man 3 exit The exit() function causes normal process termination and the least significant byte of status (i.e., status & 0xFF) is returne

man 3 exit The exit() function causes normal process termination and the least significant byte of status (i.e., status & 0xFF) is returned to the parent (see wait(2)). All functions registered with atexit(3) and on_exit(3) are called, in the reverse order of their registration. (It is possible for one of these functions to use atexit(3) or on_exit(3) to register an additional function to be executed during exit processing; the new registration is added to the front of the list of functions that remain to be called.) If one of these functions does not return (e.g., it calls _exit(2), or kills itself with a signal), then none of the remaining functions is called, and further exit processing (in particular, flushing of stdio(3) streams) is abandoned. If a function has been registered multiple times using atexit(3) or on_exit(3), then it is called as many times as it was registered. All open stdio(3) streams are flushed and closed. Files created by tmpfile(3) are removed. These all happen inside userspace. The kernel doesn't necessarily know it's exiting. It's _exit() that tells the kernel the process is exiting. The kernel then does some more processing like closing file descriptors. man 2 _exit _exit() terminates the calling process "immediately". Any open file descriptors belonging to the process are closed. Any children of the process are inherited by init(1) (or by the nearest "subreaper" process as defined through the use of the prctl(2) PR_SET_CHILD_SUBREAPER operation). The process's parent is sent a SIGCHLD signal. The value status & 0xFF is returned to the parent process as the process's exit status, and can be collected by the parent using one of the wait(2) family of calls.