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       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 attempts 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_arg associates three storage-management functions with a field
       type.  The make_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 make_arg function, the other  two  are
       optional, you may supply NULL for them.  In this case it is assumed that make_arg does not
       allocate memory but simply loads the argument into a single scalar value.

       The function link_fieldtype creates a new field type from the two given types.   They  are
       connected by an logical 'OR'.

       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.  They set errno according to their

       E_OK The routine succeeded.

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

            System error occurred, e.g., malloc failure.

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

       E_OK The routine succeeded.

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

            The field is already connected to a form.

            The field is the current field.

            System error occurred (see errno(3)).


       ncurses(3NCURSES), form(3FORM).


       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.


       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. Raymond.