#include <stdio.h>

void reverse ( char* cp_str )
{
	char* cp_rev_ptr = NULL;
	cp_rev_ptr = cp_str;
	while ( ( cp_rev_ptr ) && ( *cp_rev_ptr != '\0' ) ) 
cp_rev_ptr++;
	cp_rev_ptr--;

	while ( cp_str < cp_rev_ptr ) {
		*cp_str ^= *cp_rev_ptr ^= *cp_str ^= 
*cp_rev_ptr;
		cp_str++;
		cp_rev_ptr--;
	}
}

int main ( int argc, char* argv [] )
{
	char array [] = {"dlroW olleH"};

	printf ("\nBefore :%s", array );
	reverse ( array );
	printf ("\nAfter :%s", array );

	return ( 0 );
}

Output
======
Before :dlroW olleH
After :Hello World