step 4. The signature says that it expects a command body implementation under the name UDFreverse, shown below. The C-signature is a direct mapping, where arguments are passed by reference and the return value(s)  references are the first in the arguments list. The body should return a (malloced) string to denote an exception being raised or MAL_SUCCEED upon access.
#include "udf.h"

static str
reverse(const char *src)
{
    size_t len;
    str ret, new;

    /* The scalar function returns the new space */
    len = strlen(src);
    ret = new = GDKmalloc(len + 1);
    if (new == NULL)
        return NULL;
    new[len] = 0;
    while (len > 0)
        *new++ = src[--len];
    return ret;
}

str
UDFreverse(str *ret, str *src)
{
    if (*src == 0 || strcmp(*src, str_nil) == 0)
        *ret = GDKstrdup(str_nil);
    else
        *ret = reverse(*src);
    return MAL_SUCCEED;
}

I don't know why the argument name of  "command reverse(ra1:str):str" different from function definition UDFreverse(str *ret, str *src)?

how does ret come from? is it according to the naming rules that the first argument is the result, must it be a string type?

Thanks!

Meng