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

int main(void)
{
	char temp[] = "!dlrow olleH";
	char str[80], *p1, *p2;

    p1 = temp + strlen(temp)-1;
	p2 = str;
	 
	while(p1>=temp) 
	{
	   *p2++=*p1--;
	}

	*p2 = '\0';

	printf("%s", str);

	return 0;
}

Hello world!