Process status and termination

During the lifetime of a process, it traverses through many states before it ultimately terminates. Users must have proper mechanisms to be updated with all that happens to a process during its lifetime. Linux provides a set of functions for this purpose.

wait

For processes and threads created by a parent, it might be functionally useful for the parent to know the execution status of the child process/thread. This can be achieved using the wait family of system calls:

#include <sys/types.h>
#include <sys/wait.h>
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, intoptions);
int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options)

These system calls update the calling process with the state change events of a child. The following state change events are notified:

• Termination of child
• Stopped by a signal
• Resumed by a signal

In addition to reporting the status, these APIs allow the parent process to reap a terminated child. A process on termination is put into zombie state until the immediate parent engages the wait call to reap it.

exit

Every process must end. Process termination is done either by the process calling exit() or when the main function returns. A process may also be terminated abruptly on receiving a signal or exception that forces it to terminate, such as the KILL command, which sends a signal to kill the process, or when an exception is raised. Upon termination, the process is put into exit state until the immediate parent reaps it.

The exit calls the sys_exit system call, which internally calls the do_exit routine. The do_exit primarily performs the following tasks (do_exit sets many values and makes multiple calls to related kernel routines to complete its task):

• Takes the exit code returned by the child to the parent.
• Sets the PF_EXITING flag, indicating process exiting.
• Cleans up and reclaims the resources held by the process. This includes releasing mm_struct, removal from the queue if it is waiting for an IPC semaphore, release of filesystem data and files, if any, and calling schedule() as the process is no longer executable.

After do_exit, the process remains in zombie state and the process descriptor is still intact for the parent to collect the status, after which the resources are reclaimed by the system.