Program to reverse words in string

reverse_words.c

#include <stdio.h>
#include <stdlib.h>

/* This function reverses the whole string */
void reverse(char s[]){
        int i=0, j;
        int end =  strlen(s);

        for(i=0, j=end-1; i<j; i++, j--){
                char temp = s[i];
                s[i] = s[j];
                s[j] = temp;
        }

}
/* This function reverses a given word, we need to provide stat and end of the word */
void reverse_word(char * start, char * end){
        while(start < end){
                char temp = *start;
                *start++ = *end;
                *end-- = temp;
        }
}

void reverse_words(char *string){
        char * source = string;
        char *begin = string;

        if(string == NULL) return;

        /* Step 1 : Reverse the whole string */
        reverse(string);

        /* Now reverse individual words in reversed string */
        while(*source != '\0'){
                if(*(source +1) == ' ') {
                        reverse_word(begin, source);
                        begin =  source + 2;
                        source++;
                        
                }
             /*Case when it is last word of string */
                else if(*(source + 1) == '\0'){
                        reverse_word(begin, source);
                }
                source++;
        }       
}

/* driver program to run above code */
#define MAX_LENGTH 100
int main(){
    char string[MAX_LENGTH] = "This is a test string";
    reverse_words(string);
    return 0;
}