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.

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