Processes

A running instance of a program is called a process.

• When you invoke a command from a shell, the corresponding program is executed in a new process; the shell process resumes when that process completes.
• Use multiple cooperating processes in a single application to enable the application to do more than one thing at once, to increase application robustness, and to make use of already-existing programs.
• There are basically two operations to create or alter a process.
  1. you can fork to create another process that is an exact copy of the existing process,
  2. you can use execve to replace the program running in a process with another program.
• Running another program usually involves both operations, possibly altering the environment in between.
• Newer, lightweight processes (threads) provide separate threads of execution and stacks but shared data segments.
• The Linux specific clone call was created to support threads; it allows more flexibility by specifying which attributes are shared.
• The use of shared memory allows additional control over resource sharing between processes.