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

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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 ...
}

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