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

char *reverse(char *p, size_t len)
{
    char *first = p, *last = p + len;
    while (last-- > first)
    {
        char tmp = *last;
        *last = *first;
        *first++ = tmp;
    }
    return p;
}

int main()
{
    char msg[] = "This is a test message.";
    printf("%s\n", reverse(msg, strlen(msg)));
    return 0;
}