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 fields 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 doesnt allocate memory but simply loads the argument into a single scalar value.
The form driver requests REQ_NEXT_CHOICE and REQ_PREV_CHOICE assume that the possible values of a field form an ordered set, and provide the forms user with a way to move through the set. The set_fieldtype_choice function allows forms programmers to define successor and predecessor functions for the field type. These functions take the field pointer and an argument-block structure as arguments.
The pointer-valued routines return NULL on error.
The integer-valued routines return one of the following codes on error: