Change pointer adress inside function in C

Question

I'm trying to make a program that deletes extra white spaces from a string. It should keep one space but delete any extra.

I wrote this code, which works up until the point I have to change the passed pointer's address to the new string I made in the function. I have read that this is not possible without making a pointer to a pointer. The problem is, for my assignment, I can't change the argument types.

Is there any way to do this without making a new string, or is there a way to change the pointer address of s to point to newstr?

My code:

void str_trim(char *s)
{
    // your code
    char newstr[200];
    char *newstrp = newstr;

    while (*s != '\0')
    {
        if (*s != ' ')
        {
            *newstrp = *s;
            *newstrp++;
            *s++;
        }

        else if (*s == ' ')
        {
            *newstrp = *s;
            *newstrp++;
            *s++;

            while (*s == ' ')
            {
                s++;
            }
        }
        else
        {
            *newstrp = *s;
            *newstrp++;
            *s++;
        }
    }
    s = &newstr;
}

Edit: This is what I ended up using

char *p = s, *dp = s;

    while (*dp == ' ')
        dp++;
    ;

    while (*dp)
    {
        if (*dp == ' ' && *(dp - 1) == ' ')
        {
            dp++;
        }

        else
        {
            *p++ = *dp++;
        }
    }

    if (p > s && *(p - 1) == ' ')
    {
        p--;
    }

    *p = '\0';

Answer 1

Is there any way to do this without making a new string, or is there a way to change the pointer address of s to point to newstr?

There is no way to change the caller's variable without changing the argument type of s to char**. So, that leaves you with two choices:

You can change your function to return the new char* pointer. However, you can't return a pointer to a local variable, which means you will have to malloc() the new string, and then the caller will have to free() it (probably not what you want to do), eg:

char* str_trim(char *s)
{
    char *newstr = malloc(200);
    //...
    return newstr;
}

char *s = str_trim(...);
...
free(s);

Online Demo

Otherwise, simply get rid of the local char[] buffer altogether, and just modify the contents of s in-place (this is your only option if you can't change the signature of str_trim() at all), eg:

void str_trim(char *s)
{
    char *newstr = s;
    while (*s != '\0')
    {
        if (*s != ' ')
        {
            *newstr = *s;
            ++newstr;
            ++s;
        }
        else
        {
            *newstr = *s;
            ++newstr;
            ++s;

            while (*s == ' ')
            {
                ++s;
            }
        }
    }
    *newstr = '\0';
}

char s[] = "...";
str_trim(s);

Online Demo