xxx_reset()

This function is called when MySQL finds the first row in a new group. It should reset any internal summary variables and then use the given UDF_ARGS argument as the first value in your internal summary value for the group. Declare xxx_reset() as follows:

void xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
               char *is_null, char *error);

xxx_reset() is not needed or used in MySQL 5.5, in which the UDF interface uses xxx_clear() instead. However, you can define both xxx_reset() and xxx_clear() if you want to have your UDF work with older versions of the server. (If you do include both functions, the xxx_reset() function in many cases can be implemented internally by calling xxx_clear() to reset all variables, and then calling xxx_add() to add the UDF_ARGS argument as the first value in the group.)

xxx_clear()

This function is called when MySQL needs to reset the summary results. It is called at the beginning for each new group but can also be called to reset the values for a query where there were no matching rows. Declare xxx_clear() as follows:

void xxx_clear(UDF_INIT *initid, char *is_null, char *error);

is_null is set to point to CHAR(0) before calling xxx_clear().

If something went wrong, you can store a value in the variable to which the error argument points. error points to a single-byte variable, not to a string buffer.

xxx_clear() is required by MySQL 5.5.

xxx_add()

This function is called for all rows that belong to the same group. You should use it to add the value in the UDF_ARGS argument to your internal summary variable.

void xxx_add(UDF_INIT *initid, UDF_ARGS *args,
             char *is_null, char *error);