Character set conversions

The g_convert() family of function wraps the functionality of iconv(). In addition to pure character set conversions, GLib has functions to deal with the extra complications of encodings for file names.

File Name Encodings

Historically, UNIX has not had a defined encoding for file names: a file name is valid as long as it does not have path separators in it (“/”). However, displaying file names may require conversion: from the character set in which they were created, to the character set in which the application operates. Consider the Spanish file name “Presentación.sxi”. If the application which created it uses ISO-8859-1 for its encoding,

Character:  P  r  e  s  e  n  t  a  c  i  ó  n  .  s  x  i
Hex code:   50 72 65 73 65 6e 74 61 63 69 f3 6e 2e 73 78 69

However, if the application use UTF-8, the actual file name on disk would look like this:

Character:  P  r  e  s  e  n  t  a  c  i  ó     n  .  s  x  i
Hex code:   50 72 65 73 65 6e 74 61 63 69 c3 b3 6e 2e 73 78 69

Glib uses UTF-8 for its strings, and GUI toolkits like GTK that use GLib do the same thing. If you get a file name from the file system, for example, from readdir() or from g_dir_read_name(), and you wish to display the file name to the user, you will need to convert it into UTF-8. The opposite case is when the user types the name of a file they wish to save: the toolkit will give you that string in UTF-8 encoding, and you will need to convert it to the character set used for file names before you can create the file with open() or fopen().

By default, GLib assumes that file names on disk are in UTF-8 encoding. This is a valid assumption for file systems which were created relatively recently: most applications use UTF-8 encoding for their strings, and that is also what they use for the file names they create. However, older file systems may still contain file names created in “older” encodings, such as ISO-8859-1. In this case, for compatibility reasons, you may want to instruct GLib to use that particular encoding for file names rather than UTF-8. You can do this by specifying the encoding for file names in the G_FILENAME_ENCODING environment variable. For example, if your installation uses ISO-8859-1 for file names, you can put this in your ~/.profile:


GLib provides the functions g_filename_to_utf8() and g_filename_from_utf8() to perform the necessary conversions. These functions convert file names from the encoding specified in G_FILENAME_ENCODING to UTF-8 and vice-versa. This diagram illustrates how these functions are used to convert between UTF-8 and the encoding for file names in the file system.

Conversion between file name encodings

Checklist for Application Writers

This section is a practical summary of the detailed things to do to make sure your applications process file name encodings correctly.

  1. If you get a file name from the file system from a function such as readdir() or gtk_file_chooser_get_filename(), you do not need to do any conversion to pass that file name to functions like open(), rename(), or fopen() — those are “raw” file names which the file system understands.
  2. If you need to display a file name, convert it to UTF-8 first by using g_filename_to_utf8(). If conversion fails, display a string like “Unknown file name”. Do not convert this string back into the encoding used for file names if you wish to pass it to the file system; use the original file name instead.
  3. For example, the document window of a word processor could display “Unknown file name” in its title bar but still let the user save the file, as it would keep the raw file name internally. This can happen if the user has not set the G_FILENAME_ENCODING environment variable even though he has files whose names are not encoded in UTF-8.
  4. If your user interface lets the user type a file name for saving or renaming, convert it to the encoding used for file names in the file system by using g_filename_from_utf8(). Pass the converted file name to functions like fopen(). If conversion fails, ask the user to enter a different file name. This can happen if the user types Japanese characters when G_FILENAME_ENCODING is set to ISO-8859-1, for example.