The file id-lang.map, installed by default in $(prefix)/share/id-lang.map, contains rules for mapping file names to source languages. Each rule comprises three parts: a shell glob pattern, a language name, and language-specific scanner options.
The special pattern ‘**’ denotes the default source language. This is the language that's assigned to file names that don't match any other pattern.
The special pattern ‘***’ should be followed by a file name. The named file should contain more language-map rules and is included at this point.
The order in which rules are presented in a language-map file is significant. This order influences the order in which files are displayed as the result of queries. For example, the distributed language-map file places all rules for C .h files ahead of .c files, so that in general, declarations will precede definitions in query output. The same thing is done for C++ and its many different source file name extensions.
Here is a pared-down version of the id-lang.map file distributed with the ID utilities:
# Default language
** IGNORE # Although this is listed first,
# the default language pattern is
# logically matched last.
# Backup files
*~ IGNORE
*.bak IGNORE
*.bk[0-9] IGNORE
# SCCS files
[sp].* IGNORE
# list header files before code files
*.h C
*.h.in C
*.H C++
*.hh C++
*.hpp C++
*.hxx C++
# list C `meta' files next
*.l C
*.lex C
*.y C
*.yacc C
# list C code files after header files
*.c C
*.C C++
*.cc C++
*.cpp C++
*.cxx C++
# list assembly language after C
*.[sS] asm --comment=;
*.asm asm --comment=;
# [nt]roff
*.[0-9] roff
*.ms roff
*.me roff
*.mm roff
# TeX and friends
*.tex TeX
*.ltx TeX
*.texi texinfo
*.texinfo texinfo