threads are complete before continuing. The WAIT verb causes the thread to wait for the specified thread to finish or send a message. The following example illustrates this process:

77  THREAD-1   USAGE HANDLE OF THREAD.

PERFORM THREAD OPEN-FILES, HANDLE IN THREAD-1
PERFORM GET-PASSWORD
WAIT FOR THREAD THREAD-1
PERFORM VALIDATE-PASSWORD

In this example, the paragraphs OPEN-FILES and GET-PASSWORD run in parallel. The WAIT statement ensures that OPEN-FILES is finished before the main thread goes on to validate the password.