## NAME

           form_fieldtype - define validation-field types

## SYNOPSIS

           #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);

## 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-man-
           agement 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  argu-
           ment-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 func-
           tion allows forms programmers to define successor and pre-
           decessor  functions  for  the field type.  These functions
           take the field pointer and an argument-block structure  as
           arguments.

## RETURN VALUE

           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.

           E_SYSTEM_ERROR
                System error occurred (see errno).

           E_BAD_ARGUMENT
                Routine detected an incorrect or  out-of-range  argu-
                ment.

           E_CONNECTED
                The field is already connected to a form.

## SEE ALSO

           curses(3x), form(3x).

## NOTES

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

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

## PORTABILITY

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

## AUTHORS

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