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:

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.

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):

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.