What is the difference between using _exit() & exit() in a conventional Linux fork-exec?

You should use _exit (or its synonym _Exit) to abort the child program when the exec fails, because in this situation, the child process may interfere with the parent process' external data (files) by calling its atexit handlers, calling its signal handlers, and/or flushing buffers.

For the same reason, you should also use _exit in any child process that does not do an exec, but those are rare.

In all other cases, just use exit. As you partially noted yourself, every process in Unix/Linux (except one, init) is the child of another process, so using _exit in every child process would mean that exit is useless outside of init.

switch (fork()) {
  case 0:
    // we're the child
    execlp("some", "program", NULL);
    _exit(1);  // <-- HERE
  case -1:
    // error, no fork done ...
  default:
    // we're the parent ...
}

exit() flushes io buffers and does some other things like run functions registered by atexit(). exit() invokes _end( )

_exit() just ends the process without doing that. You call _exit() from the parent process when creating a daemon for example.

Ever notice that main() is a function? Ever wonder what called it in the first place? When a c program runs the shell you are running in provides the executable path to 'exec' system call and the control is passed to kernel which in turn calls the startup function of every executable _start(), calls your main(), when main() returns it then calls _end() Some implementations of C use slightly different names for _end() & _start() ...

exit() and _exit() invoke _end()

Normally - for every main() there should be one & only one exit() call. (or return at the end of main() )

exit() is on the top of _exit(), using conventional C library.

There are the differences:

1. _exit() won't flushes the stdio buffer while exit() flushes the stdio buffer prior to exit.

2. _exit() can not perform clean-up process while exit() can be registered with some function ( i.e on_exit or at_exit) to perform some clean-up process if anything is required before existing the program.

exit(status) simply passes the exit status to _exit(status). It is recommended that whenever to perform fork(), one of them between child and parent, one use _exit() and another use exit().