Answer by chux for Does C have an equivalent of std::less from C++?

Does C offer something with similar functionality which would allow safely comparing arbitrary pointers.

No

First let us only consider object pointers. Function pointers bring in a whole other set of concerns.

2 pointers p1, p2 can have different encodings and point to the same address so p1 == p2 even though memcmp(&p1, &p2, sizeof p1) is not 0. Such architectures are rare.

Yet conversion of these pointer to uintptr_t does not require the same integer result leading to (uintptr_t)p1 != (uinptr_t)p2.

(uintptr_t)p1 < (uinptr_t)p2 itself is well legal code, by may not provide the hoped for functionality.

If code truly needs to compare unrelated pointers, form a helper function less(const void *p1, const void *p2) and perform platform specific code there.

Perhaps:

// return -1,0,1 for <,==,> 
int ptrcmp(const void *c1, const void *c1) {
  // Equivalence test works on all platforms
  if (c1 == c2) {
    return 0;
  }
  // At this point, we know pointers are not equivalent.
  #ifdef UINTPTR_MAX
    uintptr_t u1 = (uintptr_t)c1;
    uintptr_t u2 = (uintptr_t)c2;
    // Below code "works" in that the computation is legal,
    //   but does it function as desired?
    // Likely, but strange systems lurk out in the wild. 
    // Check implementation before using
    #if tbd
      return (u1 > u2) - (u1 < u2);
    #else
      #error TBD code
    #endif
  #else
    #error TBD code
  #endif 
}