(autoconf.info)Autoconf Language

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3.1.2 The Autoconf Language

The Autoconf language differs from many other computer languages
because it treats actual code the same as plain text.  Whereas in C,
for instance, data and instructions have different syntactic status, in
Autoconf their status is rigorously the same.  Therefore, we need a
means to distinguish literal strings from text to be expanded:

   When calling macros that take arguments, there must not be any white
space between the macro name and the open parenthesis.  Arguments should
be enclosed within the M4 quote characters `[' and `]', and be
separated by commas.  Any leading blanks or newlines in arguments are
ignored, unless they are quoted.  You should always quote an argument
that might contain a macro name, comma, parenthesis, or a leading blank
or newline.  This rule applies recursively for every macro call,
including macros called from other macros.

   For instance:

                     [AC_DEFINE([HAVE_STDIO_H], [1],
                        [Define to 1 if you have <stdio.h>.])],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

is quoted properly.  You may safely simplify its quotation to:

                     [AC_DEFINE([HAVE_STDIO_H], 1,
                        [Define to 1 if you have <stdio.h>.])],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

because `1' cannot contain a macro call.  Here, the argument of
`AC_MSG_ERROR' must be quoted; otherwise, its comma would be
interpreted as an argument separator.  Also, the second and third
arguments of `AC_CHECK_HEADER' must be quoted, since they contain macro
calls.  The three arguments `HAVE_STDIO_H', `stdio.h', and `Define to 1
if you have <stdio.h>.' do not need quoting, but if you unwisely
defined a macro with a name like `Define' or `stdio' then they would
need quoting.  Cautious Autoconf users would keep the quotes, but many
Autoconf users find such precautions annoying, and would rewrite the
example as follows:

                     [AC_DEFINE(HAVE_STDIO_H, 1,
                        [Define to 1 if you have <stdio.h>.])],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

This is safe, so long as you adopt good naming conventions and do not
define macros with names like `HAVE_STDIO_H', `stdio', or `h'.  Though
it is also safe here to omit the quotes around `Define to 1 if you have
<stdio.h>.' this is not recommended, as message strings are more likely
to inadvertently contain commas.

   The following example is wrong and dangerous, as it is underquoted:

                     AC_DEFINE(HAVE_STDIO_H, 1,
                        Define to 1 if you have <stdio.h>.),
                     AC_MSG_ERROR([Sorry, can't do anything for you]))

   In other cases, you may have to use text that also resembles a macro
call.  You must quote that text even when it is not passed as a macro

     echo "Hard rock was here!  --[AC_DC]"

which results in:

     echo "Hard rock was here!  --AC_DC"

When you use the same text in a macro argument, you must therefore have
an extra quotation level (since one is stripped away by the macro
substitution).  In general, then, it is a good idea to _use double
quoting for all literal string arguments_:

     AC_MSG_WARN([[AC_DC stinks  --Iron Maiden]])

   You are now able to understand one of the constructs of Autoconf that
has been continually misunderstood...  The rule of thumb is that
_whenever you expect macro expansion, expect quote expansion_; i.e.,
expect one level of quotes to be lost.  For instance:

     AC_COMPILE_IFELSE([char b[10];], [], [AC_MSG_ERROR([you lose])])

is incorrect: here, the first argument of `AC_COMPILE_IFELSE' is `char
b[10];' and is expanded once, which results in `char b10;'.  (There was
an idiom common in Autoconf's past to address this issue via the M4
`changequote' primitive, but do not use it!)  Let's take a closer look:
the author meant the first argument to be understood as a literal, and
therefore it must be quoted twice:

     AC_COMPILE_IFELSE([[char b[10];]], [], [AC_MSG_ERROR([you lose])])

Voila`, you actually produce `char b[10];' this time!

   On the other hand, descriptions (e.g., the last parameter of
`AC_DEFINE' or `AS_HELP_STRING') are not literals--they are subject to
line breaking, for example--and should not be double quoted.  Even if
these descriptions are short and are not actually broken, double
quoting them yields weird results.

   Some macros take optional arguments, which this documentation
represents as [ARG] (not to be confused with the quote characters).
You may just leave them empty, or use `[]' to make the emptiness of the
argument explicit, or you may simply omit the trailing commas.  The
three lines below are equivalent:

     AC_CHECK_HEADERS([stdio.h], [], [], [])

   It is best to put each macro call on its own line in `configure.ac'.
Most of the macros don't add extra newlines; they rely on the newline
after the macro call to terminate the commands.  This approach makes
the generated `configure' script a little easier to read by not
inserting lots of blank lines.  It is generally safe to set shell
variables on the same line as a macro call, because the shell allows
assignments without intervening newlines.

   You can include comments in `configure.ac' files by starting them
with the `#'.  For example, it is helpful to begin `configure.ac' files
with a line like this:

     # Process this file with autoconf to produce a configure script.

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