field_validation(3FORM)

💖 field_validation(3FORM) — the manual page, but make it make sense 💖

okay bestie, so this is a page from the ncurses manual, section 3FORM. It's filed under field_validation, which lowkey gives away the whole plot: it's all about data type validation for fields. Yes, form fields. Yes, the cursed terminal UI kind. We're doing this together, hand in hand. 🫶

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✨ 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 🌸

So here's the tea: by default, no validation happens at all on form fields. Like, the library simply does not care what you type in there. Nothing. Zero. It's giving "unsupervised toddler with a keyboard". 💀

But! You can associate a form with a field type, and then the form library will actually validate the input for you. Iconic behaviour, honestly.

🎀 field_arg

Returns a pointer to the field's argument block. That argument block is an opaque structure holding a copy of the arguments you handed over in a set_field_type call. Opaque = you don't peek inside, bestie. Respect the mystery. 🙈

🎀 field_type

Returns a pointer to the field type associated with the form field — you know, the one you set by calling set_field_type. Matching set, cute. 💅

🎀 set_field_type

The function set_field_type associates a field type with a given form field. This is the type that the validation functions actually check against. Most field types are configurable, through arguments that the caller provides right there in the set_field_type call. You bring the args, the type brings the rules. 🤝

🌈 FIELD TYPES 🌈

The form library defines several field types. Under the hood they're implemented via the FIELDTYPE data structure, which contains several pointers to functions. Very struct, very function-pointer chic. ✨

Go look at the fieldtype(3FORM) manual page — that one describes the functions you can use to construct a field-type dynamically, if the built-in ones aren't serving your main character moment.

The predefined types are as follows, and yes, we're going through every single one because I don't gatekeep:

⭐ 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.

Good news, bestie: the library copies the string list, so you may use a list that lives in automatic variables on the stack. No dangling-pointer heartbreak. 🫶

⭐ 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). Go read it, I'll wait. 🍓

⭐ 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). Same drill. 💜

⭐ 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. No partial-credit vibes here.

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. Spaces said "we're part of the fit too". 👑

⭐ TYPE_IPV4

An Internet Protocol Version 4 address. Required parameter:

• 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. So it checks the shape, not the truth. Shape-checking is not the same as honesty, bestie. 😭

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

And finally — yes — it is possible to set up new programmer-defined field types. Build your own, diva. 🧚

🔥 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)). Not slay. 💀

🌍 PORTABILITY 🌍

These routines emulate the System V forms library. They were not supported on Version 7 or BSD versions. Ancient history behaviour, fr fr.

👑 AUTHORS 👑

Juergen Pfeifer. Manual pages and adaptation for new curses by Eric S. Raymond. Legends, both of them, we stan. 🙌

👀 SEE ALSO 👀

• ncurses(3NCURSES)
• form(3FORM)
• fieldtype(3FORM)
• form_variables(3FORM)

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aaaand that's the whole page, bestie, you ate it. 🎀 This was ncurses version 6.6, dated 2025-08-16, and it's filed as field_validation(3FORM). Go forth and validate responsibly. 💖✨🦋