1.3 Thread Execution

You can view multiple threads in a program as executing simultaneously. You cannot make any assumptions about the relative start or finish times of threads or the sequence in which they execute. Nevertheless, you can influence the scheduling of threads.

Each thread has its own thread identifier, which allows it to be uniquely identified. Also associated with a thread are its scheduling policy and priority, thread-specific data values, and the required system resources to support a flow of control.

A thread changes states during the course of its execution. A thread is in one of the following states:

• Waiting-The thread is not eligible to execute because it is synchronizing with another thread or with an external event, such as I/O.

• Ready-The thread is eligible to be executed by a processor.

• Running-The thread is currently being executed by a processor.

• Terminated-The thread has completed all of its work.

Note

A multithreaded program must be reentrant. Therefore, be sure that your compiler generates reentrant code before you do multithreading design or coding work. (Digital's C, C++, Ada, Pascal, and BLISS compilers generate reentrant code by default.) If your program is nonreentrant, it may be impossible to keep the program's threads from interfering with each other. See Section 3.2.1 for more information about thread reentrancy. In general, when using threads, be aware of language common practices that are inherently not thread-safe. For example, FORTRAN typically relies heavily upon static storage. These factors need to be addressed when writing threaded applications and thread-safe libraries.

Figure 1-3 shows the transitions between states for a typical thread implementation.

Figure 1-3 Thread State Transitions