DESCRIPTION
The function new_fieldtype creates a new field type usable for data validation. You supply it with field_check, a predicate to check the validity of an entered data string whenever the user attempt to leave a field. The (FIELD *) argument is passed in so the validation predicate can see the field’s buffer, sizes and other attributes; the second argument is an argument-block structure, about which more below. You also supply new_fieldtype with char_check, a function to validate input characters as they are entered; it will be passed the character to be checked and a pointer to an argument-block structure.
The function free_fieldtype frees the space allocated for a given validation type.
The function set_fieldtype associates three storage-management functions with a field type. The mak_arg function is automatically applied to the list of arguments you give set_field_type when attaching validation to a field; its job is to bundle these into an allocated argument-block object which can later be passed to validation predicated. The other two hook arguments should copy and free argument-block structures. They will be used by the forms-driver code. You must supply the mak_arg function, the other two are optional, you may supply NULL for them. In this case it is assumed, that mak_arg doesn’t allocate memory but simply loads the argument into a single scalar value.