(m4.info)Limits control

---

Next: Frozen state Prev: Preprocessor features Up: Invoking m4

---

Enter node , (file) or (file)node

2.3 Command line options for limits control
===========================================

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.  Note: 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 Note:
     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.