This chapter describes the differences between this implementation of
m4, and the implementation found under UNIX, notably System V,
There are also differences in BSD flavors of
m4. No attempt
is made to summarize these here.
This version of
m4 contains a few facilities, that do not exist
in System V
m4. These extra facilities are all suppressed by
using the `-G' command line option, unless overridden by other
command line options.
$n notation for macro arguments, n can contain several digits, while the System V
m4only accepts one digit. This allows macros in GNU
m4to take any number of arguments, and not only nine (see section Arguments to macros).
sincludeare sought in a user specified search path, if they are not found in the working directory. The search path is specified by the `-I' option and the `M4PATH' environment variable (see section Searching for include files).
undivertcan be non-numeric, in which case the named file will be included uninterpreted in the output (see section Undiverting output).
formatbuiltin, which is modeled after the C library function
printf(see section Formatted output).
regexp(see section Searching for regular expressions) and
patsubst(see section Substituting text by regular expression) builtins.
esyscmd(see section Reading the output of commands).
builtin(see section Indirect call of builtins).
indir(see section Indirect call of macros).
__line__(see section Printing error messages).
dumpdefand macro tracing can be controlled with
debugmode(see section Controlling debugging output).
debugfile(see section Saving debugging output).
In addition to the above extensions, GNU
m4 implements the
following command line options: `-F', `-G', `-I',
`-L', `-R', `-V', `-W', `-d',
`-l', `-o' and `-t'. See section Invoking
m4, for a
description of these options.
Also, the debugging and tracing facilities in GNU
m4 are much
more extensive than in most other versions of
m4not in GNU
The version of
m4 from System V contains a few facilities that
have not been implemented in GNU
m4supports multiple arguments to
defn. This is not implemented in GNU
m4. Its usefulness is unclear to me.
There are a few other incompatibilities between this implementation of
m4, and the System V version.
m4implements sync lines differently from System V
m4, when text is being diverted. GNU
m4outputs the sync lines when the text is being diverted, and System V
m4when 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
m4regards all the diverted text as being generated by the source line containing the
undivertcall, whereas GNU
m4regards the diverted text as being generated at the time it is diverted. I expect the sync line option to be used mostly when using
m4as 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.
m4makes 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
m4user to hang himself! Rescanning hangs may be avoided through careful programming, a little like for endless loops in traditional programming languages.
m4without `-G' option will define the macro
__gnu__to expand to the empty string. On UNIX systems, GNU
m4without the `-G' option will define the macro
__unix__, otherwise the macro
unix. Both will expand to the empty string.