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