Hi everyone, In http://www.monetdb.org/Documentation/Cookbooks/SQLrecipes/UserDefinedFunctio... In thesection about how to extend SQL with a simple scalar function to reverse a string step 3. Extension starts with a definitin of the MAL signatures. See the example given, or browse through the files in monetdb5/modules/mal/*.mal to get a glimpse on how to write them. The MonetDB kernel documentation provides more details. The file contains the MAL snippet: command reverse(ra1:str):str address UDFreverse comment "Reverse a string"; 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