Cursed string reversal in C

/*
============================================================
  String reversal challenge but it's cursed
============================================================

Write a function that reverses a null-terminated string in-place, but with a few constraints :
    1. You only get a single char*, which is initially at the start of the string.
    2. No extra variables can be declared. You only have your one blessed char*.
    3. No std functions.
    4. You can only write helper functions that take a single char** to your blessed char*.

============================================================
*/

#include <stdio.h>

// helpers

void swapWithNext(char** ptrRef)
{
    (*ptrRef)[0] = (*ptrRef)[0] + (*ptrRef)[1];
    (*ptrRef)[1] = (*ptrRef)[0] - (*ptrRef)[1];
    (*ptrRef)[0] = (*ptrRef)[0] - (*ptrRef)[1];
}

void rewindToStart(char** ptrRef)
{
    (*ptrRef)--;
    for( ; **ptrRef >= 0; (*ptrRef)--) {}
}

void markEnds(char** ptrRef)
{
    **ptrRef *= -1;

    for( ; (*ptrRef)[1] != '\0'; (*ptrRef)++) {}
    **ptrRef *= -1;

    rewindToStart(ptrRef);
}

void leftShiftTillEndCh(char** ptrRef)
{
    swapWithNext(ptrRef);
    **ptrRef *= -1;
    (*ptrRef)++;
    **ptrRef *= -1;

    while(1)
    {
        swapWithNext(ptrRef);

        if(**ptrRef < 0)
        {
            break;
        }

        (*ptrRef)++;
    }

    rewindToStart(ptrRef);
}

// the actual function
void cursedReverse(char* ptr)
{
    if(ptr[0] == '\0')
    {
        return;
    }

    if(ptr[1] == '\0')
    {
        return;
    }

    markEnds(&ptr);

    while(1)
    {
        if(ptr[0] < 0 && ptr[1] < 0)
        {
            swapWithNext(&ptr);
            ptr[0] *= -1;
            ptr[1] *= -1;
            break;
        }

        leftShiftTillEndCh(&ptr);
    }
}

// test
void main(int argc, char* argv[])
{
    cursedReverse(argv[1]);
    printf("%s", argv[1]);
}