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

void reverse(char *f, char *l){
	char c = *f;
	if(f != l+(strlen(l)-1)){
		reverse(f+1, l);
	}
	*(l+(strlen(f)-1))=c;
}

void main(){
	char *s = "12345";
	reverse(s, s);
	printf("%s\n", s);
}

and someone elses:
void reverse(char *p, char *q)
{
	if(q + 1 == p || p == q)
		return;
	int c;
	c = *p;
	*p = *q;
	*q = c;
	reverse(p+1, q-1);
}