Extensions in GNU m4

This version of m4 contains a few facilities, that do not exist in System V m4. These extra facilities are all suppressed by using the `-G' command line option, unless overridden by other command line options.

• In the $n notation for macro arguments, n can contain several digits, while the System V m4 only accepts one digit. This allows macros in GNU m4 to take any number of arguments, and not only nine (see section Arguments to macros).
• Files included with include and sinclude are sought in a user specified search path, if they are not found in the working directory. The search path is specified by the `-I' option and the `M4PATH' environment variable (see section Searching for include files).
• Arguments to undivert can be non-numeric, in which case the named file will be included uninterpreted in the output (see section Undiverting output).
• Formatted output is supported through the format builtin, which is modeled after the C library function printf (see section Formatted output).
• Searches and text substitution through regular expressions are supported by the regexp (see section Searching for regular expressions) and patsubst (see section Substituting text by regular expression) builtins.
• The output of shell commands can be read into m4 with esyscmd (see section Reading the output of commands).
• There is indirect access to any builtin macro with builtin (see section Indirect call of builtins).
• Macros can be called indirectly through indir (see section Indirect call of macros).
• The name of the current input file and the current input line number are accessible through the builtins __file__ and __line__ (see section Printing error messages).
• The format of the output from dumpdef and macro tracing can be controlled with debugmode (see section Controlling debugging output).
• The destination of trace and debug output can be controlled with debugfile (see section Saving debugging output).

In addition to the above extensions, GNU m4 implements the following command line options: `-F', `-G', `-I', `-L', `-R', `-V', `-W', `-d', `-l', `-o' and `-t'. See section Invoking m4, for a description of these options.

Also, the debugging and tracing facilities in GNU m4 are much more extensive than in most other versions of m4.

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