Node:Active Characters, Next:One Macro Call, Up:M4 Quotation

Active Characters

To fully understand where proper quotation is important, you first need to know what are the special characters in Autoconf: # introduces a comment inside which no macro expansion is performed, , separates arguments, [ and ] are the quotes themselves, and finally ( and ) (which m4 tries to match by pairs).

In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are "obvious-ified", although 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 will try hopelessly to use her macro array:

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

How can you correctly output the intended results¹?

Footnotes

1. Using defn.