Process Termination or destroy a process

Processes may request their own termination by making the exit( ) system call, typically returning an int. This int is passed along to the parent if it is doing a wait( ), and is typically zero on successful completion and some non-zero code in the event of problems.

child code:

            int exitCode;
            exit( exitCode );  // return exitCode; has the same effect when executed from main( )

parent code:

            pid_t pid;
            int status
            pid = wait( &status ); 
            // pid indicates which child exited. exitCode in low-order bits of status
            // macros can test the high-order bits of status for why it stopped

Processes may also be terminated by the system for a variety of reasons, including:
- The inability of the system to deliver necessary system resources.
- In response to a KILL command, or other un handled process interrupt.
- A parent may kill its children if the task assigned to them is no longer needed.
- If the parent exits, the system may or may not allow the child to continue without a parent. ( On UNIX systems, orphaned processes are generally inherited by init, which then proceeds to kill them. The UNIX nohup command allows a child to continue executing after its parent has exited. )
When a process terminates, all of its system resources are freed up, open files flushed and closed, etc. The process termination status and execution times are returned to the parent if the parent is waiting for the child to terminate, or eventually returned to init if the process becomes an orphan. ( Processes which are trying to terminate but which cannot because their parent is not waiting for them are termed zombies. These are eventually inherited by init as orphans and killed off. Note that modern UNIX shells do not produce as many orphans and zombies as older systems used to. )

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