File: m4.info,  Node: Using frozen files,  Next: Frozen file format,  Up: Frozen files

15.1 Using frozen files
=======================

Suppose a user has a library of ‘m4’ initializations in ‘base.m4’, which
is then used with multiple input files:

     $ m4 base.m4 input1.m4
     $ m4 base.m4 input2.m4
     $ m4 base.m4 input3.m4

   Rather than spending time parsing the fixed contents of ‘base.m4’
every time, the user might rather execute:

     $ m4 -F base.m4f base.m4

once, and further execute, as often as needed:

     $ m4 -R base.m4f input1.m4
     $ m4 -R base.m4f input2.m4
     $ m4 -R base.m4f input3.m4

with the varying input.  The first call, containing the ‘-F’ option,
only reads and executes file ‘base.m4’, defining various application
macros and computing other initializations.  Once the input file
‘base.m4’ has been completely processed, GNU ‘m4’ produces in ‘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’, from ‘base.m4f’, _prior_ to reading any other
input files.  This means 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’.  Currently,
‘m4wrap’ and ‘sysval’ also have problems.  Also, interactions for some
options of ‘m4’, being used in one call and not in the next, have not
been fully analyzed yet.  On the other end, you may be confident that
stacks of ‘pushdef’ definitions are handled correctly, as well as
undefined or renamed builtins, and changed strings for quotes or
comments.  And future releases of GNU M4 will improve on the utility of
frozen files.

   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::).

   If the frozen file was generated with a newer version of ‘m4’, and
contains directives that an older ‘m4’ cannot parse, attempting to load
the frozen file with option ‘-R’ will cause ‘m4’ to exit with status 63
to indicate version mismatch.