form_field_validation(3X)

💅 form_field_validation(3X) — data type validation for fields 💅

okay bestie, welcome to the Library calls page for form_field_validation(3X) — aka the one where your form fields get their data-type vibe check ✨

🌸 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, and that makes 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. opaque means “don’t peek inside, bestie” 🙈

field_type

Returns a pointer to the field type associated with the form field — the one you set 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:

• a third int argument, a minimum field width.

TYPE_ALPHA

Character data. Required parameter:

• a third int argument, a minimum field width.

TYPE_ENUM

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

• a third (char **) argument pointing to a string list;
• a fourth int flag argument to enable case-sensitivity;
• 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. stack list? valid. library said “copied,” periodt 💅

TYPE_INTEGER

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

• a third int argument controlling the precision,
• a fourth long argument constraining minimum value,
• 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:

• a third int argument controlling the precision,
• a fourth double argument constraining minimum value,
• 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:

• 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:

• none — literally no extra parameter, bestie.

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.

heads up 🚨 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. DIY field-type era 💜

🔥 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)

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💖 ncurses 6.5 · 2025-08-16 · form_field_validation(3X) — k bye bestie, go validate those fields! 🫶✨