3.2 Process StateThe state of a process indicates its status at a particular time. Most operating systems allow some form of the states listed in Table 3.1. A state diagram is a graphical representation of the allowed states of a process and the allowed transitions between states. Figure 3.1 shows such a diagram. The nodes of the graph in the diagram represent the possible states, and the edges represent possible transitions. A directed arc from state A to state B means that a process can go directly from state A to state B. The labels on the arcs specify the conditions that cause the transitions between states to occur. Figure 3.1. State diagram for a simple operating system. While a program is undergoing the transformation into an active process, it is said to be in the new state. When the transformation completes, the operating system puts the process in a queue of processes that are ready to run. The process is then in the ready or runnable state. Eventually the component of the operating system called the process scheduler selects a process to run. The process is in the running state when it is actually executing on the CPU.Section 1.2, input and output can be thousands of times slower than ordinary instructions. A process performs I/O by requesting the service through a library function that is sometimes called a system call. During the execution of a system call, the operating system regains control of the processor and can move the process to the blocked state until the operation completes.A context switch is the act of removing one process from the running state and replacing it with another. The process context is the information that the operating systems needs about the process and its environment to restart it after a context switch. Clearly, the executable code, stack, registers and program counter are part of the context, as is the memory used for static and dynamic variables. To be able to transparently restart a process, the operating system also keeps track of the process state, the status of program I/O, user and process identification, privileges, scheduling parameters, accounting information and memory management information. If a process is waiting for an event or has caught a signal, that information is also part of the context. The context also contains information about other resources such as locks held by the process.The ps utility displays information about processes. By default, ps displays information about processes associated with the user. The -a option displays information for processes associated with terminals. The -A option displays information for all processes. The -o option specifies the format of the output.
Example 3.3The following is sample output from the ps -a command.
The POSIX:XSI Extension provides additional arguments for the ps command. Among the most useful are the full (-f) and the long (-l) options. Table 3.2 lists the fields that are printed for each option. An (all) in the option column means that the field appears in all forms of ps. Example 3.4The execution of the ps -la command on the same system as for Example 3.3 produced the following output.
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