How do I escape text in autoconf/m4?

Question

The following code from my configuration.ac file does not work (note the nested square brackets with [default=no]):

AC_ARG_ENABLE(debug,
    [  --enable-debug          build with debugging support [default=no].],
    [DEBUG="$enableval"],
    [DEBUG="no"]
)

How can I escape those brackets?

Mimamsa

I don't see any Bash. Why the tag? – Trexler

Autoconf scripts are parsed first by m4 and then by the bourne shell. – Mimamsa

Answer 1

Found it! From this tutorial:

M4 arguments are quoted with [ and ]. There is NO WAY to escape these, however, you have several options if you wish to insert ['s or ]'s:

1. Use a `Quadrigraph'. @<:@ gives you [ and @:>@ gives you ].
2. Balance your quotes. M4 will turn [[]] in to []. Beware of using this in arguments to macros. Sometimes, you need to double quote as well ([[[]]]).
3. Change the quoting using: changequote(<<,>>) to change the quoting to << and >>. The autoconf documentation (rightly, in my opinion) warns against the (over) use of this, since it can lead to unexpected results.
4. Avoid [ and ] where ever possible. This is my personal choice.

My new code is therefore:

AC_ARG_ENABLE(debug,
    AS_HELP_STRING(
        [--enable-debug],
        [build with debugging symbols @<:@default=no@:>@]),
    [enable_debug="$enableval"],
    [enable_debug="no"]
)

Mimamsa

You should set your own answer as THE answer then. – Freshet

Autotools absolutely sucks. – Hexameter

The quoted tutorial has a new home. – Inferential

Answer 2

Brackets are kind of escape characters, so as you do for '\', you may escape brackets [] with brackets, eg :

AC_ARG_ENABLE(debug, [ --enable-debug build with debugging support [[default=no]].], [DEBUG="$enableval"], [DEBUG="no"] )

Note: [[ ]default=no[ ]] may not work as you expect as m4 should search the end_bracket from the end. It should so be expanded to [ ]default=no[ ].

Hercule

Better to use the 'quadrigraph' escapes, @<:@ and @>:@. – Bobbyebobbysocks

Answer 3

use AC_HELP_STRING

Rome

Thanks (that is definitely cleaner) but still the same problem. In <<AS_HELP_STRING([--enable-debug],[build with debugging support [default=no].])>> the "[" and "]" are missing from output. – Mimamsa

Why the insistence on square brackets in the output? Use parentheses; it makes your life easier. – Politics

For consistency's sake, because other packages I'm using have square brackets in their output :( – Mimamsa

AC_HELP_STRING been deprecated about half a decade ago. Use AS_HELP_STRING. – Renown