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14. Fast loading of frozen states

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 (see section 8.2 Searching for include files).

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.

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