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4.1.5.1. Process creation

A new process is created because an existing process makes an exact copy of itself. This child process has the same environment as its parent, only the process ID number is different. This procedure is called forking.

After the forking process, the address space of the child process is overwritten with the new process data. This is done through an exec call to the system.

The fork-and-exec mechanism thus switches an old command with a new, while the environment in which the new program is executed remains the same, including configuration of input and output devices, environment variables and priority. This mechanism is used to create all UNIX processes, so it also applies to the Linux operating system. Even the first process, init, with process ID 1, is forked during the boot procedure in the so-called bootstrapping procedure.

This scheme illustrates the fork-and-exec mechanism. The process ID changes after the fork procedure:

Figure 4-1. Fork-and-exec mechanism

There are a couple of cases in which init becomes the parent of a process, while the process was not started by init, as we already saw in the pstree example. Many programs, for instance, daemonize their child processes, so they can keep on running when the parent stops or is being stopped. A window manager is a typical example; it starts an xterm process that generates a shell that accepts commands. The window manager then denies any further responsibility and passes the child process to init. Using this mechanism, it is possible to change window managers without interrupting running applications.

Every now and then things go wrong, even in good families. In an exceptional case, a process might finish while the parent does not wait for the completion of this process. Such an unburied process is called a zombie process.

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