program to reverse the spelling of words in a sentence

The need:

     I don't know what could be the need of this program but I found this question on many sites and thats why I wrote this program.

The code:

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#include<stdio.h>
#include<string.h>

void reverse(char *str,int start, int end)
{
    char temp;
    int i,j;
    i=end-start;
    for(j=0;j<=i/2;j++)
    {       
        temp=*(str+start+j);
        *(str+start+j)=*(str+start+i-j);
        *(str+start+i-j)=temp;
    }
}

rev_word(char *str)
{
    int start=0,end=0,i=0;   
    while(*(str+i)!='\0')
    {
        if(*(str+i)==' ')
        {
            end=i-1;
            reverse(str, start, end);
            i++;
            start=i;
        }
        else
            i++;
    }
    end=i-1;
    reverse(str, start, end);
}

int main()
{
    char a[]="I dont know why useless programs are asked";
    printf("before\n%s\n",a);
    rev_word(a);
    printf("after\n%s\n",a);
    return 0;
}

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Approach:
   The approach is simple. make a note of start and end points (indices) of each word and reverse with the help of those points. The UDF reverse() does the job here.