Question

I wrote the following code.

<?php
function f(){
  return f();
}
f();

and get the output

$ php test.php
Segmentation fault

Why? I didn’t use any pointers.

This is StackOverflow ?

Answer 1

This is a case of infinite recursion, but that is not specifically the cause. It is a stack overflow. When you have recursion, whether infinite or not, there is a max amount of depth you can recurse (add to the stack) which is based on the size of your stack (in bytes).

Technically this is infinite, but you won’t get any errors for quite a while:

<?php
function f(){
  sleep(1);
  return f();
}
f();

What you need is known as a base case in your recursion in order to stop it before it consumes the entire stack.

<?php
function f($i){
  if($i == 10)
    return;

  echo $i;

  return f(++$i);
}
f(0);

Which will print 0 to 9.

The segmentation fault error is coming from the operating system, reporting the PHP application has encountered an issue adding to the stack. That won’t make it to your script because at the system level the binary which makes up PHP has failed.