m4: Limits control
2.3 Command line options for limits control
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There are some limits within 'm4' that can be tuned. For compatibility,
'm4' also accepts some options that control limits in other
implementations, but which are automatically unbounded (limited only by
your hardware and operating system constraints) in GNU 'm4'.
'-g'
'--gnu'
Enable all the extensions in this implementation. In this release
of M4, this option is always on by default; it is currently only
useful when overriding a prior use of '--traditional'. However,
having GNU behavior as default makes it impossible to write a
strictly POSIX-compliant client that avoids all incompatible GNU M4
extensions, since such a client would have to use the non-POSIX
command-line option to force full POSIX behavior. Thus, a future
version of M4 will be changed to implicitly use the option
'--traditional' if the environment variable 'POSIXLY_CORRECT' is
set. Projects that intentionally use GNU extensions should
consider using '--gnu' to state their intentions, so that the
project will not mysteriously break if the user upgrades to a newer
M4 and has 'POSIXLY_CORRECT' set in their environment.
'-G'
'--traditional'
Suppress all the extensions made in this implementation, compared
to the System V version. ⇒Compatibility, for a list of
these.
'-H NUM'
'--hashsize=NUM'
Make the internal hash table for symbol lookup be NUM entries big.
For better performance, the number should be prime, but this is not
checked. The default is 509 entries. It should not be necessary
to increase this value, unless you define an excessive number of
macros.
'-L NUM'
'--nesting-limit=NUM'
Artificially limit the nesting of macro calls to NUM levels,
stopping program execution if this limit is ever exceeded. When
not specified, nesting defaults to unlimited on platforms that can
detect stack overflow, and to 1024 levels otherwise. A value of
zero means unlimited; but then heavily nested code could
potentially cause a stack overflow.
The precise effect of this option is more correctly associated with
textual nesting than dynamic recursion. It has been useful when
some complex 'm4' input was generated by mechanical means, and also
in diagnosing recursive algorithms that do not scale well. Most
users never need to change this option from its default.
This option does _not_ have the ability to break endless rescanning
loops, since these do not necessarily consume much memory or stack
space. Through clever usage of rescanning loops, one can request
complex, time-consuming computations from 'm4' with useful results.
Putting limitations in this area would break 'm4' power. There are
many pathological cases: 'define(`a', `a')a' is only the simplest
example (but ⇒Compatibility). Expecting GNU 'm4' to detect
these would be a little like expecting a compiler system to detect
and diagnose endless loops: it is a quite _hard_ problem in
general, if not undecidable!
'-B NUM'
'-S NUM'
'-T NUM'
These options are present for compatibility with System V 'm4', but
do nothing in this implementation. They may disappear in future
releases, and issue a warning to that effect.
'-N NUM'
'--diversions=NUM'
These options are present only for compatibility with previous
versions of GNU 'm4', and were controlling the number of possible
diversions which could be used at the same time. They do nothing,
because there is no fixed limit anymore. They may disappear in
future releases, and issue a warning to that effect.
Info Catalog
m4: Preprocessor features
m4: Invoking m4
m4: Frozen state