What is a Thread?

• Technically, a thread is defined as an independent stream of instructions that can be scheduled to run by the operating system (OS).
• Suppose that a main program (a.out) that contains a number of procedures.
• Then suppose all of these procedures being able to be scheduled to run simultaneously and/or independently.
• That would describe a ``multi-threaded'' program.
• Before understanding a thread, one first needs to understand a UNIX process.
• Processes contain information about program resources and program execution state.

Figure 1: Left: Unix process. Right: Threads within a Unix process.

• Threads use and exist within these process resources,
• To be scheduled by the OS,
• Run as independent entities.
• This independent flow of control is accomplished because a thread maintains its own:
  • Stack pointer
  • Registers
  • Scheduling properties (such as policy or priority)
  • Set of pending and blocked signals
  • Thread specific data.
• A thread has its own independent flow of control as long as its parent process exists (dies if the parent process dies!).
• A thread duplicates only the essential resources it needs.
• A thread is "lightweight" because most of the overhead has already been accomplished through the creation of its process.