encoding

Section: Tcl Built-In Commands (n)
Updated: 8.1
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NAME

encoding - Manipulate encodings

SYNOPSIS

encoding option ?arg arg ...?

INTRODUCTION

Strings in Tcl are logically a sequence of 16-bit Unicode characters. These strings are represented in memory as a sequence of bytes that may be in one of several encodings: modified UTF-8 (which uses 1 to 3 bytes per character), 16-bit ``Unicode'' (which uses 2 bytes per character, with an endianness that is dependent on the host architecture), and binary (which uses a single byte per character but only handles a restricted range of characters). Tcl does not guarantee to always use the same encoding for the same string.

Different operating system interfaces or applications may generate strings in other encodings such as Shift-JIS. The encoding command helps to bridge the gap between Unicode and these other formats.

DESCRIPTION

Performs one of several encoding related operations, depending on option. The legal options are:

encoding convertfrom ?encoding? data: Convert data to Unicode from the specified encoding. The characters in data are treated as binary data where the lower 8-bits of each character is taken as a single byte. The resulting sequence of bytes is treated as a string in the specified encoding. If encoding is not specified, the current system encoding is used.
encoding convertto ?encoding? string: Convert string from Unicode to the specified encoding. The result is a sequence of bytes that represents the converted string. Each byte is stored in the lower 8-bits of a Unicode character (indeed, the resulting string is a binary string as far as Tcl is concerned, at least initially). If encoding is not specified, the current system encoding is used.
encoding dirs ?directoryList?: Tcl can load encoding data files from the file system that describe additional encodings for it to work with. This command sets the search path for *.enc encoding data files to the list of directories directoryList. If directoryList is omitted then the command returns the current list of directories that make up the search path. It is an error for directoryList to not be a valid list. If, when a search for an encoding data file is happening, an element in directoryList does not refer to a readable, searchable directory, that element is ignored.
encoding names: Returns a list containing the names of all of the encodings that are currently available. The encodings ``utf-8'' and ``iso8859-1'' are guaranteed to be present in the list.
encoding system ?encoding?: Set the system encoding to encoding. If encoding is omitted then the command returns the current system encoding. The system encoding is used whenever Tcl passes strings to system calls.

EXAMPLE

It is common practice to write script files using a text editor that produces output in the euc-jp encoding, which represents the ASCII characters as singe bytes and Japanese characters as two bytes. This makes it easy to embed literal strings that correspond to non-ASCII characters by simply typing the strings in place in the script. However, because the source command always reads files using the current system encoding, Tcl will only source such files correctly when the encoding used to write the file is the same. This tends not to be true in an internationalized setting. For example, if such a file was sourced in North America (where the ISO8859-1 is normally used), each byte in the file would be treated as a separate character that maps to the 00 page in Unicode. The resulting Tcl strings will not contain the expected Japanese characters. Instead, they will contain a sequence of Latin-1 characters that correspond to the bytes of the original string. The encoding command can be used to convert this string to the expected Japanese Unicode characters. For example,


set s [encoding convertfrom euc-jp "\xA4\xCF"]

would return the Unicode string ``\u306F'', which is the Hiragana letter HA.

KEYWORDS

encoding, unicode