---
name: "field_arg"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/field_arg"
---

# field\_arg(3)

data type validation for fields

## 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:**

- •

  **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.**

### TYPE\_INTEGER

**Integer data, parsable to an integer by [atoi(3)](/atoi/3.md).** **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)](/printf/3.md).**

### 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)](/printf/3.md).**

### 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**

**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)](/errno/3.md)).**

## 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)**