Simple worked example of Anton's suggestion:

void reverse_rec(char *begin, char *end)
{
    if (begin < end) {
        char swp = *begin;
        *begin = *end;
        *end = swp;
        reverse_rec(begin+1, end+1);
    }
}

void reverse(char s[])
{
    if (s)
        reverse_rec(s, s+strlen(s)-1);
}

Note that you don't need a while, because recurse_rec stops recursing when begin >= end.

Simple worked example of Anton's suggestion:

void reverse_rec(char *begin, char *end)
{
    if (begin < end) {
        char swp = *begin;
        *begin = *end;
        *end = swp;
        reverse_rec(begin+1, end-1);
    }
}

void reverse(char s[])
{
    if (s)
        reverse_rec(s, s+strlen(s)-1);
}

Note that you don't need a while, because recurse_rec stops recursing when begin >= end.