### 8.5 Saving text until end of input ¶

It is possible to ‘save’ some text until the end of the normal input has been seen. Text can be saved, to be read again by `m4` when the normal input has been exhausted. This feature is normally used to initiate cleanup actions before normal exit, e.g., deleting temporary files.

To save input text, use the builtin `m4wrap`:

Builtin: m4wrap(string, …) ¶

  Stores string in a safe place, to be reread when end of input is reached. As a GNU extension, additional arguments are concatenated with a space to the string.

  The expansion of `m4wrap` is void. The macro `m4wrap` is recognized only with parameters.

    define(`cleanup', `This is the `cleanup' action.
    ')
    ⇒
    m4wrap(`cleanup')
    ⇒
    This is the first and last normal input line.
    ⇒This is the first and last normal input line.
    ^D
    ⇒This is the cleanup action.

The saved input is only reread when the end of normal input is seen, and not if `m4exit` is used to exit `m4`.

It is safe to call `m4wrap` from saved text, but then the order in which the saved text is reread is undefined. If `m4wrap` is not used recursively, the saved pieces of text are reread in the opposite order in which they were saved (LIFO—last in, first out). However, this behavior is likely to change in a future release, to match POSIX, so you should not depend on this order.

It is possible to emulate POSIX behavior even with older versions of GNU M4 by including the file `m4-1.4.21/examples/wrapfifo.m4` from the distribution:

    $ m4 -I examples
    undivert(`wrapfifo.m4')dnl
    ⇒dnl Redefine m4wrap to have FIFO semantics.
    ⇒define(`_m4wrap_level', `0')dnl
    ⇒define(`m4wrap',
    ⇒`ifdef(`m4wrap'_m4wrap_level,
    ⇒       `define(`m4wrap'_m4wrap_level,
    ⇒               defn(`m4wrap'_m4wrap_level)`$1')',
    ⇒       `builtin(`m4wrap', `define(`_m4wrap_level',
    ⇒                                  incr(_m4wrap_level))dnl
    ⇒m4wrap'_m4wrap_level)dnl
    ⇒define(`m4wrap'_m4wrap_level, `$1')')')dnl
    include(`wrapfifo.m4')
    ⇒
    m4wrap(`a`'m4wrap(`c
    ', `d')')m4wrap(`b')
    ⇒
    ^D
    ⇒abc

It is likewise possible to emulate LIFO behavior without resorting to the GNU M4 extension of `builtin`, by including the file `m4-1.4.21/examples/wraplifo.m4` from the distribution. (Unfortunately, both examples shown here share some subtle bugs. See if you can find and correct them; or see Answers).

    $ m4 -I examples
    undivert(`wraplifo.m4')dnl
    ⇒dnl Redefine m4wrap to have LIFO semantics.
    ⇒define(`_m4wrap_level', `0')dnl
    ⇒define(`_m4wrap', defn(`m4wrap'))dnl
    ⇒define(`m4wrap',
    ⇒`ifdef(`m4wrap'_m4wrap_level,
    ⇒       `define(`m4wrap'_m4wrap_level,
    ⇒               `$1'defn(`m4wrap'_m4wrap_level))',
    ⇒       `_m4wrap(`define(`_m4wrap_level', incr(_m4wrap_level))dnl
    ⇒m4wrap'_m4wrap_level)dnl
    ⇒define(`m4wrap'_m4wrap_level, `$1')')')dnl
    include(`wraplifo.m4')
    ⇒
    m4wrap(`a`'m4wrap(`c
    ', `d')')m4wrap(`b')
    ⇒
    ^D
    ⇒bac

Here is an example of implementing a factorial function using `m4wrap`:

    define(`f', `ifelse(`$1', `0', `Answer: 0!=1
    ', eval(`$1>1'), `0', `Answer: $2$1=eval(`$2$1')
    ', `m4wrap(`f(decr(`$1'), `$2$1*')')')')
    ⇒
    f(`10')
    ⇒
    ^D
    ⇒Answer: 10*9*8*7*6*5*4*3*2*1=3628800

Invocations of `m4wrap` at the same recursion level are concatenated and rescanned as usual:

    define(`aa', `AA
    ')
    ⇒
    m4wrap(`a')m4wrap(`a')
    ⇒
    ^D
    ⇒AA

however, the transition between recursion levels behaves like an end of file condition between two input files.

    m4wrap(`m4wrap(`)')len(abc')
    ⇒
    ^D
    error→m4:stdin:1: ERROR: end of file in argument list