It is been shown how
m4 expands macros when it encounters a name
that matches a defined macro in the input. There are times, however,
when you wish to defer expansion. Principally, there are three situations
when this is so:
- Free-form text
- There may be free-form text that you wish to appear at the output--and
as such, be unaltered by any macros that may be inadvertently invoked in
the input. It is not always possible to know if some particular name is
defined as a macro, so it should be quoted.
- Overcoming syntax rules
- Sometimes you may wish to form strings which would violate M4's
syntax rules -- for example, you might wish to use leading whitespace or
a comma in a macro argument. The solution is to quote the entire
- Macro arguments
- This is the most common situation for quoting: when arguments to macros
are to be taken literally and not expanded as the arguments are
collected. In the previous section, an example was given that
demonstrates the effects of not quoting the first argument to
define. Quoting macro arguments is considered a good practice
that you should emulate.
Strings are quoted by surrounding the quoted text with the ``' and
`'' characters. When
m4 encounters a quoted string--as a
type of token (21.3.1 Token scanning)--the quoted string is expanded to
the string itself, with the outermost quote characters removed.
Here is an example of a string that is triple quoted:
A more concrete example uses quoting to demonstrate how to prevent
unwanted expansion within macro definitions:
When the macro `foo' is defined,
m4 strips off the outermost
quotes and registers the definition
dnl text has
a special purpose, too, which will be covered in 21.4.1 Discarding input.
As the macro `foo' is expanded, the next pair of quote characters
are stripped off and the string is expanded to `bar'. Since the
expansion of the quoted string is the string itself (minus the quote
characters), we have prevented unwanted expansion from the string
`bar' to `zog'.
As mentioned in 21.3.1 Token scanning, the default M4 quote
characters are ``' and `''. Since these are two commonly used
characters in Bourne shell programming (51),
Autoconf reassigns these to the `[' and `]' characters--a
symmetric looking pair of characters least likely to cause problems when
writing GNU Autotools macros. From this point forward, we shall use
`[' and `]' as the quote characters and you can forget about
the default M4 quotes.
Autoconf uses M4's built-in
changequote macro to
perform this reassignment and, in fact, this built-in is still available
to you. In recent years, the common practice when needing to use the
quote characters `[' or `]' or to quote a string with an
legitimately imbalanced number of the quote characters has been to
changequote and temporarily reassign them around the
dnl Uh-oh, we need to use the apostrophe! And even worse, we have two
dnl opening quote marks and no closing quote marks.
perl -e 'print "$]\n";'
This leads to a few potential problems, the least of which is that it's
easy to reassign the quote characters and then forget to reset them,
leading to total chaos! Moreover, it is possible to entirely disable
M4's quoting mechanism by blindly changing the quote characters
to a pair of empty strings.
In hindsight, the overwhelming conclusion is that using
changequote within the GNU Autotools framework is a bad idea.
Instead, leave the quote characters assigned as `[' and `]'
and use the special strings
@:>@ anywhere you
want real square brackets to appear in your output. This is an easy
practice to adopt, because it's faster and less error prone than using
perl -e 'print "$@:>@\n";'
This, and other guidelines for using M4 in the GNU Autotools
framework are covered in detail in 21.5 Writing macros within the GNU Autotools framework.