m4: Other Incompatibilities
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.