Reverse words in sentence while keeping position of white space

Hi guys, something i couldn't figure out....

Write a function to reverse the order of words

Given a sentence of words in a character array, write a C function to reverse the order of words.

char* reverse_word (char* str);

The function gets a character array as its sole argument, reverses the order of words in it, and puts back the result into it. It returns the pointer to the argument on termination for convenience sake.

See the following example.
char str[] = "this     is very beautiful   ";
printf ("[%s]\n", reverse_word(str));   /* print [beautiful     very is this   ] */

Be ware that the position of space characters has not moved; the result of the example above is [beautiful     very is this   ], not [beautiful very is     this   ].

The following constraints apply:
No external function calls. (strlen, strcpy, etc).
No explicit memory allocation. (malloc, realloc, dynamic arrays, etc)

I managed only managed to reverse the order of words which results in [beautiful very is     this   ], however I didn't know how to maintain the position of space characters [beautiful     very is this   ].

this is the code that i wrote:

#include <stdio.h>
char* reverse_word(char* s)
{
    char *str = s;
    int len = string_len(s);
    int i=0;
    int count=0;
   
    reverse_substring(str,0,pos_lastChar(str));
    while(i<=len){
        if(str[i] ==' ' || i==len){
            reverse_substring(str,i-count,i-1);
            count=0;    
        }  
        if(str[i]!=' '){
            count++;
        }
        i++;
    }
    return str;
}

int pos_lastChar(char* s)
{
    int i;
    for(i=string_len(s); i>0;i--){
        if (s[i]!=' ' && s[i]!='\0'){
            return i;            
        }
    }
}

reverse_substring(char* s, int start, int end)
{
    char temp;
    int i,j;
    for(i=start, j=end; i<j; i++,j--)
    {
        temp = s[i];
        s[i]=s[j];
        s[j]=temp;
    }
}    
int string_len(char* str)
{
    int x=0;
    while(*str++){
        x++;
    }
    return (x);
}

int main(int argc, char **argv)
{
    char str[] = "this is   very beautiful   ";
    printf("Length of string: %d\n",string_len(str));
    printf("[%s]\n", reverse_word(str));
}