Understanding the Linux Kernel

3.4. Destroying Processes

Most processes "die" in the sense that they terminate the execution of the code they were supposed to run. When this occurs, the kernel must be notified so that it can release the resources owned by the process; this includes memory, open files, and any other odds and ends that we will encounter in this book, such as semaphores.

The usual way for a process to terminate is to invoke the exit( ) system call. This system call may be inserted by the programmer explicitly. Additionally, the exit( ) system call is always executed when the control flow reaches the last statement of the main procedure (the main( ) function in C programs).

Alternatively, the kernel may force a process to die. This typically occurs when the process has received a signal that it cannot handle or ignore (see Chapter 9) or when an unrecoverable CPU exception has been raised in Kernel Mode while the kernel was running on behalf of the process (see Chapter 4).

3.4.1. Process Termination

All process terminations are handled by the do_exit( ) function, which removes most references to the terminating process from kernel data structures. The do_exit( ) function executes the following actions:

1. Sets the PF_EXITING flag in the flag field of the process descriptor to denote that the process is being eliminated.

2. Removes, if necessary, the process descriptor from an IPC semaphore queue via the sem_exit( ) function (see Chapter 18) or from a dynamic timer queue via the del_timer( ) function (see ...