### 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 (see Searching for include files).

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.