Well a segfault is when your program tries to access an area of memory out of the area of memory assigned to it, the kernel throws a wobbler and throws a segmentation fault.

If you ran an infinite recursive program eventually you would probably run out of stack space, thus get a segfault. I've not tried it (for obvious reasons).

The classic cause of a segfaults is de-referencing a null pointer, amongst other things.

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)