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Compatibility with other versions of m4

This chapter describes the differences between this implementation of m4, and the implementation found under UNIX, notably System V, Release 3.

There are also differences in BSD flavors of m4. No attempt is made to summarize these here.

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.

Facilities in System V m4 not in GNU m4

The version of m4 from System V contains a few facilities that have not been implemented in GNU m4 yet.

• System V m4 supports multiple arguments to defn. This is not implemented in GNU m4. Its usefulness is unclear to me.

Other incompatibilities

There are a few other incompatibilities between this implementation of m4, and the System V version.

• GNU m4 implements sync lines differently from System V m4, when text is being diverted. GNU m4 outputs the sync lines when the text is being diverted, and System V m4 when the diverted text is being brought back. The problem is which lines and filenames should be attached to text that is being, or has been, diverted. System V m4 regards all the diverted text as being generated by the source line containing the undivert call, whereas GNU m4 regards the diverted text as being generated at the time it is diverted. I expect the sync line option to be used mostly when using m4 as a front end to a compiler. If a diverted line causes a compiler error, the error messages should most probably refer to the place where the diversion were made, and not where it was inserted again.
• GNU m4 makes no attempt at prohiting autoreferential definitions like:

  define(`x', `x')
  define(`x', `x ')
  

  There is nothing inherently wrong with defining `x' to return `x'. The wrong thing is to expand `x' unquoted. In m4, one might use macros to hold strings, as we do for variables in other programming languages, further checking them with:

  ifelse(defn(`holder'), `value', ...)
  

  In cases like this one, an interdiction for a macro to hold its own name would be a useless limitation. Of course, this leave more rope for the GNU m4 user to hang himself! Rescanning hangs may be avoided through careful programming, a little like for endless loops in traditional programming languages.
• GNU m4 without `-G' option will define the macro __gnu__ to expand to the empty string. On UNIX systems, GNU m4 without the `-G' option will define the macro __unix__, otherwise the macro unix. Both will expand to the empty string.

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