Chapter 7. Output to Display

65

Concerning the translated messages by

gettext

, the software does not need anything. It works

well if the user properly set LC_CTYPE and LC_MESSAGES locale.

If you are handling a string in non ASCII encoding (using multibyte character, UTF 8 directly,

and so on), you will have to care about points which you don't have to care about if you are using

ASCII.

  8 bit cleanness. I think everyone understand this.

  Continuity of multibyte characters. In multibyte encodings such as EUC JP and UTF 8, one

character may consist from more than two bytes. These bytes should be outputed continued.

Insertion of additional codes between the continuing bytes can break the character. I have

seen a software which outputs location control code everytime it outputs one byte. It breaks

multibyte character.

7.1.2 Number of Columns

Internationalized console software cannot assume that a character always occupy one column.

You can get the number of column of a character of a string using

wcwidth()

and

wcswidth()

.

Note that you have to use

wchar_t

 style programming since these functions have a

wchar_t

parameter.

Additional cares have to be taken not to destroy multicolumn characters. For example, imagine

your software displayed a double column character at (row, column) = (1, 1). What will occur

when your software then display a single column character at (row, column) = (1, 2) or at (1, 1) ?

The single column character erases the half of the double column character? Nobody knows the

answer. It depends on the implementation of the console. All what I can tell is that your software

should avoid such cases.

If your software inputs a string from keyboard, you will have to take more cares. All of numbers

of characters, bytes, and columns differ. For example, in UTF 8 encoding, one character of 'a'

with acute accent occupies two bytes and one column. One character of CJK ideograph occupies

three bytes and two columns. For example, if the user types 'Backspace', how many backspace

code (0x08) should the software outputs? How many bytes should the software erase from the

internal buffer? Don't be nervous; you can use

wchar_t

which assures one character occupy one

wchar_t

everytime and you can use

wcwidth()

to know the number of columns. Note that

control codes such as 'backspace' (0x08) and so on are column oriented everytime. It backs 'one'

column even if the character at the position is a doublewidth character.