form_field(3X)                                                  form_field(3X)

       form_fieldtype - define validation-field types

       #include <form.h>
       FIELDTYPE *new_fieldtype(
           bool (* const field_check)(FIELD *, const void *),
           bool (* const char_check)(int, const void *));
       int free_fieldtype(FIELDTYPE *fieldtype);
       int set_fieldtype_arg(
           FIELDTYPE *fieldtype,
           void *(* const make_arg)(va_list *),
           void *(* const copy_arg)(const void *),
           void  (* const free_arg)(void *));
       int set_fieldtype_choice(
           FIELDTYPE *fieldtype
           bool (* const next_choice)(FIELD *, const void *),
           bool (* const prev_choice)(FIELD *, const void *));
       FIELDTYPE *link_fieldtype(FIELDTYPE *type1,
                                 FIELDTYPE *type2);

       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.

       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

       The pointer-valued routines return NULL on error.

       The integer-valued routines return one of the following codes on error:

       E_OK The routine succeeded.

            System error occurred (see errno).

            Routine detected an incorrect or out-of-range argument.

            The field is already connected to a form.

       curses(3X), form(3X).

       The header file <form.h> automatically includes the header file

       All of the (char *) arguments of these functions should actually be
       (void *).  The type has been left uncorrected for strict compatibility
       with System V.

       These routines emulate the System V forms library.  They were not
       supported on Version 7 or BSD versions.

       Juergen Pfeifer.  Manual pages and adaptation for new curses by Eric S.