Does C have an equivalent of std::less from C++?

Question

I was recently answering a question on the undefined behaviour of doing p < q in C when p and q are pointers into different objects

Answer 1

I once tried to find a way around this and I did find a solution that works for overlapping objects and in most other cases assuming the compiler does the "usual" thing.

You can first implement the suggestion in How to implement memmove in standard C without an intermediate copy? and then if that doesn't work cast to uintptr (a wrapper type for either uintptr_t or unsigned long long depending on whether uintptr_t is available) and get a most-likely accurate result (although it probably wouldn't matter anyway):

#include 
#ifndef UINTPTR_MAX
typedef unsigned long long uintptr;
#else
typedef uintptr_t uintptr;
#endif

int pcmp(const void *p1, const void *p2, size_t len)
{
    const unsigned char *s1 = p1;
    const unsigned char *s2 = p2;
    size_t l;

    /* Check for overlap */
    for( l = 0; l < len; l++ )
    {
        if( s1 + l == s2 || s1 + l == s2 + len - 1 )
        {
            /* The two objects overlap, so we're allowed to
               use comparison operators. */
            if(s1 > s2)
                return 1;
            else if (s1 < s2)
                return -1;
            else
                return 0;
        }
    }

    /* No overlap so the result probably won't really matter.
       Cast the result to `uintptr` and hope the compiler
       does the "usual" thing */
    if((uintptr)s1 > (uintptr)s2)
        return 1;
    else if ((uintptr)s1 < (uintptr)s2)
        return -1;
    else
        return 0;
}