(autoconf.info)Active Characters

Next: One Macro Call Up: M4 Quotation

8.1.1 Active Characters

To fully understand where proper quotation is important, you first need
to know what the special characters are in Autoconf: `#' introduces a
comment inside which no macro expansion is performed, `,' separates
arguments, `[' and `]' are the quotes themselves, `(' and `)' (which M4
tries to match by pairs), and finally `$' inside a macro definition.

   In order to understand the delicate case of macro calls, we first
have to present some obvious failures.  Below they are "obvious-ified",
but when you find them in real life, they are usually in disguise.

   Comments, introduced by a hash and running up to the newline, are
opaque tokens to the top level: active characters are turned off, and
there is no macro expansion:

     # define([def], ine)
     =># define([def], ine)

   Each time there can be a macro expansion, there is a quotation
expansion, i.e., one level of quotes is stripped:

     int tab[10];
     =>int tab10;
     [int tab[10];]
     =>int tab[10];

   Without this in mind, the reader might try hopelessly to use her
macro `array':

     define([array], [int tab[10];])
     =>int tab10;

How can you correctly output the intended results(1)?

   ---------- Footnotes ----------

   (1) Using `defn'.

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