form_field_validation(3X)

💖 form_field_validation — bestie edition 💖

okay so this is form_field_validation, a library call from the curses family (that's 3X, iykyk), and it's literally all about making your form fields check their own data before letting garbage through. no cap. 🎀

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✨ NAME ✨

form_field_validation — data type validation for fields 🦋

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✨ 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;

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✨ DESCRIPTION ✨

so like, by default, nobody validates anything. form fields are just out here living their truth, accepting whatever. 💀 you can associate a form with a field type though, and then the form library starts doing the vibe check on input. slay.

🌸 field_arg

returns a pointer to the field's argument block. and what's an argument block, you ask? it's an opaque structure holding a copy of the arguments you passed in a set_field_type call. opaque = don't peek, bestie, it's not for you. 🙈

🌸 field_type

returns a pointer to the field type associated with the form field — aka the one you set by calling set_field_type. 👑

🌸 set_field_type

set_field_type associates a field type with a given form field. this is the type that the validation functions check. most field types are configurable, via arguments that the caller provides when calling set_field_type. so you're not just picking a vibe, you're customizing it. periodt. 💅

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✨ FIELD TYPES ✨

the form library defines several field types, and they're implemented via the FIELDTYPE data structure, which contains several pointers to functions. very struct, very demure. 🧚

see the form_fieldtype(3X) manual page — it describes functions you can use to construct a field-type dynamically. DIY field types, iconic behavior. 🔥

the predefined types are as follows, and yes, we're going through all of them:

🌈 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. (so it just picks the first one, no drama. 😌)

the library copies the string list, so you may use a list that lives in automatic variables on the stack. go off, stack-allocated bestie. 🫶

🌈 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. (yeah. just ignored. moving on. 💀)

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. like, the WHOLE thing, bestie, not just a lil piece.

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. chef's kiss, honestly. 👑

🌈 TYPE_IPV4

an Internet Protocol Version 4 address. required parameter:

• none 🎀 (yes, truly, no extra args, she's low maintenance)

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. so it checks the shape, not the truth. 💀

this is an ncurses extension; this field type may not be available in other curses implementations. so if you're on something else, don't come for her.

it is possible to set up new programmer-defined field types. make your own, main character energy. 🌟

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✨ 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. slay. ✅
• E_SYSTEM_ERROR — system error occurred (see errno(3)). 🚨

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✨ PORTABILITY ✨

these routines emulate the System V forms library. they were not supported on Version 7 or BSD versions. so, like, vintage, but not that vintage. 🦋

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✨ AUTHORS ✨

Juergen Pfeifer. manual pages and adaptation for new curses by Eric S. Raymond. legends, honestly. 👑💖

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✨ SEE ALSO ✨

curses(3X), form(3X), form_fieldtype(3X), form_variables(3X) — go read them, they're all part of the same friend group. 🫶

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okay that's the whole thing, bestie. this page came from ncurses 6.6, dated 2025-08-16, and it's form_field_validation(3X) — now go validate something. mwah. 💅✨🎀