Pointer increment with ++

Question

I am trying to brush up on my C and I have the following code it works when I use the i offset but not with hold++, I don't see why I thought they did same thing? Here is the version I can't get to work:

char* reversestring2(char* s)
{
    int mysize = strlen(s);
    char* temp = s+mysize;
     //I do +1 for the terminator
    char* hold = (char*) malloc(mysize+1);

    while(temp > s)
    {
        *(hold) = *--temp;
        hold++;
    }
    *(hold++) = '\0';
    //printf("pre cpy %s", hold);
    strcpy(s,hold);
    //printf("%s", hold);
    return(s);
}

Thanks

char* reversestring(char* s)
{
    int mysize = strlen(s);
    char* temp = s+mysize;
    char* hold = (char*) malloc(mysize+1);
    int i=0;
    while(temp > s)
    {
        *(hold+i) = *--temp;
        //hold++;
        i++;
    }
    *(hold+i) = '\0';
    //printf("pre cpy %s", hold);
    strcpy(s,hold);
    //printf("%s", hold);
    return(s);
}

int main()
{
    //printf("%s\n", "you suck");
    char test[100] = "you suck";
    reversestring(test);
    printf("%s\n", test);
    //or
    char* hold = (char*) malloc(100);
    hold = reversestring(test);
    if(hold == NULL)
        printf("%s\n", "ERROR");
    printf("Second run: %s\n", hold);
}

Answer 1

hold is a pointer variable which points to the beginning (first char element) of the block of memory that you allocate. hold++ will make it point to the next char in that block of memory. hold = hold + 5 will make it point to the 6-th char in that block of memory and so on.

if you want to use hold++ instead, you need to delete i++, and replace *(hold+i) = *--temp; with *hold = *--temp; if you want so.

*hold is equivalent to *(hold + 0), I think you get the idea

Answer 2

When you use hold++, the pointer hold advances to the end of the allocated array. Then you do this:

*(hold+i) = '\0';

which inserts a value into memory outside the bounds of the allocated space (e.g 200 steps from the beginning of an array of length 100), which causes undefined behavior. Then this:

strcpy(s,hold);

which copies who-knows-what from uncontrolled memory into the string.