# form_field_validation 3x 2025-08-16 ncurses 6.6 Library calls

    form_field_validation(3x)        Library calls       form_field_validation(3x)

## NAME

           form_field_validation - data type validation for fields

## SYNOPSIS

           #include <form.h>

           void *field_arg(const FIELD *field);
           FIELDTYPE *field_type(const FIELD *field);
           int set_field_type(FIELD *field, FIELDTYPE *type, ...);

           /* field types */
           FIELDTYPE *TYPE_ALNUM;
           FIELDTYPE *TYPE_ALPHA;
           FIELDTYPE *TYPE_ENUM;
           FIELDTYPE *TYPE_INTEGER;
           FIELDTYPE *TYPE_NUMERIC;
           FIELDTYPE *TYPE_REGEXP;
           FIELDTYPE *TYPE_IPV4;

## DESCRIPTION

           By  default, no validation is done on form fields.  You can associate a
           form with a field type, making the form library validate input.

### field_arg

           Returns a pointer to the field's argument block.  The argument block is
           an opaque structure containing a copy of the arguments  provided  in  a
           set_field_type call.

### field_type

           Returns  a  pointer  to  the field type associated with the form field,
           i.e., by calling set_field_type.

### set_field_type

           The function set_field_type associates a field type with a  given  form
           field.   This  is the type checked by validation functions.  Most field
           types are configurable, via arguments which the  caller  provides  when
           calling set_field_type.

## FIELD TYPES

           The form library defines several field types.  They are implemented via
           the  FIELDTYPE  data  structure,  which  contains  several  pointers to
           functions.

           See the form_fieldtype(3x) manual page, which describes functions which
           can be used to construct a field-type dynamically.

           The predefined types are as follows:

### TYPE_ALNUM

           Alphanumeric data.  Required parameter:

           o   a third int argument, a minimum field width.

### TYPE_ALPHA

           Character data.  Required parameter:

           o   a third int argument, a minimum field width.

### TYPE_ENUM

           Accept one of a specified set of strings.  Required parameters:

           o   a third (char **) argument pointing to a string list;

           o   a fourth int flag argument to enable case-sensitivity;

           o   a fifth int flag argument specifying whether a partial  match  must
               be  a  unique one.  If this flag is off, a prefix matches the first
               of any set of more than one list elements with that prefix.

           The library copies the string list, so you may use a list that lives in
           automatic variables on the stack.

### TYPE_INTEGER

           Integer data, parsable to an integer by atoi(3).  Required parameters:

           o   a third int argument controlling the precision,

           o   a fourth long argument constraining minimum value,

           o   a fifth long constraining maximum value.  If the maximum  value  is
               less  than  or  equal  to  the  minimum  value, the range is simply
               ignored.

           On return, the field buffer is formatted according to the printf format
           specification ".*ld", where  the  "*"  is  replaced  by  the  precision
           argument.

           For details of the precision handling see printf(3).

### TYPE_NUMERIC

           Numeric data (may have a decimal-point part).  Required parameters:

           o   a third int argument controlling the precision,

           o   a fourth double argument constraining minimum value,

           o   and  a  fifth  double  constraining  maximum value.  If your system
               supports locales, the decimal  point  character  must  be  the  one
               specified  by  your  locale.   If the maximum value is less than or
               equal to the minimum value, the range is simply ignored.

           On return, the field buffer is formatted according to the printf format
           specification ".*f",  where  the  "*"  is  replaced  by  the  precision
           argument.

           For details of the precision handling see printf(3).

### TYPE_REGEXP

           Regular expression data.  Required parameter:

           o   a  third  argument, a regular expression (char *) string.  The data
               is valid if the regular expression matches it.

           Regular expressions are in the format of regcomp and regexec.

           The regular expression must match the whole field.   If  you  have  for
           example, an eight character wide field, a regular expression "^[0-9]*$"
           always means that you have to fill all eight positions with digits.  If
           you  want  to  allow fewer digits, you may use for example "^[0-9]* *$"
           which is good for trailing spaces (up to an empty field), or "^ *[0-9]*
           *$" which is good for leading and trailing spaces around the digits.

### TYPE_IPV4

           An Internet Protocol Version 4 address.  Required parameter:

           o   none

           The form library checks whether or not the buffer has the form a.b.c.d,
           where a, b, c, and d are numbers in  the  range  0  to  255.   Trailing
           blanks in the buffer are ignored.  The address itself is not validated.

           This  is  an ncurses extension; this field type may not be available in
           other curses implementations.

           It is possible to set up new programmer-defined field types.

## RETURN VALUE

           The functions field_type and  field_arg  return  NULL  on  error.   The
           function set_field_type returns one of the following:

           E_OK The routine succeeded.

           E_SYSTEM_ERROR
                System error occurred (see errno(3)).

## 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.

## SEE ALSO

           curses(3x), form(3x), form_fieldtype(3x), form_variables(3x)

    ncurses 6.6                       2025-08-16         form_field_validation(3x)