(m4.info.gz) Other Incompatibilities

 
 15.3 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 file names 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.
 
      The sync line option is 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 prohibiting self-referential
      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 leaves 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.
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