Thread properties

A thread is the schedulable entity.

It has only those properties that are required to ensure its independent control of flow. These include the following properties:

• Stack
• Scheduling properties (such as policy or priority)
• Set of pending and blocked signals
• Some thread-specific data

An example of thread-specific data is the errno error indicator. In multithreaded systems, errno is no longer a global variable, but usually a subroutine returning a thread-specific errno value. Some other systems may provide other implementations of errno.

Threads within a process must not be considered as a group of processes. All threads share the same address space. This means that two pointers having the same value in two threads refer to the same data. Also, if any thread changes one of the shared system resources, all threads within the process are affected. For example, if a thread closes a file, the file is closed for all threads.