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Not thread-safe. Also, from Alex in the comments: "Sounds good! Although probably we will run out of stack waay before that".

It won't run out of stack space with a high enough optimization level so that it uses tail recursion. https://en.wikipedia.org/wiki/Tail_call

You can try this yourself by compiling:

#include <stdio.h>

int i = 0; int x() {--i && x(printf("x"));} int main() {x();}

using gcc with and without '-02'.