form_fieldtype - define validation-field types
You also supply new_fieldtype with char_check, a function to validate input characters as they are entered; it will be passed the character to be checked and a pointer to an argument-block structure.
The function free_fieldtype frees the space allocated for a given validation type.
The function set_fieldtype_arg associates three storage-management functions with a field type. The make_arg function is automatically applied to the list of arguments you give set_field_type when attaching validation to a field; its job is to bundle these into an allocated argument-block object which can later be passed to validation predicated. The other two hook arguments should copy and free argument-block structures. They will be used by the forms-driver code. You must supply the make_arg function, the other two are optional, you may supply NULL for them. In this case it is assumed that make_arg does not allocate memory but simply loads the argument into a single scalar value.
The function link_fieldtype creates a new field type from the two given types. They are connected by an logical 'OR'.
The form driver requests REQ_NEXT_CHOICE and REQ_PREV_CHOICE assume that the possible values of a field form an ordered set, and provide the forms user with a way to move through the set. The set_fieldtype_choice function allows forms programmers to define successor and predecessor functions for the field type. These functions take the field pointer and an argument-block structure as arguments.
The integer-valued routines return one of the following codes on error:
All of the (char *) arguments of these functions should actually be (void *). The type has been left uncorrected for strict compatibility with System V.
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