Interview Question [2]: The Difference Between 'exit()' and '_exit()'

Firstly, the _exit() function's role is the simplest: it directly stops the process, clears its memory space, and destroys its various data structures in the kernel (user-mode constructs). The exit() function, however, adds some wrappers on top of this, performing several additional steps before exiting (calling user-defined cleanup routines). For this reason, some people no longer consider exit() to be a pure system call.

The biggest difference between the exit() function and the _exit() function is that exit() checks for open files and writes the contents of file buffers back to the files before calling the _exit() system call, which is known as "flushing I/O buffers."

Secondly, the numerous differences between exit() and _exit() become particularly prominent when using fork(), and especially vfork().