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(m4.info)Frozen files

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Fast loading of frozen states
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   Some bigger `m4' applications may be built over a common base
containing hundreds of definitions and other costly initializations.
Usually, the common base is kept in one or more declarative files,
which files are listed on each `m4' invocation prior to the user's
input file, or else, `include''d from this input file.

   Reading the common base of a big application, over and over again,
may be time consuming.  GNU `m4' offers some machinery to speed up the
start of an application using lengthy common bases.  Presume the user
repeatedly uses:

     m4 base.m4 input.m4

with a varying contents of `input.m4', but a rather fixed contents for
`base.m4'.  Then, the user might rather execute:

     m4 -F base.m4f base.m4

once, and further execute, as often as needed:

     m4 -R base.m4f input.m4

with the varying input.  The first call, containing the `-F' option,
only reads and executes file `base.m4', so defining various application
macros and computing other initializations.  Only once the input file
`base.m4' has been completely processed, GNU `m4' produces on
`base.m4f' a "frozen" file, that is, a file which contains a kind of
snapshot of the `m4' internal state.

   Later calls, containing the `-R' option, are able to reload the
internal state of `m4''s memory, from `base.m4f', _prior_ to reading
any other input files.  By this mean, instead of starting with a virgin
copy of `m4', input will be read after having effectively recovered the
effect of a prior run.  In our example, the effect is the same as if
file `base.m4' has been read anew.  However, this effect is achieved a
lot faster.

   Only one frozen file may be created or read in any one `m4'
invocation.  It is not possible to recover two frozen files at once.
However, frozen files may be updated incrementally, through using `-R'
and `-F' options simultaneously.  For example, if some care is taken,
the command:

     m4 file1.m4 file2.m4 file3.m4 file4.m4

could be broken down in the following sequence, accumulating the same
output:

     m4 -F file1.m4f file1.m4
     m4 -R file1.m4f -F file2.m4f file2.m4
     m4 -R file2.m4f -F file3.m4f file3.m4
     m4 -R file3.m4f file4.m4

   Some care is necessary because not every effort has been made for
this to work in all cases.  In particular, the trace attribute of
macros is not handled, nor the current setting of `changeword'.  Also,
interactions for some options of `m4' being used in one call and not
for the next, have not been fully analyzed yet.  On the other end, you
may be confident that stacks of `pushdef''ed definitions are handled
correctly, so are `undefine''d or renamed builtins, changed strings for
quotes or comments.

   When an `m4' run is to be frozen, the automatic undiversion which
takes place at end of execution is inhibited.  Instead, all positively
numbered diversions are saved into the frozen file.  The active
diversion number is also transmitted.

   A frozen file to be reloaded need not reside in the current
directory.  It is looked up the same way as an `include' file (Note:
Search Path).

   Frozen files are sharable across architectures.  It is safe to write
a frozen file one one machine and read it on another, given that the
second machine uses the same, or a newer version of GNU `m4'.  These
are simple (editable) text files, made up of directives, each starting
with a capital letter and ending with a newline (<NL>).  Wherever a
directive is expected, the character `#' introduces a comment line,
empty lines are also ignored.  In the following descriptions, LENGTHs
always refer to corresponding STRINGs.  Numbers are always expressed in
decimal.  The directives are:

`V NUMBER <NL>'
     Confirms the format of the file.  NUMBER should be 1.

`C LENGTH1 , LENGTH2 <NL> STRING1 STRING2 <NL>'
     Uses STRING1 and STRING2 as the beginning comment and end comment
     strings.

`Q LENGTH1 , LENGTH2 <NL> STRING1 STRING2 <NL>'
     Uses STRING1 and STRING2 as the beginning quote and end quote
     strings.

`F LENGTH1 , LENGTH2 <NL> STRING1 STRING2 <NL>'
     Defines, through `pushdef', a definition for STRING1 expanding to
     the function whose builtin name is STRING2.

`T LENGTH1 , LENGTH2 <NL> STRING1 STRING2 <NL>'
     Defines, though `pushdef', a definition for STRING1 expanding to
     the text given by STRING2.

`D NUMBER, LENGTH <NL> STRING <NL>'
     Selects diversion NUMBER, making it current, then copy STRING in
     the current diversion.  NUMBER may be a negative number for a
     non-existing diversion.  To merely specify an active selection,
     use this command with an empty STRING.  With 0 as the diversion
     NUMBER, STRING will be issued on standard output at reload time,
     however this may not be produced from within `m4'.