File: m4.info,  Node: Other Incompatibilities,  Prev: Incompatibilities,  Up: Compatibility

16.3 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 was made, and not where it was inserted again.

          divert(2)2
          divert(1)1
          divert`'0
          ⇒#line 3 "stdin"
          ⇒0
          ^D
          ⇒#line 2 "stdin"
          ⇒1
          ⇒#line 1 "stdin"
          ⇒2

     The current ‘m4’ implementation has a limitation that the syncline
     output at the start of each diversion occurs no matter what, even
     if the previous diversion did not end with a newline.  This goes
     contrary to the claim that synclines appear on a line by
     themselves, so this limitation may be corrected in a future version
     of ‘m4’.  In the meantime, when using ‘-s’, it is wisest to make
     sure all diversions end with newline.

   • 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, because that would cause
     an infinite rescan loop.  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.