What does “ulimit -s unlimited” do?

Question

There are understandably many related questions on stack allocation

What and where are the stack and heap?

Why is there a limit on the stack size?

Si

Answer 1

stack size can indeed be unlimited. _STK_LIM is the default, _STK_LIM_MAX is something that differs per architecture, as can be seen from include/asm-generic/resource.h:

/*
 * RLIMIT_STACK default maximum - some architectures override it:
 */
#ifndef _STK_LIM_MAX
# define _STK_LIM_MAX           RLIM_INFINITY
#endif

As can be seen from this example generic value is infinite, where RLIM_INFINITY is, again, in generic case defined as:

/*
 * SuS says limits have to be unsigned.
 * Which makes a ton more sense anyway.
 *
 * Some architectures override this (for compatibility reasons):
 */
#ifndef RLIM_INFINITY
# define RLIM_INFINITY          (~0UL)
#endif

So I guess the real answer is - stack size CAN be limited by some architecture, then unlimited stack trace will mean whatever _STK_LIM_MAX is defined to, and in case it's infinity - it is infinite. For details on what it means to set it to infinite and what implications it might have, refer to the other answer, it's way better than mine.